Physiological Role and Regulation
Free radical reactions are a natural and necessary component of metabolism. In healthy cells, free radicals facilitate electron transport in the respiratory chain, phagocytosis, cell proliferation and differentiation, catecholamine metabolism, and prostaglandin synthesis.
Specifically, LPO is critical for renewing the lipid composition of biomembranes and regulating enzymes. This regulation can be direct (reaction products directly modifying enzyme molecules) or indirect (via altering the physicochemical state of membranes).
The intensity of the process is tightly controlled by the balance of three factors:
- Prooxidants — activate oxidation (naphthoquinones, vitamin D, catecholamines, NADPH₂, NADH₂).
- Antioxidants — suppress the process.
- Substrates — targets for oxidation (membrane phospholipids, proteins, nucleic acids).
Pathological Cascade and Stages of Lipoperoxidation
When pathogenic factors trigger an excess of prooxidants, the process spirals out of control, progressing through three stages:
- Oxygen Initiation. The process starts with the superoxide radical. By acquiring hydrogen ions, it forms hydrogen peroxide, which then generates the hydroxyl radical—one of the most aggressive reactive oxygen species. This stage accumulates an excess of reactive oxygen species (singlet oxygen, superoxide anion radical).
- Lipid Free Radical Formation. The hydroxyl radical attacks an intact lipid molecule. By abstracting a hydrogen atom, it turns into water, leaving behind a lipid alkyl radical. This produces a pronounced cytotoxic effect in the cell and interstitium.
- Peroxide Formation (Chain Reaction). The alkyl radical binds oxygen to become a peroxyl radical. This radical attacks a new lipid molecule, forming a lipid hydroperoxide and releasing another radical. The chain reaction branches, and hydroperoxides can break down to form alkoxyl radicals. This leads to irreversible denaturation of organic compounds.
Antioxidant Defense System (AOS)
To limit peroxidation reactions, the cell employs enzymatic (playing a leading role) and non-enzymatic mechanisms. These convert active radicals into inactive compounds. Defense operates at three levels:
- Antioxygen defense: decreases cellular oxygen content by enhancing its utilization (retinol, carotenoids, riboflavin).
- Antiradical defense: quenches free radicals. During the initiation phase, electron acceptors (tocopherol, vitamin K) and superoxide dismutase (SOD) are active. During the lipid radical formation phase, traps are engaged (ascorbic acid, ubiquinone, tocopherol).
- Antiperoxide defense: inactivates hydroperoxides. This involves enzymes (glutathione peroxidase, catalase) and metal chelators (EDTA), which bind transition metal ions and prevent the generation of new radicals.
Integral Membrane Damage
Excessive LPO intensification is one of the five primary mechanisms of cell membrane injury. This pathogenetic cascade leads to severe consequences:
- Detergent Effect Formation. The combined action of LPO and hydrolase activation leads to the accumulation of free fatty acids and lysophospholipids. These amphiphilic compounds act as detergents (soaps), inserting themselves into the lipid bilayer and disrupting it.
- Cluster Formation. Structural defects—primitive channels—form within the membrane. Membrane permeability increases uncontrollably, disrupting ion transport and cellular homeostasis.
- Suppression of Enzyme Systems. Alterations in the conformation of lipoprotein complexes block receptor function, humoral regulation, and nerve impulse generation. The end result is cellular dysfunction and death.