What Is Superoxide Dismutase?

Superoxide dismutase (SOD) is an important member of the antioxidant enzyme system in biological systems, and is widely distributed in microorganisms, plants and animals ^[1] .

Chinese name: Superoxide dismutase

Foreign name: Superoxide Dismutase
Abbreviation: SOD

Basic introduction of superoxide dismutase

Superoxide dismutase concept

Superoxide dismutase (SOD) is an antioxidant metal enzyme that exists in the body. It can catalyze the disproportionation of superoxide anion free radicals to generate oxygen and hydrogen peroxide. It plays an important role in the balance between oxidation and antioxidant in the body. The important role is inseparable from the occurrence and development of many diseases ^[2] .

Superoxide dismutase classification

According to the different metal auxiliary groups in SOD, SOD can be roughly divided into three categories, namely Cu / Zn-SOD, Mn-SOD, and Fe-SOD ^[3] .

Cu / Zn-SOD: It is blue-green and mainly exists in the cytoplasm of eukaryotic cells. It is considered to be the most widely distributed species in relatively primitive biological groups ^[3] .

Mn-SOD: It is pink and mainly exists in mitochondria of prokaryotes and eukaryotes ^[3] .

Fe-SOD: It is yellow-brown and mainly exists in prokaryotic cells. They can effectively remove superoxide anion free radicals (with 1 unpaired electron and 1 negative charge), avoid excessive damage to cells, and have functions such as anti-oxidation, anti-radiation and anti-aging ^{[ 3]} .

Superoxide dismutase distribution

Cu / Zn-SOD exists in most primitive invertebrate cells, and vertebrates generally contain Cu / Zn-SOD and Mn-SOD. Cu / Zn-SOD is contained in red blood cells and liver cells such as human, mouse, pig, and bovine, and it is mainly present in the cytoplasm and also between the outer and inner membranes of the mitochondria. Mn-SOD has also been purified from human and animal liver cells, which are generally found in the mitochondrial matrix.

Fe-SOD in plant cells mainly exists in chloroplasts.

Fungi generally contain Mn-SOD and Cu / Zn-SOD. Most eukaryotic algae have Fe-SOD in their chloroplast matrix, Mn-SOD is bound to thylakoid membranes, and most algae do not contain Cu / Zn-SOD.

Superoxide dismutase structure

Cu / Zn-SOD: Its active center includes a Cu ion and a Zn ion. Studies have shown that the presence of Cu is necessary for the activity of Cu / Zn-SOD. It directly interacts with superoxide anion radicals, while the surrounding environment of Zn is crowded, not directly exposed in the reaction solution, and does not directly interact with superoxide anion radicals. , Stabilizing the environment around the active center. The bivalent copper ion is bonded to the nitrogen atoms on the four histidines around it by coordination bonds, and its configuration is a distorted, near-planar square. Around Zn, there are three histidines coordinated by nitrogen atoms, one of which is shared by Cu and Zn, forming an "imidazole bridge" structure. In addition, Zn also coordinated with an aspartic acid residue, which caused Zn to form an irregular tetrahedral coordination configuration ^[4] .

Mn-SOD: It consists of 203 amino acid residues. The active center is Mn (), the coordination structure is a five-coordinated triangular double cone, one of the axial ligands is a water molecule, and the other axial position of the ligand is His-28 protein auxiliary group, in the equatorial plane Above are the protein prosthetic groups His-83, Asp-166 and His-170. The active site of the enzyme is an environment mainly composed of hydrophobic residues. Two subunit chains form a channel, which constitutes the only way for substrates or other internal ligands to approach Mn () ions ^[5] .

Superoxide dismutase reaction mechanism

The catalytic effect of SOD is achieved through the alternating electron gain and loss of the metal ions M ^{n + 1} (oxidized state) and M ⁿ (reduced state). It is generally believed that a superoxide anion radical first forms an inner-bound complex with a metal ion. M ^{n + 1} is reduced to M ⁿ by the superoxide anion radical in the body, and O ₂ is generated at the same time, and M ⁿ is oxidized to M ⁿ by HO ₂ ^· ^{+1 to} generate H ₂ O _{2 ^[3]} at the same time. SOD is oxidized to SOD in the initial oxidation state. Finally, H ₂ O ₂ is catalytically decomposed into water (H ₂ O) and O _{2 ^[1]} by the action of catalase.

Determination of superoxide dismutase

The main methods for measuring the activity of superoxide dismutase include direct method, pyrogallol auto-oxidation method, cytochrome C reduction method, chemiluminescence method and fluorescence kinetic method. In recent years, a number of new methods have been established, such as immunological methods, simple gel filtration diffusion methods, polarographic oxygen electrode methods, and micro-assay methods ^[2] .

1. The principle of the direct method is to determine the disproportionation of O2 ^.- according to the properties of O2 ^.- or the substance that produces O2 ^.- , so as to determine the activity of SOD. Classical direct methods include: pulse radiation decomposition, electron paramagnetic resonance (EPR), and nuclear magnetic resonance. Because the required equipment is expensive, it is rarely used.

2. Pyrogallol auto-oxidation method: The principle is based on classic spectrophotometry. Under basic conditions, pyrogallol is auto-oxidized to citrin, and the wavelength is tracked at 325nm, 420nm, or 650nm using ultraviolet-visible spectrum The classic is 420nm), and O2 ^.- is produced at the same time ^. SOD catalyzes the disproportionation reaction of O2 ^.- to inhibit the auto-oxidation of pyrogallol. This method has the advantages of strong specificity, small sample size (only 50 l), fast and simple operation, good repeatability, high sensitivity, simple reagents and so on.

3. Cytochrome C reduction method: The principle is O2 produced in the xanthine-xanthine oxidase system.- ^Reduces a certain amount of oxidized cytochrome C to reduced cytochrome C, which has a maximum light absorption at 550nm. In the presence of SOD, as a part of O2 ^.- is disproportionated by SOD catalysis, the reaction speed of O2 ^.- reducing cytochrome C is correspondingly reduced, that is, its reaction is inhibited. Plotting the percentage of the inhibition response versus the SOD concentration gives an inhibition curve, from which the SOD activity in the sample is calculated. This method is the classic method of indirect method, but the sensitivity of this method is low.

4. Chemiluminescence method: The principle is that under aerobic conditions, xanthine oxidase catalyzes the oxidation reaction of the substrate xanthine or hypoxanthine to generate uric acid and produces O2 ^.- . The latter can react with the chemiluminescence agent luminol to cause it to excite. SOD can remove O2 ^{.- and} thus inhibit the chemiluminescence of luminox. This method can be applied to the trace determination of SOD. It is not only highly sensitive, easy to implement, but also specific and accurate at least similar to the cytochrome C reduction method.

5. Immunological method: It measures SOD activity, and immunological method can measure the quality of SOD in the sample, so it has good specificity. It is an ideal method for measuring SOD. Immunoassay includes radioimmunoassay and chemiluminescent immunoassay Method, ELISA method, etc. However, the disadvantage is that only the corresponding antigen of the antibody can be measured. For the detection of different types of SOD, corresponding specific antibodies must be prepared, which is cumbersome.

Application fields of superoxide dismutase

Superoxide dismutase food

SOD is high in vegetables and fruits, such as bananas, hawthorns, prickly pears, kiwis, garlic, etc. Others are also distributed in scallops, chicken, etc. The activity of SOD is higher in the peel than in the pulp, and higher in fresh fruits than after placement. It is processed into health products and food additives in various forms for use, such as milk, beer, fudge and other types of food nutrition fortifiers with added SOD ^[6] .

Superoxide dismutase in the daily chemical industry

Skin aging and damage are important characteristics of human aging, and human aging is caused by the accumulation or removal of reactive oxygen species free radicals. Excess free radicals in the body can cause cell damage and pigmentation. Because human skin is in direct contact with oxygen, it can cause skin aging and damage. Supplementation of exogenous SOD is beneficial to delay skin aging, anti-oxidation, and discoloration. Therefore, many cosmetics manufacturers at home and abroad have added a certain proportion of SOD to their products. Such as France's Estee Lauder pomegranate water, Japan's SKII fairy water, and domestic Dabao SOD honey, etc. ^[1] .

Anti-inflammatory dismutase

Because SOD is a specific disproportionation catalyst acting on superoxide anion free radicals, SOD as a pharmaceutical product has significant effects in treating inflammation, autoimmunity, and cardiovascular and cerebrovascular diseases caused by the action of free radicals. SOD can use its anti-oxidant effect to inhibit inflammation types such as arthritis, pleurisy, and acute bronchitis ^[7] .

Antitumor aspects of superoxide dismutase

In humans, SOD can effectively suppress cancer cells by eliminating superoxide anion free radicals, and the expression of Mn-SOD is more prominent ^[7] .