Mechanisms of Enzymatic S-Oxygenation of Thioanisole Derivatives and O-Demethylation of Anisole Derivatives Promoted by Both Microsomes and a Reconstituted System with Purified Cytochrome P-450

Abstract

Abstract Oxygenations of thioanisole derivatives have been shown to be promoted with phenobarbital induced rabbit liver microsomes. In order to understand the mechanistic details of the oxygenations, a kinetic study on the S-oxygenation promoted by a reconstituted system with purified cytochrome P-450 has been carried out. Log(Vmax) were correlated with the one electron oxidation potentials of the sulfides (Ep). The oxygenation is considered to proceed via one electron transfer from the sulfides to the iron-bound “oxenoid” intermediate of the enzyme. A similar O-demethylation of anisoles has been investigated. In the oxidative O-demethylation of p-methoxy-d3-anisole with rabbit liver microsomes, an intramolecular kinetic isotope effect of 3.4 was observed. Meanwhile, a large isotope effect of 5.1 was observed in the competitive O-demethylation of p-dimethoxybenzene and p-di(methoxy-d3) benzene. In order to clarify the oxidation pathway of O-demethylation with the enzymatic system of cytochrome P-450, the rates of the O-demethylation of p-substituted anisoles have been measured. Unlike the enzymatic oxygenation of sulfides, there was no correlation between the rates of O-demethylation reaction and one electron oxidation potentials of anisoles. These observations suggest that the O-demethylation of anisoles proceeds via the initial hydrogen abstraction with the iron-bound “oxenoid” intermediate of the cytochrome P-450.