What Is Chemical Toxicology?

Toxicology (toxicology) is an applied discipline that studies the harmful effects of exogenous factors (chemical, physical, and biological factors) on biological systems. It is a science that studies the toxicity, severity, frequency, and mechanism of toxic effects of a chemical substance on an organism, as well as the science of qualitative and quantitative evaluation of toxic effects. It is a discipline that predicts its harm to the human body and the ecological environment, and provides a scientific basis for determining safety limits and taking control measures.

toxicology

(Subject)

Toxicology is the study of exogenous factors (chemical, physical,

main

The currently accepted definition of toxicology is the science that studies the harm of exogenous chemicals to organisms. Since the purpose of toxicology research is to protect the health or safety of organisms, toxicology belongs to the nature of the discipline

It can be divided into three parts: descriptive toxicology, mechanism toxicology, and management toxicology (also called regulatory toxicology).

According to the standard disciplines, it can be divided into: forensic toxicology, clinical toxicology, management toxicology or regulatory toxicology, research toxicology, and so on.

From applied toxicology can be divided into: food toxicology, industrial toxicology, pesticide toxicology, military toxicology, radiation toxicology, environmental toxicology, ecological toxicology and other branches.

Research objects can be divided into: insect toxicology, veterinary toxicology, human toxicology and phytotoxicology.

From the field of research can be divided into: drug toxicology, environmental toxicology, food toxicology, industrial toxicology, clinical toxicology, forensic toxicology, analytical toxicology, military toxicology, management toxicology, and so on.

The target organs or systems studied can be divided into: organ toxicology, liver toxicology, kidney toxicology, ophthalmology, ototoxicology, neurotoxicology, reproductive toxicology, and immunotoxicology.

First, the concept

A toxicology is a science that studies the harmful effects of chemical substances on biological organisms from the medical perspective. However, in recent years, the scope of toxicology research has been expanded to various harmful factors, such as physical factors such as nuclide, microwave, and biological factors, etc., not only limited to chemical substances; but the main research content of classic toxicology is still the chemical substances on the body Biological function and its mechanism.

Second, health toxicology is a discipline that studies the harmful effects and mechanisms of foreign compounds that humans may be exposed to during production and life from the perspective of preventive medicine. It is an important branch of toxicology and a basic discipline of preventive medicine. It provides toxicological theory and research methods for labor hygiene, environmental hygiene and food hygiene, and is an important part of preventive medicine.

Three foreign compounds are some chemical substances that exist in human life and the external environment and may come into contact with and enter the body. Foreign compounds are not a component of the human body, nor are they required by the human body, nor are they necessary to maintain the body's normal physiological functions and life. Present a certain biological role in the body. Common foreign compounds are agricultural chemicals, industrial chemicals, drugs, food additives, daily chemicals, various environmental pollutants and mycotoxins.

Second, the task of health toxicology research

In the field of preventive medicine, there are three main research tasks in health toxicology. One is to study the law of interaction between the body and foreign compounds, that is, the mechanism of poisoning. The other is to evaluate the safety of foreign compounds. The third is to formulate relevant health standards and management schemes. Provide scientific evidence.

3. Research methods of health toxicology

An animal experiment

1.In vivo experiments

It is usually carried out in whole animals, so that the experimental animals are exposed to a certain amount of the test foreign compound in a certain period of time according to the actual human contact, and then the animals may be observed for morphological or functional changes. Most experiments use mammals, such as rats, mice, guinea pigs, rabbits, hamsters, dogs and monkeys. The general toxicity of foreign compounds is usually tested, such as acute toxicity tests, subchronic toxicity tests and chronic toxicity tests.

2. In vitro experiments

Most use free organs, primary cultured cells, cell lines and organelles. Organ perfusion technology can be used to perfuse the liver, kidneys, lungs, and brains, thereby keeping isolated organs in a living state for a certain period of time, contacting the foreign compounds under test, and observing the changes in morphology and function of the organs. At the same time, the metabolism of the test substance in the organ can also be observed; free cells and organelles are mostly used for preliminary screening of various harmful effects of foreign compounds on the body, in-depth study of the mechanism of action and the process of metabolic transformation, and have many advantages.

The above-mentioned studies at the overall, organ, cell, and subcellular levels each have certain characteristics and limitations. In actual work, the most appropriate methods should be adopted mainly according to the purpose and requirements of experimental research, and mutually verified.

Two population survey

In order to verify the results of animal experiments on humans, crowd surveys are sometimes required. According to the results of animal tests and the nature of foreign compounds, appropriate observation indicators are selected and epidemiological methods are used to conduct population surveys. The characteristics of the crowd survey are that the data can be directly observed in the human body, but it is susceptible to the influence and interference of many other confounding factors; the results and assessment must be deducted and stored truthfully, from the surface and inside, and comprehensively considered and analyzed with the results of animal experiments, to obtain More in line with actual conclusions.

I. Toxicity

Toxicity is the ability of a substance to cause damage to the body. A relatively small amount of a substance can cause certain damage to the body; a relatively low amount of a substance requires a relatively large amount before it becomes toxic. The level of substance toxicity is only of relative significance. In a certain sense, as long as a certain amount is reached, any substance is toxic to the body; in general, if it is lower than a certain amount, no substance is toxic; the key is the amount of such substance in contact with the body. In addition to the amount of contact between the substance and the body, it is also related to the physical and chemical properties of the substance itself and the way it contacts the body.

Second, the dose

Dose is an important factor that determines the harmful effects of foreign compounds on the body. The concept of dose is broader, which can refer to the amount given to the body, or the amount of foreign compounds in contact with the body, the amount of foreign compounds absorbed into the body, the concentration or content of the foreign compounds in the target organ's action site or body fluid. Because the internal dose is not easy to determine, the general dose concept refers to the amount of foreign compounds administered to the body or the amount of body contact. Dosage units are expressed as the amount of foreign compound exposed per unit weight, such as mg / kg body weight.

1. Lethal dose

The lethal dose is the dose that can cause the body to die. However, in a group, the number of dead individuals varies greatly, and the required doses are also inconsistent. Therefore, the lethal quantity has the following different concepts.

Absolute lethal dose (LD100) refers to the lowest dose that can cause all deaths in a group. ^[1]

. Half lethal dose (LD50) refers to the dose required to cause 50% of a group of individuals to die, also known as the medium lethal dose. It means the unit mg / kg body weight of LD50. The smaller the value of LD50, the stronger the toxicity of foreign compounds; conversely, the larger the value of LD50, the lower the toxicity.

2.Maximal no-effect level

The maximum non-action dose is that within a certain period of time, a foreign compound contacts the body in a certain way or route. According to the current level of knowledge, the most sensitive test methods and observation indicators have not been able to observe any harmful effects on the body. The highest dose.

The determination of the maximum non-action dose is based on the results of subchronic toxicity or chronic toxicity tests, and is the main basis for assessing the harmful effects of foreign compounds on the body. Based on this, an acceptable daily imtarie (intake, ADI) and a maximum allowable concentration (MAC) of a foreign compound can be formulated. ADI refers to the dosage of this foreign compound that does not cause any damaging effects in humans daily. MAC refers to the concentration of a foreign compound that can exist in the environment without causing any harmful effects to the human body.

3. Minimum effective level

The minimum effective dose is the minimum dose required for a foreign compound to contact the body in a certain manner or route in a certain period of time, which can cause an abnormal change in an observation index or cause the body to begin to have a damaging effect. Poisoning threshold dose, or poisoning threshold. In theory, the maximum no-acting dose and the minimum effective dose should be very different, because any small, or even infinitely small, dose increase should have a corresponding increase in damage to the body in theory. However, because the observation index of the damage effect is limited by the sensitivity of the observation method of this index, slight changes may not be detected. Only when the difference between the two doses reaches a certain degree can the difference in the degree of damage be clearly observed. Therefore, there is still a certain gap between the maximum non-effective dose and the minimum effective dose.

When the time, method or route of contact between the foreign compound and the body changes and the observation index changes, the maximum non-acting dose and the minimum effective dose will also change. Therefore, when indicating the maximum and minimum effective dose of a foreign compound, the species strain, contact mode or route, duration of exposure, and observation indicators of the test animal must be stated. For example, after a certain organophosphorus compound is administered to rats (Wistar strain) for 3 months, the maximum ineffective dose of cholinesterase activity in whole blood is reduced by 50% is 10 mg / kg body weight.

Effects and responses

An effect indicates a biological change that can be caused by a certain dose of a foreign compound in contact with the body. The degree of such change is expressed in units of measurement, such as several, milligrams, units, and so on.

The second response is the ratio of a certain amount of foreign compounds to the body after contact with the body, or to a certain degree, or the ratio of the number of individuals that have an effect in a certain group, generally expressed in% or ratio.

Dose-response relationship and dose-response relationship

The dose-response relationship or the dose-response relationship is an important concept in toxicology. If a certain damaging effect in the body is caused by a certain foreign compound, there must be a clear dose-effect or dose-response relationship, otherwise it cannot be sure.

Both the dose effect and the dose-response relationship can be represented by a curve, that is, a unit of measurement indicating the intensity of the effect or a percentage or ratio of the response is plotted on the ordinate, and the dose is plotted on the abscissa. Different foreign compounds cause different types of effects or reactions under different specific conditions, mainly because the correlation between the effect or response and the dose is inconsistent, and different types of curves can be presented. In general, there are the following basic types of dose-response or dose-response curves:

1. The linear effect or response intensity has a linear relationship with the dose; as the dose increases, the intensity of the effect or response also increases and is proportional. However, in biological organisms, this kind of linear relationship rarely occurs, only in certain in vitro experiments, within a certain dose range.

2. The parabolic dose has a non-linear relationship with the effect or response, that is, as the dose increases, the intensity of the effect or response also increases, but initially increases rapidly, then becomes slow, so that the curve first steepens, then flattens, and becomes parabolic . If the dose is changed to a logarithmic value, a straight line is formed. The relationship between the dose and the effect or response is changed into a straight line, which can facilitate the mutual estimation between low dose and high dose, or low response intensity and high response intensity.

3. S-shaped curve This kind of curve is characterized by the low dose range. As the dose increases, the response or effect intensity increases slowly, and then when the dose is higher, the response or effect intensity also increases rapidly. As it continues to increase, the intensity of the reaction or effect increases and tends to slow. The curve started flat, followed by steepness, and then flattened, forming an irregular S-shape. The middle part of the curve, that is, the response rate is about 50%, the slope is the largest, the dose is slightly changed, and the response is greatly increased or decreased. It is more common in the dose-response relationship, and some dose-response relationships have also appeared. S-shaped curves are divided into two types: symmetrical or asymmetric. An asymmetric S-shaped curve is asymmetric at both ends, with one end longer and the other shorter. If the abscissa (dose) of the asymmetric S-shaped curve is expressed in logarithm, it becomes a symmetric S-shaped curve; if the response rate is replaced by a probability unit, it becomes a straight line.

V. Damage and non-damage

One non-damaging effect is generally considered that the non-damaging effect does not cause changes in the body's functional morphology, growth, development and lifespan; it does not cause a reduction in the body's certain functional capacity, nor does it cause the body's ability to compensate for additional stress states. All biological changes that occur in the body should be within the scope of the body's ability to compensate. After the body stops contacting the foreign toxic compound, the body's ability to maintain homeostasis should not be reduced. The body's vulnerability to other external adverse factors The sensitivity should not be increased. Steady state is a tendency or ability of the body to keep the internal environment stable.

II. Damage effects Damage effects are the opposite of non-damage effects and should have the following characteristics:

1. The normal morphology, growth and development of the body are severely affected, and the life span will also be shortened.

2. The functional capacity of the body or the compensatory capacity of the extra stress state is reduced.

3. The body's ability to maintain stability has declined.

4. The susceptibility of the body to the adverse effects of certain other factors is increased.

It should be pointed out that both damaging and non-damaging effects are biological effects caused by foreign compounds in the body, and in biological effects, changes in quantity often cause qualitative changes, so the damaging and non-damaging effects have only a certain relative significance. In addition, observational indicators to determine the harmful and non-damaging effects are also constantly developing.

6. The determination of normal and non-damaging effects often involves the range of normal values of many indicators of the body, and sometimes it is necessary to measure the normal values. It must first be clear that "normal values" have only relative significance. In actual work, according to the current level of recognition, individuals who are considered to be "healthy" or "normal" will be measured with an observation index, and the average value ± 2 standard deviations will be used as the normal value range. Statistical methods can be used to determine whether the change of this indicator deviates from the normal value range. Any observation indicator that meets one of the following conditions can be considered to have deviated from the normal value range and is considered to be damaging or non-damaging.

Compared with the control group, there is a statistically significant difference (P <0.05), and its value is not within the normal range.

Compared with the control group, there is a statistically significant difference (P <0.05), but its value is within the generally accepted "normal value" range; but if the difference persists for a period of time after stopping contact, it belongs to Harmful effect.

Compared with the control group, there is a statistically significant difference (P <0.05), but its value is within the generally accepted "normal value" range; but if the body is in a functional or biochemical stress state, this difference More obvious, it is harmful.

First, the concept of biological transport

Absorption, distribution and metabolism of foreign compounds in the body are collectively referred to as biological transport.

Second, the biological transport mechanism

The biological transport of foreign compounds in the body mainly through the following mechanisms:

A simple diffusion of a foreign compound in the body depends on its concentration gradient to determine the diffusion direction of the substance, that is, from the higher molecular concentration side of the biofilm to the lower concentration side. When both sides reach dynamic equilibrium, Diffusion ceases. The simple diffusion process does not need to consume energy, foreign compounds do not chemically react with the membrane, and the biological membrane has no initiative, which is equivalent to a physical process only, so it is called simple diffusion. Simple diffusion is the main mechanism for the biological transport of foreign compounds in the body. In general, most foreign compounds are biotransported by simple diffusion. In addition to the two differences in concentration gradients of biofilms that can affect simple diffusion, there are other factors that can also affect simple diffusion processes.

1. The solubility of foreign compounds in lipids can be expressed by the lipid-water partition coefficient, that is, the ratio of the concentration of foreign compounds in the lipid phase to the concentration in the water phase (concentration in the lipid phase / concentration in the water phase). The larger the lipid-water partition coefficient, the easier it is to diffuse through the biofilm. However, in the process of biological transport, foreign compounds must pass through the water phase in addition to the lipid phase. Because the structure of the biofilm includes the lipid phase and the water phase, the solubility of a foreign compound in water is too low, even if the lipid-water partition coefficient It is very large, and it is not easy to diffuse through the biofilm. Only foreign compounds that are both soluble in fat and water are most likely to diffuse through the biofilm.

2. Ionization or dissociation of foreign compounds. Foreign compounds in the ionic state are not easy to pass through the biofilm; conversely, foreign compounds in the non-dissociated state are easy to pass through. The degree of dissociation of a foreign compound depends on its dissociation constant (pK) and the pH (pH) of the medium in which it is located. In addition to the two main factors mentioned above, there are many other factors that can also affect simple diffusion.

Second filtration

Filtration is the process by which foreign compounds pass through the hydrophilic channels on a biofilm. A large amount of water can enter the cells through the pores through the osmotic pressure gradient and hydrostatic pressure. Foreign compounds can be transported passively with water as a carrier.

Three active transfers

The process of foreign compounds moving through a biofilm from a low concentration to a high concentration. Its main characteristics are: reversible concentration gradient transport, so it consumes a certain amount of metabolic energy; the transport process requires the participation of carriers. The carrier is often a protein on the biofilm, which can form a complex with the foreign compound being transported and transport to the other side of the membrane, and then release the foreign compound. The carrier returns to the original location and continues the second transport; Since it is a component of the biofilm, it has a certain capacity; when the concentration of the compound reaches a certain level, the carrier can be saturated, and the transport reaches its limit; active transport has a certain selectivity. That is, compounds must have a certain basic structure in order to be transferred; slight changes in the structure can affect the progress of the transfer; If the basic structure of the two compounds is similar, the same transfer system is required during the biological transfer process, and the two compounds can appear Competition, and competition inhibition.

Four-carrier diffusion

Foreign compounds that are not easily soluble in lipids utilize a process in which the carrier moves from a high concentration to a low concentration. Because it is impossible to reverse the concentration gradient from low concentration to high concentration, it does not consume metabolic energy. Because of the use of the carrier, the biofilm has a certain degree of initiative or selectivity, but it cannot reverse the concentration gradient, so it belongs to the diffusive nature. It can also be called to facilitate diffusion or promote diffusion. Water-soluble glucose enters the bloodstream from the gastrointestinal tract, from plasma to red blood cells, and from blood to nerve tissues.

Five Drinks and Devour

Liquid or solid foreign compounds are surrounded by the extended biofilm, and then the enclosed droplets or larger particles are incorporated into the cell to achieve the purpose of transport. The former is called cytosolic and the latter is called phagocytosis. Elimination of foreign bodies in the body, such as leukocytes engulfing microorganisms, and elimination of toxic foreign bodies by liver reticuloendothelial cells are related to this.

Third, the concept of absorption and ways of absorption

The concept of absorption

Absorption is the process by which foreign compounds pass through the body's biofilm into the blood through various pathways.

Two absorption pathway

1. Absorbed through the gastrointestinal tract

The gastrointestinal tract is the main route of absorption of foreign compounds. Many foreign compounds can enter the digestive tract with food or water and be absorbed in the gastrointestinal tract. Generally, the absorption process of foreign compounds in the gastrointestinal tract is mainly through simple diffusion. Only a few kinds of foreign compounds are absorbed through a dedicated active transport system that absorbs nutrients and endogenous compounds.

Absorption of foreign compounds in the gastrointestinal tract can occur anywhere, but mainly in the small intestine. Absorption of foreign compounds in the stomach is mainly through a simple diffusion process. Because gastric juice has a very high acidity (pH 1.0), weak organic acids are mostly in the form of undissociated, so it is easy to absorb; but weak organic bases have a high degree of dissociation in the stomach and are generally not easily absorbed.

Absorption in the small intestine is also mainly through simple diffusion. The pH in the small intestine is relatively neutral (pH 6.6), and the dissociation of the compound is different from that in the stomach. For example, weak organic bases are mainly non-dissociated in the small intestine and are therefore easily absorbed. Weak organic acids are contrary to this mechanism, for example, benzoic acid is not easily absorbed in the small intestine. But in fact, due to the large surface area of the small intestine, villus and microvilli can increase its surface area by about 600 times, so the small intestine can also absorb a considerable amount of benzoic acid. In addition, the small intestinal mucosa can also absorb small molecules with a molecular weight of 100 to 200 or less through filtration. Gastrointestinal epithelial cells can also absorb some particulate matter through cytotoxic or phagocytosis.

2.Respiratory absorption

The lung is the main absorbing organ in the respiratory tract. The alveolar epithelial cell layer is extremely thin and the blood vessels are abundant. Therefore, gas, volatile liquid vapor, and fine aerosols are quickly and completely absorbed in the lungs. The fastest absorption is gas, small particles of aerosols and substances with higher lipid-water partition coefficients. The foreign compounds absorbed through the lungs are different from those absorbed through the gastrointestinal tract. The former do not enter the liver with the portal vein blood flow, and go directly into the systemic circulation and distribute the whole body without biotransformation in the liver. The absorption of gases, volatile liquids, and aerosols in the respiratory tract is mainly through simple diffusion and is affected by many factors, mainly the difference in concentration between alveolar gas and plasma. The concentration of a gas in the alveolar gas can be expressed by its partial pressure in the alveoli, and the partial pressure of a gas is the percentage of the total pressure of the alveolar gas. The higher the partial pressure, the greater the amount of body contact and the easier it is to absorb. As the absorption process progresses, the partial pressure of the gas in the blood will gradually increase, and the partial pressure difference will decrease accordingly. The partial pressure of this gas in the blood will gradually approach the partial pressure of alveolar gas, and finally reach equilibrium and become saturated. In the saturated state, the ratio of the concentration of the gas in the blood (mg / L) to the concentration in the alveolar gas (mg / L) is called the blood / gas distribution coefficient, which is the concentration of the gas in the blood / the ratio of the gas in the alveoli Concentration ratio. The larger the blood / gas partition coefficient, that is, the higher the solubility, the more easily the gas is absorbed.

The rate of gas absorption in the respiratory tract is also related to its solubility and molecular weight. In general, absorption rate is directly proportional to solubility. When non-fat-soluble substances are absorbed through hydrophilic pores, their absorption rate is mainly affected by the molecular weight; substances with large molecular weights are relatively slow to absorb, and vice versa. Substances that dissolve in biofilm lipids have little relationship between absorption rate and molecular weight, but mainly depend on their lipid / water partition coefficient. The larger the lipid / water partition coefficient, the higher the absorption rate.

The factors that affect the absorption of chemicals through the respiratory tract are also the alveolar ventilation and blood flow. The ratio of alveolar ventilation to blood flow is called the ventilation / blood flow ratio, especially related to the ratio of alveolar ventilation to blood flow.

3.Transdermal absorption

Absorption of foreign compounds through the skin can generally be divided into two stages. The first stage is the process of the foreign compounds penetrating the epidermis of the skin, that is, the stratum corneum, which is the penetration stage. The second stage is that the stratum corneum enters the nipple layer and the dermis and is absorbed into the blood, which is the absorption stage.

The main mechanism of transdermal absorption is simple diffusion, and the diffusion rate is related to many factors. The main relevant factors during the penetration phase are the molecular weight of the foreign compound, the thickness of the stratum corneum, and the fat solubility of the foreign compound. The speed of fat-soluble non-polar compounds passing through the epidermis is proportional to the level of fat-solubility, that is, the size of the fat / water partition coefficient is proportional to the rate of penetration. The fat-soluble ones penetrate faster, but inversely proportional to molecular weight.

In the absorption phase, foreign compounds must have a certain water solubility to be easily absorbed, because plasma water is an aqueous solution. At present, it is believed that the fat / water partition coefficient is close to 1, that is, compounds that have both a certain fat solubility and water solubility are easily absorbed into the blood.

In addition, temperature, humidity and skin damage can also affect skin absorption.

Fourth, the concept of distribution and the main factors affecting distribution

A distribution concept

Distribution is the process by which foreign compounds are absorbed into blood or other body fluids and then dispersed to cells in various tissues throughout the body with the flow of blood or lymph fluid.

The main factors affecting the distribution

1. Binding of foreign compounds to plasma proteins Foreign compounds often bind to plasma proteins, especially plasma albumin, after entering the bloodstream. This binding is reversible, and it can be regarded as a process in which foreign compounds are distributed and transported in the body. The foreign compound bound to plasma albumin is in dynamic equilibrium with unbound free chemicals, and because the specificity of plasma albumin bound to chemicals is not strong, when another foreign compound or drug or physiological metabolite exists At times, competition can occur. For example, DDE (DDT metabolite) can competitively replace bilirubin that has been bound to albumin and free it in the blood.

2. Combination of foreign compounds with other tissue components Foreign compounds can also bind with other tissue components, such as a variety of proteins, mucopolysaccharides, nuclear proteins, phospholipids, and the like. These combinations have distributional and toxicological significance. For example, carbon monoxide has a high affinity for hemoglobin, resulting in hypoxia and poisoning. Another example is the herbicide paraquat, which can be concentrated in the lungs and cause damage regardless of the contact.

3. Storage and storage of foreign compounds in adipose tissue and bone Fat-soluble foreign compounds can be stored in adipose tissue and do not exhibit biological activity. Biological effects only occur when fat is used passively and foreign compounds return to a free state. This is the case when DDT is stored in adipose tissue.

Bone can also serve as a storage and deposition site for many foreign compounds. For example, lead can replace calcium in bones, and 40% of lead absorbed by the body can be deposited in bones, which is relatively less harmful to the body. However, under certain conditions, it can be released freely and enter the systemic circulation, causing damage to the body.

4. The effects of various barriers in the body There are several membrane barriers in the body, which is of great significance for protecting some organs. It is of great toxicological significance to study whether the distribution of foreign compounds in the body can penetrate these barriers.

Blood-brain barrier

A special functional structure consisting of capillary endothelial cells and the pia mater of astrocytes surrounding the capillaries & frac34; & frac34; blood-brain barrier. The importance of the blood-brain barrier lies in ensuring the exchange of normal metabolic substances between the blood and brain tissues, preventing the entry of unwanted substances, and thus maintaining the normal function of the brain. Generally, foreign compounds can penetrate only with small molecular weight and high fat solubility. Ionized, ionic, water-soluble chemicals are difficult to penetrate the blood-brain barrier. For example, inorganic mercury does not easily enter the brain tissue, while methylmercury easily penetrates the blood-brain barrier, causing damage to the central nervous system function.

Placental barrier

In addition to the exchange of nutrients, oxygen, carbon dioxide, and metabolites between the mother and the fetus, the placenta also has the function of preventing some foreign compounds from entering the embryo through the placenta through the placenta and ensuring the normal growth and development of the fetus. The anatomical basis of the placental barrier is composed of several layers of cells located between the maternal blood circulation system and the embryo. The number of placental cell layers is different in different species of animals and in different gestational stages of the same species. For example, pigs and horses have 6 layers, rats and guinea pigs have only one layer; rabbits have 6 layers in the early stages of pregnancy, and only one layer to the end of pregnancy. Thinner placenta, that is, those with fewer cell layers, foreign compounds are relatively easy to penetrate. For example, rat placenta is thinner than humans, and foreign compounds are easier to penetrate. Therefore, teratogenicity tests in pregnant rats may be more complicated.

The mechanism by which most foreign compounds penetrate the placenta is simple diffusion, and nutrients necessary for embryo development enter the embryo through active transport.

V. The concept of excretion and the main ways

The concept of excretion Excretion is the process by which foreign compounds and their metabolites are transported out of the body, and is the last link in the entire process of the body's material metabolism.

Two main ways of excretion

1. The excretion of foreign compounds by the kidney through the kidney is extremely efficient and the most important organ for excretion. There are three main mechanisms for excretion: glomerular filtration, simple diffusion of glomeruli, and active tubule transport, which is simple. Proliferation and active transport are even more important.

Glomerular filtration is a kind of passive transport. Glomerular capillaries have pores, about 40 ° A in diameter, and substances with a molecular weight below 70,000 can be filtered. Therefore, most foreign compounds or their metabolites can be filtered out. Only the chemical substances that bind to plasma proteins are too large to pass through the channels. However, it should be pointed out that any chemical substance or its metabolite with a large lipid / water partition coefficient can be reabsorbed into the blood by renal tubular epithelial cells in a simple diffusion manner. Only water-soluble substances or ionic substances enter urine.

Active renal tubular transport is actually the active secretion of renal tubules. This active transport can be divided into two systems, one for the transport of organic anionic chemicals and the other for the transport of organic cationic chemicals. Both systems are located in the proximal tubules of the renal tubules. Both of these transport systems can transport substances that bind to proteins, and there is a competitive effect when two chemicals pass through the same transport system.

2. Excreted by the liver with bile

Excretion of the body with the bile through the liver is another excretory route for the elimination of foreign compounds in the body, second only to the kidneys. The blood from the gastrointestinal tract carries the absorbed foreign compounds into the liver through the portal vein, then flows through the liver and enters the systemic circulation. Foreign compounds first undergo biotransformation in the liver. Part of the metabolites formed during the biotransformation process can be directly excreted into bile by liver cells, and then mixed into feces and excreted.

After foreign compounds enter the small intestine with bile, there may be two ways to go: Some of the easily absorbed foreign compounds and their metabolites can be reabsorbed in the small intestine, and then returned to the liver through the portal vein system, and then excreted with the bile. Liver circulation. Enterohepatic circulation has important physiological significance, and can make some of the compounds needed by the body to be reused. For example, an average of 95% of various bile acids are reabsorbed by the intestinal wall and reused. In terms of toxicology, some foreign compounds are reabsorbed, which prolongs the residence time in the body, and the toxic effect will also increase. Another part of the foreign compound forms a conjugate during the biotransformation process and appears in the bile in the form of the conjugate; the intestinal flora and glucuronidase existing in the intestine can hydrolyze a part of the conjugate, and the foreign compound can It is reabsorbed and enters the enterohepatic circulation.

3. Exhale with the lungs

Many gaseous foreign compounds can be excreted through the respiratory tract. Such as carbon monoxide, certain alcohols and volatile organic compounds can be excreted through the lungs. The main mechanism of its excretion through the lungs is simple diffusion, and the rate of excretion is mainly determined by the solubility of the gas in the blood, the breathing rate, and the speed of the blood flowing through the lungs. Gases that are less soluble in blood, such as nitrous oxide, are excreted faster; substances that are more soluble in blood, such as ethanol, are excreted more slowly through the lungs, and the effects of breathing rate are slightly different for different compounds. For example, ether is highly soluble in the blood, and is excreted very quickly through the lungs when hyperventilated. The discharge of some gases that are not easily soluble in blood (such as sulfur hexafluoride) is hardly affected by excessive ventilation.

Foreign compounds dissolved in the secretions of the respiratory tract and particulate matter taken up by macrophages will be discharged with the secretions on the surface of the respiratory tract.

4. Other excretion routes

Foreign compounds can also be excreted through other routes. For example, excretion via the gastrointestinal tract, excretion with sweat and saliva, and excretion with milk. Although the proportion of such excretion routes in the entire excretion process is not important, some of them have special toxicological significance. For example with milk excretion. Many foreign compounds can enter milk through simple diffusion. Organochlorine pesticides, ethers, polyhalobiphenyls, caffeine, and certain metals can be excreted with milk. If a substance is repeatedly and repeatedly contacted with the mother for a long time, it is easy to concentrate in milk. The important significance lies in the damaging effect on the baby; because the foreign body intake of milk by the baby is usually larger than the general population in terms of unit weight.

First, the concept of biotransformation

The process of foreign compounds undergoing a series of chemical changes in the body and forming their derivatives and decomposition products is called biological transformation, or metabolic transformation. The resulting derivatives are metabolites. After foreign compounds are biotransformed, some can achieve detoxification and reduce toxicity. However, some can increase its toxicity and even produce teratogenic and carcinogenic effects. Therefore, the metabolic transformation should not be regarded only as a detoxification process, but the metabolic process is dual in toxicity to foreign compounds.

Types of biotransformation reactions

Nitric oxide

Oxidation can be divided into two types of oxidation reactions catalyzed by microsomal mixed functional oxidase and non-microsomal mixed functional oxidase.

Microsomes are fragments formed by the endoplasmic reticulum during cell homogenization, and are not independent organelles. The endoplasmic reticulum can be divided into two types: rough surface and smooth surface. Therefore, the microsomes formed are also rough surface and smooth surface, but both contain mixed-function oxidase, which is more active.

Micrososmal mixed function oxidase (MFO), also known as mixed function oxidase or microsomal monooxygenase, can be referred to as monooxygenase for short. In this process, NADPH also needs to provide electrons to reduce cytochrome P-450 and form a complex with the substrate to complete this reaction process.

Mixed-function oxidase is an enzyme system on the endoplasmic reticulum membrane of the cell, and its composition is relatively complicated. The cytochrome P-450 oxidase, also known as cytochrome P-450 dependent monooxygenase, is now known There are reduced coenzyme -cytochrome P-450 reductase. In addition, it also contains microsome FDA-monooxygenase, which is characterized by the absence of cytochrome P-450 and the inclusion of flavin adenine dinucleotide instead of cytochrome P-450 to participate in the monooxygenase reaction. During the oxidation of foreign compounds catalyzed by FAD monooxygenase, NADPH and oxygen molecules are also required.

Many foreign compounds can be catalyzed by mixed function oxidases and oxygenated to form various hydroxyl compounds. The hydroxyl compound will be further decomposed to form various products, so the oxidation reaction may have the following various types:

Aliphatic hydroxylation: also known as aliphatic oxidation, is the oxidation of the penultimate or second carbon atom at the end of the side chain (R) of an aliphatic compound and forms a hydroxyl group.

Aromatic hydroxylation: Hydrogen on the aromatic ring is oxidized. For example, benzene can form phenol, and aniline can form p-aminophenol or o-aminophenol. In the measurement of microsomal mixed function oxidase activity, this reaction can be used, that is, aniline is used as a substrate to be hydroxylated by MFO to form p-aminophenol, and the content is measured to indicate the aniline hydroxylase activity. During the hydroxylation process, o-aminophenol can also be formed.

(3) Epoxidation reaction: A bridge structure is formed between two carbon atoms of a foreign compound, that is, an epoxide. Epoxides are generally only intermediate products and will continue to decompose. But after PAHs, such as benzo (a) pyrene, form epoxides, they can covalently bind with cell biological macromolecules, induce mutations and form tumors.

N-dealkylation reaction: The alkyl group on the oxygen N of the amine compound is oxidized to remove an alkyl group to form an aldehyde or a ketone. Carbamate insecticides, such as carbamide, carcinogen azo pigment cream yellow, and dimethylnitrosamine, can all occur in this reaction. Dimethylnitrosamine can also form a free methyl group [CH3 +] after N-dealkylation, which can cause guanine methylation (or alkylation) on nucleic acid molecules in the nucleus to induce mutation or carcinogenesis.

O-dealkylation and S-dealkylation reaction: Similar to N-dealkylation reaction, but the alkyl group connected to oxygen atom or sulfur atom is removed after oxidation.

O-dealkylation can occur in p-nitroanisole. The latter is catalyzed by microsomal mixed function oxidase, and the content of p-nitrophenol formed is determined, which can represent the activity of mixed function oxidase.

N-hydroxylation reaction: The hydroxylation is carried out on the N atom, for example, aniline and carcinogen 2-acetamidinium can occur. Aniline undergoes N-hydroxylation to form N-hydroxyaniline, which can oxidize hemoglobin to methemoglobin.

Alkyl metal dealkylation reaction: Tetraethyl lead can remove one alkyl group under the catalysis of mixed function oxidase to form triethyl lead. With this, tetraethyl lead can exhibit toxic effects in the body.

Desulfurization reaction: Desulfurization reactions often occur in many organophosphorus compounds. In this reaction, sulfur atoms are oxidized to sulfate and fall off. Such as parathion oxidative desulfurization into paraoxon, toxicity is enhanced.

The oxidation reaction catalyzed by non-microsomal mixed functional oxidase. In the cytosol, plasma and mitochondria of liver tissue, there are some less specific enzymes that can catalyze the oxidation and reduction of certain foreign compounds, such as alcohol dehydrogenase. Aldehyde dehydrogenase, catalase, xanthine oxidase, etc.

Hepatocyte cytosol contains monoamine oxidase and diamine oxidase, which can catalyze the oxidation of amines to form aldehydes and ammonia. The oxidation reaction catalyzed by diamine oxidase mainly involves the formation of biological amines in the body, and has less relationship with the metabolism and conversion of foreign compounds .

Co-oxidation during prostaglandin biosynthesis In the oxidation reaction of foreign compounds, in addition to the oxidation reactions catalyzed by the aforementioned microsomal mixed-function oxidase and non-microsomal mixed-function oxidase, an oxidation reaction has been observed in recent years. In the prostaglandin biosynthesis process, some foreign compounds can be simultaneously oxidized, which is called a co-oxidation reaction.

Two reduction reaction

Foreign compounds containing nitro, azo, and carbonyl groups, as well as disulfides and sulfoxides, can be reduced in the body. For example, both nitrobenzene and azobenzene can be reduced to form aniline. Carbon tetrachloride can be catalyzed and reduced by NADPH-cytochrome P-450 reductase in the body to form chloroform free radicals (CCl3 +), which will destroy the lipid structure of liver cell membrane, cause liver steatosis and necrosis. Arsenic in pentavalent arsenic compounds can also be reduced to trivalent arsenic. Trivalent arsenic compounds have higher solubility in water, so they are more toxic than pentavalent arsenic compounds.

Trihydrolysis

Many foreign compounds, such as esters, amides, and phosphate substituents containing ester linkages are extremely prone to hydrolysis. There are many hydrolases in plasma, liver, kidney, intestinal mucosa, muscle, and god tissues, as well as in microsomes. Esterase is a widespread hydrolase. Esterase and amidase can hydrolyze esters and amines, respectively.

The hydrolysis reaction is the main metabolic mode of many organophosphorus pesticides in the body, such as dichlorvos, parathion, dimethoate, and malathion. The toxicity decreases or disappears after hydrolysis. Some insects are resistant to malathion, that is, because of its high carboxylase activity in the body, malathion is easily deactivated. In addition, pyrethroid insecticides are also detoxified by hydrolytic enzyme-catalyzed degradation.

Quadruple reaction

A binding reaction is a biosynthetic reaction of a foreign compound entering the body with certain other endogenous compounds or groups during metabolism. Especially foreign organic compounds and their metabolites containing hydroxyl, amino, carbonyl and epoxy groups are most likely to occur. The products formed by the combination of foreign compounds and their metabolites with certain endogenous compounds or groups in the body are called conjugates. In the binding reaction, coenzyme and transferase are required and metabolic energy is consumed. The source of the so-called endogenous compound or group is the product of the normal metabolic process in the body, and the endogenous compound must be an endogenous compound to participate in the binding reaction, which cannot be performed directly by the in vitro importer.

Foreign compounds can undergo binding reactions directly during metabolism, or they can go through the above-mentioned first-stage biological transformation reactions (first-phase reactions) such as oxidation, reduction, or hydrolysis, and then undergo binding reactions (second-phase reactions). Generally, Under the circumstances, through the binding reaction, on the one hand, certain functional groups on the foreign compound molecule can lose their activity and toxicity; on the other hand, most of the foreign compounds can increase their polarity, reduce their fat solubility, and accelerate their binding reactions. Excretion process by the body.

According to the mechanism of the binding reaction, the binding reaction can be divided into the following types:

Glucuronide binding is probably the most common binding reaction, mainly foreign compounds and their metabolites bind to glucuronide. The source of glucuronic acid is uridine diphosphate glucose (UDPG) generated during the metabolism of sugars. UDPG is oxidized to produce uridine diphosphate glucuronic acid; UCPGA is the donor of glucuronic acid. Under the action of uronic acid transferase, it is combined with the hydroxyl, amino and carboxyl groups of foreign compounds and their metabolites, and the reaction product is -glucuronide. Glucuronic acid must be an endogenous metabolite, and binding reactions cannot be performed directly by an in vitro importer.

The glucuronic acid binding effect is mainly carried out in liver microsomes. In addition, it can also occur in the kidney, intestinal mucosa and skin. Foreign compounds are excreted with bile after a binding reaction in the liver. But sometimes part of it is in the lower part of the intestinal tract, which can be hydrolyzed by -glucuronidase in the intestinal flora, and this foreign compound can be reabsorbed and enter the enterohepatic circulation, so that its residence time in the body is prolonged.

Sulfuric acid binds foreign compounds and their alcohol, phenol or amine compounds in metabolites, which can be combined with sulfuric acid to form sulfate esters. The source of endogenous sulfuric acid is a metabolite of sulfur-containing amino acids, but it must be activated by adenosine triphosphate to become 3'-adenosine-5'-phosphate sulfate (PAPS), and then interact with phenols under the action of sulfotransferase. , Alcohols or amines are combined into sulfate esters. The combination of phenol and sulfuric acid is more common.

The sulfuric acid binding reaction is mostly performed in liver, kidney, gastrointestinal and other tissues; due to the limitation of the source of sulfuric acid in the body, it cannot be provided sufficiently, so it is less than the glucuronic acid binding reaction.

In general, the original toxicity of foreign compounds can be reduced by the sulfuric acid binding reaction. However, some foreign compounds have higher toxicity after sulfuric acid binding reaction. For example, a carcinogen belonging to aromatic amines, 2-acetylaminohydrazone (FAA or AAF for short), undergoes N-hydroxylation reaction in the body to form N-hydroxy-2-acetamidofluorene, and its hydroxyl group can be combined with sulfuric acid to form Sulfate. This AAF sulfate has strong carcinogenicity, which is stronger than AAF itself. This reaction occurs in rats, mice and dogs. However, some animals lack sulfatase in the liver to form sulfate esters.

Glutathione binds toxic metals and epoxides in the body and can be detoxified by binding to glutathione. The glutathione binding reaction is catalyzed by glutathione transferase. Glutathione is contained in the liver and kidney, and the cytosol content of liver cells is large. In recent years, it has been found in liver microsomes. Microsomal glutathione transferase is in direct contact with foreign compounds, which may be more important in the glutathione binding reaction.

The binding of glutathione to epoxide is very important. Many foreign compounds, such as many carcinogens and liver poisons, can form epoxides in the body, and most of these epoxides have a strong damaging effect on cells. For example, brominated benzene is metabolized into epoxide, and bromobenzene epoxide is a strong liver poison that can cause liver necrosis; but after being combined with glutathione, it will be detoxified and excreted. There is a limit to the production and storage of glutathione in the body. If a large number of epoxides are formed in a short period of time, glutathione depletion may occur and still cause serious damage.

Glycine binds to some foreign compounds containing carboxyl groups, for example organic acids can bind to amino acids. The nature of this binding reaction is a peptide binding, and the most common binding with glycine, in fact, other amino acids can also perform this binding. For example, toluene is metabolized in the body to produce benzoic acid, which can be combined with glycine to form hippuric acid and be excreted from the body. Cyanohydrogen is bound by cysteine and excreted by saliva and urine.

Acetyl binds aromatic amines in foreign compounds. For example, aniline can react with acetyl-CoA through its amino group and acetyltransferase catalyzes the aromatic amines to form their acetyl derivatives. In addition, aliphatic amines have a similar response. Acetase A is derived from the metabolites of sugar, fat, and protein.

methyl-binding biological amines in the body and methyl-binding reaction, also known as methylation. The methyl group is derived from methionine. The methyl group of methionine is activated by ATP to become S-adenosylmethionine, which is then catalyzed by methyltransferase to combine biological amines with methyl groups and be detoxified and excreted. In the detoxification of foreign compounds, methyl bonding is not important.

Factors affecting biotransformation

One species difference and individual difference

The speed of biotransformation of the same foreign compound can be quite different in different animals. For example, the biological half-life of aniline is 35 minutes in mice and 167 minutes in dogs. The metabolism of the same foreign compound in animals of different species can be completely different. As mentioned earlier, N-2-acetamidinium can be N-hydroxylated in rats, mice, and dogs, and then combined with sulfuric acid to form sulfates, which exhibits a strong carcinogenic effect; while N- generally does not occur in guinea pigs. It is hydroxylated, so it cannot be combined into sulfates, and has no carcinogenic effect or very weak carcinogenic effect.

The individual differences of foreign compounds in the biotransformation process in vivo are also manifested in the activity of certain enzymes involved in metabolism in each body. For example, arylhydrocarbon hydroxylase (AHH) can hydroxylate aromatic hydrocarbon compounds and produce carcinogenic activity, and their activity varies significantly among individuals. With the same amount of smoking, people with higher AHH vitality are 36 times more likely to develop lung cancer than those with low vitality; those with moderate vitality in the body are 16 times more likely to develop lung cancer than those with low vitality.

Inhibition and induction of metabolizing enzymes of two foreign compounds

Inhibition of the biotransformation of one foreign compound can be inhibited by another compound, and this inhibition is related to enzymes that catalyze biotransformation. The enzyme systems involved in biological transformation generally do not have high substrate specificity. Several different compounds can be used as substrates of the same enzyme system, that is, the biological transformation process of several foreign compounds is catalyzed by the same enzyme system. . Therefore, when one foreign compound appears or increases in the body, it can affect the catalytic effect of an enzyme on another foreign compound, that is, the competitive inhibition of the two compounds.

Induction of some foreign compounds can increase the activity of catalytic enzymes or increase the content of enzymes in certain metabolic processes. This phenomenon is called the induction of enzymes. Any compound with an inducing effect is called an inducer. The result of the induction can promote other foreign compounds. The biotransformation process makes it enhanced or accelerated. During the induction of microsomal mixed-function oxidase, proliferation of the endoplasmic reticulum of the slippery surface was also observed; enhanced enzyme activity and the promotion of other compounds' metabolic transformation were related to this.

Triple metabolic saturation

The saturation state of a foreign compound in the body's metabolism has a considerable impact on its metabolism, and therefore its toxic effect. For example, brominated benzene is first converted into brominated benzene epoxide with liver toxicity in the body; if the input dose is small, about 75% of the brominated benzene epoxide can be converted into a glutathione conjugate, and It is excreted in the form of bromophenyl sulfide; however, if a larger dose is entered, only 45% of the side can be excreted in the form described above. When the dose is too large, because the amount of glutathione is insufficient, and even the glutathione is depleted, the binding reaction is reduced, so the reaction of unbound bromobenzene epoxide with DNA or RNA and protein is enhanced, showing Toxic effect.

Four other influencing factors

Mainly manifested in age and gender and nutritional status. The nutritional status of protein, ascorbic acid, riboflavin, vitamin A and vitamin E can all affect the activity of mixed function oxidase in microsomes. If the protein supply is insufficient in animal experiments, the microsomal enzyme activity decreases. When ascorbic acid is deficient, the hydroxylation of aniline is weakened. Lack of riboflavin can reduce azo compound reductase activity and enhance the carcinogenic effect of carcinogen cream yellow. The decrease in the activity of the above enzymes may cause the conversion process of foreign compounds to weaken or slow down.

The effect of age on the metabolic conversion of foreign compounds is manifested in that liver microsomal enzyme function has not yet matured in newborns and adolescents, and has begun to decline after old age. Its functions are lower than those of adults. weak. For example, 30 days after the rat was born, the mixed function oxidase of liver microsomes reached the adult level, and then began to decline after 250 days. The glucuronide-binding response weakened in older animals, but the monoamine oxidase activity of rats increased with age. In general, the ability of young and old organisms to metabolize and convert foreign compounds is weaker than that of adults, so the harmful effects of foreign compounds are also strong.