Interest of photochemical methods for induction of lipid peroxidation.

Lipid peroxidation, which plays a part in a wide variety of biological processes, is an integral process in the oxidation of unsaturated fatty acids via a radical chain reaction. Among the various species which may induce this reaction in vivo, reactive forms of oxygen such as peroxide anion, the hydroxyl radical and singlet oxygen are of cardinal importance. These species may be generated enzymatically, chemically or by various radiochemical and photochemical reactions. We present here the advantages of photochemical induction of peroxidation, and we describe the principles of the reactions, the photosensitizers that can be employed to generate the various reactive species of oxygen, and the techniques, direct (ESR) or indirect (specific traps), used to detect the reactive species. Photosensitization can induce the formation of a whole gamut of products of lipid peroxidation: conjugated dienes, aldehydes, hydroperoxides, etc. The relative proportions of the various hydroperoxides of fatty acids or cholesterol depend on the nature of the reactive species involved. Utilization of photochemical reactions is an effective and clean way of inducing peroxidation, allowing fine control of both initiation and orientation.