Reactive oxygen species (ROS) are single-electron reduction products of oxygen in the body. Electrons leak out of the respiratory chain before they are passed to the terminal oxidase and consume about 2% of oxygen, including oxygen. The reduction product superoxide anion (O2·-), the two electron reduction product hydrogen peroxide (H 2 O 2 ), the trielectron reduction product hydroxyl radical (·OH), and nitric oxide. [1] The production of ROS is mainly caused by the conversion of mitochondria from state III to state IV to a high oxygen environment and a highly reduced state of the respiratory chain to cause a large amount of electrons to leak out and reduce oxygen molecules.

Overview

Oxygen is an indispensable gas in the process of life movement. Once people are in an environment of lack of oxygen or oxygen supply, they will feel the pain or even death of suffocation. Therefore, since the discovery of oxygen by British Joseph Priestley in the early 1770s, oxygen has been considered a gas that is beneficial to the human body. However, today, with the rapid development of science and technology, we know that both oxygen in the air and dissolved oxygen in the water have high oxidizing properties. Like ordinary metal iron, all parts of the human body in the air are constantly It is "rusted" by the corrosion of oxygen. Of course, this corrosion is different from iron, which is reflected in the human body's cellular level. In particular, the continuous aging of various organs of the human body with age is a visual manifestation of this "rusting" of corrosion. In 1969, McCord and Fridovich discovered that during the biochemical reaction, O2 obtained an electron reduction to form superoxide radical (O-2), which was then purified by red blood cells to obtain O-2 scavenging inactivation enzyme, and named superoxidation. Superoxide dismultase (SOD). This discovery has inspired a large number of scientific researchers to focus on the O-2 production process, reactivity, toxicity, physiology and pathology, to explore the physiological significance of SOD. At the same time, hydrogen peroxide (H2O2), hydroxyl radical (·OH), and excited oxygen (monooxygen or singlet oxygen, O2) derived from O-2 have also received attention.

The so-called active oxygen, in a nutshell, refers to a general term for oxygen-containing and active substances in the body or in the natural environment: there is mainly an excited state of oxygen molecules, that is, a singlet oxygen molecule or singlet oxygen. Molecular (O2); three kinds of oxygen-containing free radicals, namely superoxide anion radical (O-2), hydroxyl radical (·OH) and hydroperoxy radical (HO2); Hydrogen peroxide (H2O2) and lipid peroxide (ROOH), and a nitrogen-containing oxide (NO). These materials are chemically reactive and have a short lifetime, such as an average lifetime of 2 μs for O2, 200 μs for ·OH radicals, and 5 s for O-2 radicals. Because of their short life span and high reactivity, the determination of other reactive oxygen species in addition to H2O2 is still an international problem, and there is no specific and effective method. In general, the analytical methods used may generally be a chemical reaction method, a capture method, and a direct measurement method.

Oxidative damage to biological macromolecules

Oxidative damage to nucleic acids

The oxidative damage of DNA mainly includes: First, the modification of bases. Hydroxyl radicals can add to the 5,6-double bond of thymine to form thymine free radicals. Alterations in bases can result in the destruction of many biochemical and protein synthesis processes under the control of their groups. The second is the break of the bond. Free radicals take hydrogen atoms from the pentose sugar of DNA, causing them to form free radicals with unpaired electrons at the C4 position, which in turn undergo chain cleavage at the β-position. O2 also decomposes nucleotides, especially guanylic acid, in a ratio of guanosine, adenosine, cytidine and uridine breakdown of 26:13:8:1. DNA damaged by oxidative damage may undergo cleavage, mutation, and changes in thermal stability, which seriously affect the normal transcription and translation process of genetic information.

Oxidative damage to proteins

The effects of reactive oxygen species on proteins include modification of amino acids, cleavage of peptide chains, formation of cross-linked polymers of proteins, changes in conformation and immunogenicity.

Modified amino acid

Amino acid components that play a key role in protein molecules are particularly sensitive to free radical damage. Aromatic amino acids and sulfur-containing amino acids are most prominent. Different free radicals have special effects on specific amino acid side chains, such as superoxide anion-mediated methionine oxidation. To become methionine sulfoxide, cysteine ​​is oxidized to sulfoalanine; hydroxyl radical can remove one hydrogen atom in the α-position of aliphatic amino acid; intermediate products such as alkoxy radical and peroxy radical It is possible to oxidize tryptophan to kynurenine, N-methyl kynurenine and quintalionine. [3]

Breaking the peptide chain

There are two ways to break the protein peptide chain caused by reactive oxygen species. One is the hydrolysis of the peptide chain, and the other is the direct cleavage from the α-carbon atom. The way it is broken depends on the type and concentration of active oxygen and protein. The rate of reaction between the two. Hydrolysis of peptide bonds often occurs at proline. The mechanism is that reactive oxygen species attack valine to introduce carbonyl groups to form α-pyrrolidone, which is broken by hydrolysis to its adjacent amino acids, and α-pyrrolidone becomes a new N-terminal. It can be further hydrolyzed to glutamine. The way in which the peptide chain is directly cleaved is that the reactive oxygen species attack the α-carbon atom to form an α-carbonperoxy group, which is converted to an imino peptide, which is hydrolyzed by weak acid to an amino acid and a dicarboxy compound.

Protein cross-linking polymer

A variety of mechanisms can lead to cross-linking and polymerization of proteins. The tyrosine in the protein molecule can form a dityrosine, and the cysteine ​​oxidizes to form a disulfide bond, both of which can form a crosslink of the protein. Crosslinking can be divided into two forms of intramolecular crosslinking and intermolecular crosslinking. The number of tyrosines and cysteines in the protein molecule can determine the form of cross-linking. In addition, malondialdehyde (MDA) produced by lipid peroxidation reacts with protein amino acid residues to form enamine, which can also cause protein cross-linking. The α-carbonylaldehyde product which is auto-oxidized by monosaccharides in the living body can be cross-linked with proteins to inactivate the enzyme, and the membrane deformability is lowered, resulting in cell senescence and death.

Change conformation

After oxidation, the protein is thermodynamically unstable, and some tertiary structures are opened, losing the original conformation. Oxidation of SOD with H2O2 and ascorbic acid-Fe(III) enhanced the UV absorption and decreased the endogenous fluorescence, indicating that the enzyme molecules tend to be loose and disordered by tightly ordered arrangement. Using spin labeling studies, the detection of lower concentrations of ascorbic acid-Fe(III) and H2O2 can affect the association of SOD molecular subunits or their surrounding structures.

Altering immunogenicity

Using H2O2, or H2O2, -Cu2+ and ascorbic acid-Fe(III) systems to act on bovine erythrocyte copper-zinc superoxide dismutase (SOD), human serum albumin (HAS) and human IgG, and found that SOD, HAS and IgG Increased antibody response suggests that reactive oxygen species may be involved in the formation of antigen-antibody complexes in certain autoimmune diseases. [3]

Biofilm damage

The damage of free radicals to biofilms is polyunsaturated fatty acids acting on cell membranes and subcellular membranes, causing lipid peroxidation, lipid peroxidation intermediates, lipid free radicals (L·), lipoxygen free radicals. (LO·), lipooxygen free radicals (LOO·) can react with membrane proteins to form protein free radicals to polymerize and crosslink proteins. In addition, lipid peroxidated carbonyl products (such as malondialdehyde) can also attack the amino group of membrane protein molecules, resulting in intramolecular cross-linking and intermolecular cross-linking of proteins. On the other hand, free radicals can also be directly covalently bound to the enzyme on the membrane or to the receptor. These oxidative damages destroy the spatial configuration of many enzymes and receptors, ion channels embedded in the membrane system, destroying the integrity of the membrane, reducing membrane fluidity, increasing membrane fragility, and exchanging substances and information inside and outside the cell or inside and outside the organelle. Barriers affect membrane function and antigen specificity, leading to extensive injury and pathology. Most of the HO in the body is produced in the organelles, especially in the mitochondria, causing damage to the mitochondrial membrane, leading to energy metabolism disorders of the cells and the body.

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