On the mechanism of action of cytochrome P-450. Spectral intermediates in the reaction with iodosobenzene and its derivatives

Cytochrome P-450 is known to catalyze the following oxygen transfer reaction: RH + PhIO----ROH + PhI where RH represents a variety of hydroxylatable substrates and PhIO a variety of iodosobenzene derivatives that serve as oxygen donors, and neither molecular oxygen nor an external electron donor is required. To determine whether the cytochrome functions in such reactions by a peroxidase-type mechanism, the kinetics of its interaction with a variety of substituted iodosobenzenes and iodobenzene diacetates have been determined by stopped flow spectrophotometry. The reaction of phenobarbital-induced rabbit liver microsomal cytochrome P-450 form 2 with iodosobenzenes or iodobenzene diacetates leads to the reversible formation of three spectral intermediates, termed E, F, and G. Complex E is characterized by a type I difference spectrum, representing the iodosobenzene-dependent partial shift of the low spin hexacoordinate form of the ferric enzyme to the high spin pentacoordinate form, F represents a transient intermediate whose spectrum cannot be determined for kinetic reasons, and G represents a blue-shifted intermediate with an absorption maximum at about 393 nm in the absolute spectrum. The striking and principal feature of thes...Continue Reading