NADPH-initiated cytochrome P450-dependent free iron-independent microsomal lipid peroxidation: specific prevention by ascorbic acid.

In this paper we demonstrate that ascorbic acid specifically prevents NADPH-initiated cytochrome P450 (P450)-mediated microsomal lipid peroxidation in the absence of free iron. Lipid peroxidation has been evidenced by the formations of conjugated dienes, lipid hydroperoxide and malondialdehyde. Other scavengers of reactive oxygen species including superoxide dismutase, catalase, glutathione, alpha-tocopherol, uric acid, thiourea, mannitol, histidine, beta-carotene and probucol are ineffective to prevent the NADPH-initiated P450-mediated free iron-independent microsomal lipid peroxidation. Using a reconstituted system comprised of purified NADPH-P450 reductase, P450 and isolated microsomal lipid or pure L-alpha-phosphatidylcholine diarachidoyl, a mechanism has been proposed for the iron-independent microsomal lipid peroxidation and its prevention by ascorbic acid. It is proposed that the perferryl moiety P450 Fe3+.O2.- initiates lipid peroxidation by abstracting methylene hydrogen from polyunsaturated lipid to form lipid radical, which then combines with oxygen to produce the chain propagating peroxyl radical for subsequent formation of lipid peroxides. Apparently, ascorbic acid prevents initiation of lipid peroxidation by interacting with P450 Fe3+.O2.-.