Free radicals and antioxidants

The discovery of free radicals and antioxidants was as significant a milestone for medical science as the discovery of microorganisms and antibiotics, as doctors received not only an explanation for many pathological processes, including aging, but also effective methods for combating them.

The last decade has been marked by advances in the study of free radicals in biological objects. These processes have proven to be a necessary metabolic link in the normal functioning of the body. They participate in oxidative phosphorylation reactions, in the biosynthesis of prostaglandins and nucleic acids, in the regulation of lipotic activity, in the processes of cell division. In the body, free radicals are most often formed during the oxidation of unsaturated fatty acids, and this process is closely related to lipid peroxidation (LPO).

What are free radicals?

A free radical is a molecule or atom that has an unpaired electron in its outer orbit, which makes it aggressive and capable of not only reacting with cell membrane molecules but also converting them into free radicals (a self-sustaining avalanche reaction).

The carbon containing radical reacts with molecular oxygen to form the peroxide free radical COO.

The peroxide radical extracts hydrogen from the side chain of unsaturated fatty acids, forming a lipid hydroperoxide and another carbon-containing radical.

Lipid hydroperoxides increase the concentration of cytotoxic aldehydes, and the carbon-containing radical supports the reaction of formation of peroxide radicals, etc. (in a chain).

There are various mechanisms by which free radicals are formed. One of them is the effect of ionizing radiation. In some situations, during the process of molecular oxygen reduction, one electron is added instead of two, and a highly reactive superoxide anion (O) is formed. The formation of superoxide is one of the defense mechanisms against bacterial infection: without oxygen free radicals, neutrophils and macrophages cannot destroy bacteria.

The presence of antioxidants both in the cell and in the extracellular space indicates that the formation of free radicals is not an episodic phenomenon caused by the effects of ionizing radiation or toxins, but a constant phenomenon that accompanies oxidation reactions under normal conditions. The main antioxidants include enzymes of the superoxide dismutase (SOD) group, whose function is to catalytically convert the peroxide anion into hydrogen peroxide and molecular oxygen. Since superoxide dismutases are ubiquitous, it is reasonable to assume that the superoxide anion is one of the main by-products of all oxidation processes. Catalases and peroxidases convert hydrogen peroxide formed during dismutation into water.

The main feature of free radicals is their extraordinary chemical activity. As if feeling their inferiority, they try to regain the lost electron, aggressively taking it from other molecules. In turn, the "offended" molecules also become radicals and begin to rob themselves, taking electrons from their neighbors. Any changes in a molecule - be it the loss or addition of an electron, the appearance of new atoms or groups of atoms - affect its properties. Therefore, free radical reactions occurring in any substance change the physical and chemical properties of this substance.

The most well-known example of a free-radical process is oil spoilage (rancidity). Rancid oil has a peculiar taste and smell, which is explained by the appearance of new substances in it, formed during free-radical reactions. The most important thing is that proteins, fats and DNA of living tissues can become participants in free-radical reactions. This leads to the development of various pathological processes that damage tissues, aging and the development of malignant tumors.

The most aggressive of all free radicals are free oxygen radicals. They can provoke an avalanche of free radical reactions in living tissue, the consequences of which can be catastrophic. Free oxygen radicals and their active forms (for example, lipid peroxides) can form in the skin and any other tissue under the influence of UV radiation, some toxic substances contained in water and air. But the most important thing is that active forms of oxygen are formed during any inflammation, any infectious process occurring in the skin or any other organ, since they are the main weapon of the immune system, with which it destroys pathogenic microorganisms.

It is impossible to hide from free radicals (just as it is impossible to hide from bacteria, but it is possible to protect yourself from them). There are substances that are distinguished by the fact that their free radicals are less aggressive than the radicals of other substances. Having given its electron to the aggressor, the antioxidant does not seek to compensate for the loss at the expense of other molecules, or rather, does so only in rare cases. Therefore, when a free radical reacts with an antioxidant, it turns into a full-fledged molecule, and the antioxidant becomes a weak and inactive radical. Such radicals are no longer dangerous and do not create chemical chaos.

What are antioxidants?

"Antioxidants" is a collective term and, like such terms as "antineoplastic agents" and "immunomodulators", does not imply belonging to any specific chemical group of substances. Their specificity is the closest connection with free-radical lipid oxidation in general and free-radical pathology in particular. This property unites different antioxidants, each of which has its own specific features of action.

The processes of free radical oxidation of lipids are of general biological nature and, in the opinion of many authors, are a universal mechanism of cell damage at the membrane level when sharply activated. In this case, in the lipid phase of biological membranes, lipid peroxidation processes cause an increase in the viscosity and orderliness of the membrane bilayer, change the phase properties of membranes and reduce their electrical resistance, and also facilitate the exchange of phospholipids between two monolayers (the so-called phospholipid flip-flop). Under the influence of peroxidation processes, the mobility of membrane proteins is also inhibited. At the cellular level, lipid peroxidation is accompanied by swelling of mitochondria, uncoupling of oxidative phosphorylation (and in advanced processes - solubilization of membrane structures), which at the level of the whole organism is manifested in the development of so-called free radical pathologies.

Free Radicals and Cell Damage

Today it has become obvious that the formation of free radicals is one of the universal pathogenetic mechanisms in various types of cell damage, including the following:

• reperfusion of cells after a period of ischemia;
• some drug-induced forms of hemolytic anemia;
• poisoning by some herbicides;
• carbon tetrachloride management;
• ionizing radiation;
• some mechanisms of cell aging (for example, the accumulation of lipid products in the cell - ceroids and lipofuscins);
• oxygen toxicity;
• atherogenesis due to oxidation of low-density lipoproteins in the cells of the arterial wall.

Free radicals participate in the processes:

• aging;
• carcinogenesis;
• chemical and medicinal damage to cells;
• inflammation;
• radioactive damage;
• atherogenesis;
• oxygen and ozone toxicity.

Effects of free radicals

Oxidation of unsaturated fatty acids in cell membranes is one of the main effects of free radicals. Free radicals also damage proteins (especially thiol-containing proteins) and DNA. The morphological outcome of cell wall lipid oxidation is the formation of polar permeability channels, which increases the passive permeability of the membrane for Ca2+ ions, the excess of which is deposited in the mitochondria. Oxidation reactions are usually suppressed by hydrophobic antioxidants such as vitamin E and glutathione peroxidase. Vitamin E-like antioxidants that break oxidation chains are found in fresh vegetables and fruits.

Free radicals also react with molecules in the ionic and aqueous environment of cellular compartments. In the ionic environment, molecules of substances such as reduced glutathione, ascorbic acid, and cysteine retain antioxidant potential. The protective properties of antioxidants become evident when, upon depletion of their reserves in an isolated cell, characteristic morphological and functional changes are observed due to oxidation of lipids in the cell membrane.

The types of damage caused by free radicals are determined not only by the aggressiveness of the radicals produced, but also by the structural and biochemical characteristics of the target. For example, in the extracellular space, free radicals destroy glycosaminoglycans of the main substance of connective tissue, which can be one of the mechanisms of joint destruction (for example, in rheumatoid arthritis). Free radicals change the permeability (and therefore the barrier function) of cytoplasmic membranes due to the formation of channels of increased permeability, which leads to a violation of the water-ion homeostasis of the cell. It is believed that it is necessary to provide patients with rheumatoid arthritis with vitamins and microelements, in particular, correction of vitamin deficiency and microelement deficiency with oligogal E. This is due to the fact that a noticeable activation of peroxidation processes and suppression of antioxidant activity have been proven, so it is very important to include bioantioxidants with high antiradical activity in complex therapy, which include antioxidant vitamins (E, C and A) and microelements selenium (Se). It has also been shown that the use of a synthetic dose of vitamin E, which is absorbed worse than natural. For example, doses of vitamin E up to 800 and 400 IU / day lead to a decrease in cardiovascular diseases (by 53%). However, the answer to the effectiveness of antioxidants will be obtained in large controlled studies (from 8,000 to 40,000 patients), which were conducted in 1997.

The protective forces that maintain the LPO rate at a certain level include enzyme systems that inhibit peroxidation and natural antioxidants. There are 3 levels of regulation of the rate of free radical oxidation. The first stage is antioxygen, it maintains a fairly low partial pressure of oxygen in the cell. This primarily includes respiratory enzymes that compete for oxygen. Despite the wide variability of O3 absorption in the body and the release of CO2 from it, pO2 and pCO2 in arterial blood normally remain fairly constant. The second stage of protection is antiradical. It consists of various substances present in the body (vitamin E, ascorbic acid, some steroid hormones, etc.), which interrupt LPO processes by interacting with free radicals. The third stage is antiperoxide, which destroys already formed peroxides with the help of appropriate enzymes or non-enzymatically. However, there is still no unified classification and unified views on the mechanisms regulating the rate of free radical reactions and the action of protective forces that ensure the utilization of the final products of lipid peroxidation.

It is believed that, depending on the intensity and duration, changes in the regulation of LPO reactions can: firstly, be reversible with a subsequent return to normal, secondly, lead to a transition to another level of autoregulation and, thirdly, some of the effects disunite this self-regulation mechanism, and, consequently, lead to the impossibility of implementing regulatory functions. That is why understanding the regulatory role of LPO reactions under conditions of exposure to extreme factors, in particular, cold, is a necessary stage of research aimed at developing scientifically based methods for managing adaptation processes and complex therapy, prevention and rehabilitation of the most common diseases.

One of the most frequently used and effective is a complex of antioxidants, which includes tocopherol, ascorbate and methionine. Analyzing the mechanism of action of each of the antioxidants used, the following was noted. Microsomes are one of the main places of accumulation of exogenously introduced tocopherol in liver cells. Ascorbic acid, which is oxidized to dehydroascorbic acid, can act as a possible proton donor. In addition, the ability of ascorbic acid to directly interact with singlet oxygen, hydroxyl radical and superoxide anion radical, as well as to destroy hydrogen peroxide has been shown. There is also evidence that tocopherol in microsomes can be regenerated by thiols and, in particular, by reduced glutathione.

Thus, there are a number of interconnected antioxidant systems in the body, the main role of which is to maintain enzymatic and non-enzymatic oxidative reactions at a steady-state level. At each stage of the development of peroxide reactions, there is a specialized system that performs these functions. Some of these systems are strictly specific, others, such as glutathione peroxidase, tocopherol, have a greater breadth of action and less substrate specificity. The additivity of the interaction of enzymatic and non-enzymatic antioxidant systems with each other ensures the body's resistance to extreme factors that have prooxidant properties, i.e. the ability to create conditions in the body that predispose to the production of activated oxygen forms and the activation of lipid peroxidation reactions. There is no doubt that the activation of lipid peroxidation reactions is observed under the influence of a number of environmental factors on the body and in pathological processes of various natures. According to V. Yu. Kulikov et al. (1988), depending on the mechanisms of activation of LPO reactions, all factors affecting the body can be divided with a certain degree of probability into the following groups.

Factors of physicochemical nature that contribute to an increase in tissue precursors and direct activators of LPO reactions:

• oxygen under pressure;
• ozone;
• nitric oxide;
• ionizing radiation, etc.

Factors of biological nature:

• phagocytosis processes;
• destruction of cells and cell membranes;
• systems for generating activated oxygen forms.

Factors that determine the activity of the body's antioxidant systems of enzymatic and non-enzymatic nature:

• activity of processes associated with the induction of antioxidant systems of enzymatic nature;
• genetic factors associated with the depression of one or another enzyme that regulates lipid peroxidation reactions (deficiency of glutathione peroxidase, catalase, etc.);
• nutritional factors (lack of tocopherol, selenium, other microelements, etc. in food);
• structure of cell membranes;
• the nature of the relationship between antioxidants of enzymatic and non-enzymatic nature.

Risk factors that potentiate the activation of LPO reactions:

• activation of the body's oxygen regime;
• state of stress (cold, high temperature, hypoxia, emotional and painful impact);
• hyperlipidemia.

Thus, activation of LPO reactions in the body is closely related to the functioning of oxygen transport and utilization systems. Adaptogens deserve special attention, including the widely used eleutherococcus. The preparation from the root of this plant has general tonic, adaptogenic, anti-stress, anti-atherosclerotic, anti-diabetic and other properties, reduces general morbidity, including influenza. When studying the biochemical mechanisms of action of antioxidants in humans, animals and plants, the range of pathological conditions for the treatment of which antioxidants are used has significantly expanded. Antioxidants are successfully used as adaptogens for protection against radiation damage, treatment of wounds and burns, tuberculosis, cardiovascular diseases, neuropsychiatric disorders, neoplasms, diabetes, etc. Naturally, interest in the mechanisms underlying such universal action of antioxidants has increased.

At present, it has been experimentally established that the efficiency of antioxidants is determined by their activity in inhibiting lipid peroxidation due to interaction with peroxide and other radicals initiating LPO, as well as due to the effects of antioxidants on the membrane structure, facilitating oxygen access to lipids. LPO can also change with a mediated system of antioxidant action through neurohormonal mechanisms. It has been shown that antioxidants affect the release of neurotransmitters and hormones, receptor sensitivity and their binding. In turn, a change in the concentration of hormones and neurotransmitters changes the intensity of LPO in target cells, which leads to a change in the rate of lipid catabolism and, as a consequence, to a change in their composition. The relationship between the LPO rate and a change in the spectrum of membrane phospholipids plays a regulatory role. A similar regulatory system has been found in the cell membranes of animals, plants and microbial organisms. As is known, the composition and fluidity of membrane lipids affect the activity of membrane proteins, enzymes, and receptors. Through this regulation system, antioxidants act on the reparation of the membrane, changed in the pathological state of the organism, normalize its composition, structure and functional activity. Changes in the activity of enzymes synthesizing macromolecules and the composition of the nuclear matrix with a change in the composition of membrane lipids caused by the action of antioxidants can be explained by their influence on the synthesis of DNA, RNA, protein. At the same time, data on the direct interaction of antioxidants with macromolecules appeared in the literature.

These data, as well as the recently discovered data on the effectiveness of antioxidants in picomolar concentrations, highlight the role of receptor pathways in their effect on cellular metabolism. In the work of V. E. Kagan (1981) on the mechanisms of structural and functional modification of biomembranes, it was shown that the dependence of the rate of LPO reactions in biomembranes depends not only on their fatty acid composition (degree of unsaturation), but also on the structural organization of the lipid phase of the membranes (molecular mobility of lipids, strength of protein-lipid and lipid-lipid interactions). It was found that as a result of the accumulation of LPO products, lipid redistribution occurs in the membrane: the amount of liquid lipids in the biolayer decreases, the amount of lipids immobilized by membrane proteins decreases, and the amount of ordered lipids in the biolayer (clusters) increases. V.

When studying the nature, composition and mechanism of homeostasis of the antioxidant system, it was shown that the manifestation of the damaging effect of free radicals and peroxide compounds is prevented by a complex multicomponent antioxidant system (AOS), which provides binding and modification of radicals, preventing the formation or destruction of peroxides. It includes: hydrophilic and hydrophobic organic substances with reducing properties; enzymes that maintain the homeostasis of these substances; antiperoxide enzymes. Among the natural antioxidants there are lipid (steroid hormones, vitamins E, A, K, flavonoids and polyphenols vitamin P, ubiquinone) and water-soluble (low molecular thiols, ascorbic acid) substances. These substances either trap free radicals or destroy peroxide compounds.

One part of tissue antioxidants has a hydrophilic character, the other - a hydrophobic one, which makes possible the simultaneous protection of functionally important molecules from oxidizing agents in both the aqueous and lipid phases.

The total amount of bioantioxidants creates a "buffer antioxidant system" in tissues, which has a certain capacity, and the ratio of prooxidant and antioxidant systems determines the so-called "antioxidant status" of the organism. There is every reason to believe that thiols occupy a special place among tissue antioxidants. This is confirmed by the following facts: high reactivity of sulfhydryl groups, due to which some thiols are oxidized at a very high rate, dependence of the rate of oxidative modification of SH groups on their radical environment in the molecule. This circumstance allows us to single out a special group of easily oxidized substances from a variety of thiol compounds, which perform specific functions of antioxidants: reversibility of the oxidation reaction of sulfhydryl groups into disulfide groups, which makes it possible, in principle, to energetically maintain the homeostasis of thiol antioxidants in the cell without activating their biosynthesis; the ability of thiols to exhibit both antiradical and antiperoxide effects. The hydrophilic properties of thiols determine their high content in the aqueous phase of the cell and the possibility of protection from oxidative damage of biologically important molecules of enzymes, nucleic acids, hemoglobin, etc. At the same time, the presence of non-polar groups in thiol compounds ensures the possibility of their antioxidant activity in the lipid phase of the cell. Thus, along with substances of lipid nature, thiol compounds take an extensive part in protecting cellular structures from the action of oxidizing factors.

Ascorbic acid is also subject to oxidation in the body tissues. Like thiols, it is part of the AOS, participating in the binding of free radicals and the destruction of peroxides. Ascorbic acid, the molecule of which contains both polar and non-polar groups, exhibits close functional interaction with SH-glutathione and lipid antioxidants, enhancing the effect of the latter and preventing lipid peroxidation. Apparently, thiol antioxidants play a leading role in protecting the main structural components of biological membranes, such as phospholipids or proteins immersed in the lipid layer.

In turn, water-soluble antioxidants - thiol compounds and ascorbic acid - exhibit their protective action mainly in an aqueous environment - the cell cytoplasm or blood plasma. It should be borne in mind that the blood system is an internal environment that plays a decisive role in non-specific and specific reactions of the body's defense, affecting its resistance and reactivity.

[ 1 ], [ 2 ], [ 3 ], [ 4 ], [ 5 ], [ 6 ]

Free radicals in pathology

The issue of cause-and-effect relationships in changes in the intensity of lipid peroxidation in the dynamics of disease development is still being discussed in the literature. According to some authors, it is the violation of the stationarity of this process that is the main cause of the indicated diseases, while others believe that the change in the intensity of lipid peroxidation is a consequence of these pathological processes initiated by completely different mechanisms.

Research conducted in recent years has shown that changes in the intensity of free radical oxidation accompany diseases of various genesis, which confirms the thesis about the general biological nature of free radical damage to cells. Sufficient evidence has accumulated of the pathogenetic participation of free radical damage to molecules, cells, organs and the body as a whole and successful treatment with pharmacological drugs that have antioxidant properties.