Mechanism of the Cytochrome P-450 Catalyzed Isomerization of Hydroperoxides

Abstract

The interactions of hydroperoxides with cytochrome P-450 have been studied extensively (O’Brien, 1982; Sligar et al., 1984). It is well known that these compounds can donate an oxygen to the ferric form of P-450. The resulting activated form of the enzyme can oxidize many substrates with results similar to those of NADPH/O₂-supported P-450 oxidations. Mechanistic details of the O-O bond cleavage step and the nature of the oxidant remain unsettled issues. Homolytic cleavage would produce an alkoxy radical and an iron-coordinated hydroxyl radical similar to peroxidase Compound II (equation 1, where Fe represents the heme iron of P-450). Heterolytic cleavage of the O-O bond would produce the alcohol and an iron-oxo species analogous to peroxidase Compound I (equation 2). Much of the early work in this area suggests that homolysis is the principal result when hydroperoxides interact with P-450. For example, Griffin (1980) successfully trapped a methyl radical from the microsomal decomposition of cumene hydroperoxide (COON). This species is produced when the cumyloxy radical undergoes β-scission. Blake and Coon (1981) invoked homolysis to explain the results of structure-activity relationships on the benzylic hydroxylation of substituted toluenes by rabbit liver P-450_LM2 that were supported by analogs of COOH. According to the mechanism proposed, the cumyloxy radical is the oxidant that abstracts H• from the substrate. The resulting carbon-centered radical then interacts with (FeOH)³⁺ forming the benzylic alcohol and the native enzyme. More recent work with P-450 and peroxyphenylacetic acid demonstrated that peroxy compounds can undergo both homolytic and heterolytic scission of the O-O bond. McCarthy and White (1983) provided evidence that hydroxylations of cosubstrates in the P-450-peroxyacid system were a consequence of heterolysis, and that (FeO)³⁺ was the oxidant. Recently, it was reported that P-450 catalyzes both types of O-O bond cleavage, and that the actual mechanism is dependent on the structure of the peroxy compound (Lee and Bruice, 1985).