BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E.S. Severin - 2004

CHAPTER 8. LIPID METABOLISM

VII. Lipid Peroxidation and Its Role in the Pathogenesis of Cell Damage

Oxygen, which is essential for the functioning of the mitochondrial Cell/36.html">Respiratory Chain and numerous other reactions, also acts as a toxic agent when converted into so-called reactive oxygen species.

Reactive oxygen species include:

OH^• — hydroxyl radical;

O₂^• — superoxide anion;

Н₂O₂ — hydrogen peroxide.

Reactive oxygen species are generated in many Cells through the sequential single-electron reduction of 1 oxygen molecule by 4 electrons. Although the final product of these reactions is Water, highly reactive chemical intermediates are formed along the way. The most reactive of these is the hydroxyl radical, which interacts with most organic molecules. It abstracts an electron from them, thereby initiating free-radical chain oxidation. While these free-radical oxidation reactions can serve useful Functions—for instance, when white Blood Cells utilize reactive oxygen species to destroy phagocytized Bacteria—in other cells, free-radical oxidation leads to the destruction of organic molecules, primarily Lipids and, consequently, Cellular Membrane Structures, frequently resulting in cell death. Therefore, the body maintains an efficient system for inhibiting Lipid Peroxidation (LPO).

A. Sources of Reactive Oxygen Species

The Respiratory Chain as a Source of Reactive Oxygen Species

Electron leakage from the mitochondrial respiratory chain and its direct interaction with oxygen is the primary pathway for the Generation of reactive oxygen species in most cells.

Coenzyme Q in the respiratory chain accepts single electrons sequentially from Donors, becoming converted into the semiquinone form (Fig. 8-55) — CoQH^• (see Chapter 6).

This radical can interact directly with oxygen to form the superoxide anion O₂^•, which, in turn, can be converted into other reactive oxygen species:

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Fig. 8-55. Sequential reduction reactions of ubiquinone in the respiratory chain.

Reactions Catalyzed by Oxidases and Oxygenases

Many oxidases—Enzymes that directly reduce oxygen—produce hydrogen peroxide (Н₂O₂). Oxidases generate hydrogen peroxide According to the following scheme:

O₂ + SH₂ —> S + Н₂O₂, where SH₂ is the oxidizable substrate.

Examples of such oxidases include amino acid oxidases, superoxide dismutase, and oxidases localized in Peroxisomes. Peroxisomal oxidases specifically oxidize very long-chain Fatty acids (containing more than 20 carbon atoms) into shorter chains, which then undergo β-oxidation in Mitochondria.

Monooxygenases (such as cytochrome P₄₅₀, which incorporates one oxygen atom into the molecule being oxidized) and Dioxygenases (which incorporate both oxygen atoms) also serve as sources of reactive oxygen species.

Hydrogen peroxide is not exceptionally reactive chemically, but it promotes The formation of the most toxic oxygen species—the hydroxyl radical (OH^•)—via the following reaction:

Fe²⁺ + Н₂O₂ —> Fe³⁺ + OH^- + OH^•.

The presence of Fe²⁺ or other transition Metal Ions within cells accelerates the generation of hydroxyl radicals and other reactive oxygen species. For instance, in erythrocytes, The oxidation of Hemoglobin iron promotes the Formation of the superoxide anion.

B. Lipid peroxidation

Lipid peroxidation (LPO) reactions are free-radical processes that constantly occur within the living Organism. Free-radical oxidation disrupts The Structure of numerous molecules. In Proteins, specific Amino Acids undergo oxidation. Consequently, protein structures are degraded, and covalent cross-links form between them. All of this activates intracellular Proteolytic Enzymes that hydrolyze damaged proteins. Reactive oxygen species also readily disrupt Introduction/20.html">DNA Structure. The nonspecific binding of Fe²⁺ to the DNA molecule facilitates the formation of hydroxyl radicals, which destroy the structure of nitrogenous bases. However, fatty acids containing double bonds separated by a CH₂ group are the most susceptible to reactive oxygen species. It is precisely from this CH₂ group that a free radical (the oxidation initiator) easily abstracts an electron, thereby converting the lipid containing this acid into a free radical.

LPO consists of chain reactions that drive the propagation of free radicals—species possessing an unpaired electron that initiate the further spread of peroxidation.

Stages of Lipid peroxidation

1) Initiation: formation of a free radical (L•)

The reaction is most commonly initiated by a hydroxyl radical, which abstracts a hydrogen atom from the CH₂ groups of a polyunsaturated fatty acid, resulting in the formation of a lipid radical.

2) Chain propagation:

Chain propagation occurs upon The addition of O₂, yielding a lipoperoxy radical (LOO•) or a lipid hydroperoxide (LOOH).

LPO represents a set of free-radical chain reactions, meaning that each newly formed radical triggers the generation of several others.

3) Disruption of lipid structure

The End products of polyunsaturated fatty acid peroxidation are malondialdehyde and acid hydroperoxide.

4) Chain termination — interaction of radicals with each other:

Chain propagation can be halted through the interaction of free radicals with one another or with various antioxidants, such as vitamin E, which donates electrons and is thereby converted into a stable oxidized form.

C. Cell damage resulting from lipid peroxidation

Reactive oxygen species damage the structure of DNA, proteins, and various cellular membrane structures. As a result of hydrophilic zones appearing in the hydrophobic membrane layer due to the formation of fatty acid hydroperoxides, water and sodium or Calcium Ions can penetrate the cells, causing Swelling of cells and Organelles and their eventual destruction. LPO activation is characteristic of numerous pathologies, including muscular dystrophy (Duchenne muscular dystrophy) and Parkinson's disease—where LPO destroys Nerve Cells in the Brainstem—as well as atherosclerosis and tumor development. Peroxidation is also activated in Tissues subjected first to ischemia and subsequently to reoxygenation, which occurs, for instance, during coronary artery spasm followed by their dilation.

A similar situation arises when a thrombus forms in a vessel supplying the myocardium. Thrombus formation leads to vascular lumen occlusion and The Development of ischemia in the corresponding myocardial region (tissue Hypoxia). If prompt therapeutic measures are taken to dissolve the thrombus, oxygen supply to the tissue is restored (reoxygenation). Research demonstrates that at the moment of reoxygenation, the generation of reactive oxygen species increases sharply, which can inflict cellular damage. Thus, despite the rapid restoration of blood flow, cell damage occurs in the affected myocardial area due to the activation of peroxidation.

Alterations in tissue structure resulting from LPO can be observed in the Skin: with Aging, the number of age spots increases, particularly on the dorsal surface of the hands. This pigment is called lipofuscin, and it represents a mixture of lipids and proteins cross-linked by covalent bonds and denatured through interaction with the reactive groups of LPO products. This pigment is phagocytosed but not hydrolyzed by lysosomal enzymes, and consequently accumulates within cells, impairing their functions.

LPO occurs not only in living organisms but also in food products, especially during improper preparation and storage. Fat rancidity, the formation of a darker layer On the surface of butter, and the appearance of a specific odor in dairy products are all signs of LPO. Food products containing unsaturated lipids are typically supplemented with antioxidants—substances that inhibit LPO and preserve the structure of food components.

D. Cellular defense systems against reactive oxygen species

Antioxidant enzymes

Enzymes that protect cells from the action of reactive oxygen species include superoxide dismutase, catalase, and Glutathione peroxidase. These enzymes are most active in the Liver, Adrenal Glands, and Kidneys, where the content of mitochondria, cytochrome P₄₅₀, and peroxisomes is exceptionally high. Superoxide dismutase (SOD) converts superoxide anions into hydrogen peroxide:

2 О2^• + 2 Н⁺ —> Н₂О₂ + О₂.

SOD Isoenzymes are located in both the Cytosol and mitochondria, acting as a primary line of defense because the superoxide anion is typically the first reactive oxygen species generated during electron leakage from the respiratory chain.

SOD is an inducible enzyme, meaning its synthesis increases when lipid peroxidation is activated within the cells.

Hydrogen peroxide, which can trigger the formation of the highly reactive OH^• species, is broken down by the enzyme catalase:

2 Н₂О₂ —> 2 Н₂О + О₂.

Catalase is localized primarily in peroxisomes, where the greatest amounts of hydrogen peroxide are produced, as well as in leukocytes, where it protects cells from the consequences of the "respiratory burst" (see Section 6).

Glutathione peroxidase is a vital enzyme responsible for the inactivation of reactive oxygen species, as it decomposes both hydrogen peroxide and lipid hydroperoxides. It catalyzes the reduction of peroxides using the tripeptide glutathione (y-glutamylcysteinylglycine). The sulfhydryl group of glutathione (GSH) acts as an electron donor and, upon oxidation, forms the disulfide-linked form of glutathione, in which two glutathione molecules are joined via a disulfide bond.

Н₂O₂ + 2 GSH —> 2 Н₂O + G-S-S-G.

Oxidized glutathione is reduced by glutathione reductase:

GS-SG + NADPH + Н⁺ —> 2 GSH + NADP⁺.

Glutathione peroxidase, which reduces membrane lipid hydroperoxides, utilizes selenium (an essential dietary trace element) as a coenzyme. A deficiency in selenium leads to a diminished antioxidant defense capacity.

Vitamins with antioxidant activity

Vitamin E (α-tocopherol), the most widespread natural antioxidant, is a lipophilic molecule capable of directly inactivating free radicals within the Hydrophobic core of cell membranes, thereby preventing the propagation of lipid peroxidation chains. There are 8 Different types of tocopherols, with α-tocopherol exhibiting the highest biological activity.

Vitamin E donates a hydrogen atom to a lipid peroxyl radical (ROO^•), reducing it to a hydroperoxide (ROOH) and thus halting the lipid peroxidation chain reaction (Fig. 8-56).

Fig. 8-56. Mechanism of the antioxidant action of vitamin E. Vitamin E (α-tocopherol) inhibits free-radical oxidation by donating an electron, which inactivates the lipid radical while converting vitamin E into stable, fully oxidized tocopherylquinone.

The vitamin E free radical generated by this reaction is stable and unable to propagate the radical chain. Instead, the vitamin E radical interacts directly with lipid peroxide radicals to reduce them, while being converted itself into a stable oxidized form—tocopherylquinone.

Vitamin C (ascorbic acid) also acts as an antioxidant, inhibiting lipid peroxidation through two distinct mechanisms. First, vitamin C reduces the oxidized form of vitamin E, thereby maintaining the required concentration of this antioxidant directly within cell membranes. Second, as a water-soluble vitamin and potent reducing agent, vitamin C interacts with water-soluble reactive oxygen species—such as O₂^•, Н₂O₂, and ОН•—and inactivates them.

β-Carotene, a precursor of vitamin A, also exhibits antioxidant properties and inhibits lipid peroxidation. Research has demonstrated that a plant-rich diet supplemented with vitamins E, C, and carotenoids significantly reduces the risk of atherosclerosis and cardiovascular diseases, suppresses cataract development (clouding of the eye lens), and displays anticarcinogenic effects. There is substantial evidence indicating that the beneficial effects of these dietary components are associated with the inhibition of lipid peroxidation and other molecules, thereby preserving the normal structural integrity of cellular components.