Reduction of benzyl halides by liver microsomes: Formation of 478 nm-absorbing σ-alkyl-ferric cytochrome P-450 complexes

Abstract

The benzyl halides benzyl bromide and 4-nitrobenzyl chloride are reduced anaerobically by NADPH and rat liver microsomes to yield toluene and 4-nitrotoluene, respectively. These reductions are cytochrome P-450-dependent since they are inhibited by CO and metyrapone, and are increased after pretreatment of rats by phenobarbital and 3-methylcholanthrene. During benzyl halide reduction, cytochrome P-450 complexes, which are very unstable to O 2 and characterized by a Soret peak at 478 nm, are formed in steady-state concentrations. These concentrations are very dependent on pretreatment of rats and on the nature of the reducing agent (NADPH or dithionite) and the benzyl halide : 4-methylbenzyl bromide and benzyl bromide lead to 478 nm absorbing complexes in the presence of NADPH whereas 4-nitrobenzyl chloride and benzyl chloride lead to such complexes only in the presence of dithionite. Microsomal reductions of 4-nitrobenzyl chloride and benzyl bromide in D 2O lead to partially deuterated 4-nitrotoluene and toluene. From these results, we propose a mechanism for anaerobic microsomal reduction of benzyl halides involving the intermediate formation of σ-alkyl cytochrome P-450-Fe(III)-CH 2Ar complexes which exhibit red-shifted Soret peaks around 478 nm. Toluenes, ArCH 3, are formed either by protonation of the σ-alkyl complexes or by hydrogen abstraction by the intermediate free radical ArCH 2.