The role of tocopherols in biomembrane lipid peroxidation.

The rate constants of elementary reactions of oxidation inhibition by tocopherols of different structures (peroxy radical destruction by antioxidant molecules, interaction of inhibitor radicals with one another and with oxidation substrate) are discussed. In contrast with most synthetic phenol antioxidants, tocopherols exhibit a high affinity to peroxy radicals (the rate constants for their reactions with these radicals are one or two orders of magnitude greater than those for reactions involving most synthetic phenols: they possess a fairly high stability of phenoxyl radicals formed and activity in the reaction of oxidation-chain transfer). The activity of tocopherol radicals in chain transfer reactions depends on their structure and manifests itself in a marked dependence of the extent of inhibition on oxidation conditions and the antioxidant concentration, up to an opposite effect. This can be used to control the oxidation rate: the radicals formed from tocopherols can serve as a buffer maintaining the oxidation rate at a certain level in systems with different antioxidant concentrations (concentration control) and with substrates of various unsaturation extent (substrate control). Tocopherols exhibit a unique set of properties which allow them to control the rate of lipid peroxidation and the physico-chemical properties of biological membranes. This dual effect provides the feedback relations and thereby maintains homeostasis in the organism.