Radical intermediates in the catalytic cycles of cytochrome P-450

Schemes are presented summarizing current knowledge of the mechanism of action of cytochrome P-450 when it functions either as a monooxygenase with molecular oxygen as the oxygen donor or as a peroxygenase with peroxy compounds as the oxygen donor. In the process, a large variety of physiologically occurring and foreign compounds undergo hydroxylation and oxy and peroxy radicals are generated. In addition, cytochrome P-450 catalyzes reductive reactions, including a recently discovered reaction in which organic hydroperoxides are cleaved to yield hydrocarbons and aldehydes or ketones. The reaction is believed to involve homolysis of the oxygen-oxygen bond and generation of an alkoxy radical, with beta-scission of the latter followed by reduction of the secondary radical to the hydrocarbon. Evidence has been obtained that lipid hydroperoxides are physiological substrates for this reductive cleavage reaction catalyzed by cytochrome P-450.