Radical intermediates in peroxide-dependent reactions catalyzed by cytochrome P-450

Abstract

Highly purified liver microsomal cytochrome P-450 acts as a peroxygenase in catalyzing the reaction, RH+ XOOH→ROH+XOH, Where RH represents any of a large variety of foreign or physiological substrates and ROH the corresponding product, and XOOH represents any of a series of peroxy compounds such as hydroperoxides or peracids serving as the oxygen donor and XOH the resulting alcohol or acid. Several experimental approaches in this and other laboratories have yielded results compatible with a homolytic mechanism of oxygen-oxygen bond cleavage but not with the heterolytic formation of a common iron-oxo intermediate from the various peroxides.

Recently, we have found a new reaction, catalyzed by the reconstituted system containing the phenobarbital-inducible form of P-450, which catalyzes the reductive cleavage of hydroperoxides: XRR’C-OOH+ NADPH+H⁺→ XR’CO + R’H+H₂O + NADP⁺ Thus, cumyl hydroperoxide yields acetophenone and methane, and 13-hydroperoxyoctadeca-9, 11-dienoic acid yields pentane and an as yet unidentified additional product. Since hydroperoxide reduction does not produce the corresponding alcohol, it is concluded that homolytic cleavage of the oxygen-oxygen bond occurs with rearrangement of the resulting alkoxy radical. Studies are in progress to determine how broad a role the new hydroperoxide cleavage reaction plays in the biological peroxidation of lipids.