The nature of the rate-limiting step in aniline hydroxylation involving cytochrome P-450 rat liver microsomes

Abstract

The kinetics of aniline hydroxylation was studied with: (1) rat liver microsomes involving NADPH and O 2 (system 1), (2) hepatic microsomes and tert-butylhydroperoxide (system 2) and (3) microsomes and cumyl hydroperoxide (system 3) at 15–37 °C. The reactions were characterized by the values of the aniline oxidation rate constants, k 2 = V/E 0, where E 0 is the initial concentration of cytochrome P-450: k 1 2 = 1.60 · 10 8 exp (−13 400/RT) sec −1, k 2 2 = 1.66 · 10 9 exp (−14500/RT) sec −1, k 3 2 = 6.83 · 10 9 exp (−15300/RT) sec −1. The values of ΔH 0 ∗ and ΔS 0 ∗, were calculated and compared for the three systems. The evidence suggests that oxygen insertion into the substrate molecule is the rate-limiting step in the reaction of aniline oxidation for the mentioned systems. The nature of aniline binding to cytochrome P-450 and that of the hydroxylating agent have been discussed.