MECHANISM OF N-OXIDATION REACTION CATALYZED BY CYTOCHROME P-450

Abstract

The purpose of the present study was to elucidate the mechanism of ALoxidation by cytochrome P-450 (P450). The N-dealkylation, ALoxygenation and reduction rates for dialkylarylamines and their NLoxide substrates, and kinetic hydrogen isotope effects for N-demethylation of N, N dimethylaniline (DMA) and N-oxidation of 1, 4-dihydropyridine were investigated. P450 was demonstrated to catalyze the formation of small amounts of N-oxide. The ratio of N-dealkylation:N-oxygenation varied from 1020 to 6. The slow decomposition of N-oxide was also observed by P450 and appears to occur via homolytic N-O bond scission. Low kinetic deuterium and tritium isotope effects were observed for DMA N-demethylation catalyzed by P450 and chloroperoxidase; high isotope effects were observed for peroxidases and hemoglobin. With all of these enzymes low isotope effects were observed for dehydrogenation of two 1, 4-dihydropyridines, where the aminium radicals have low pKas. It is concluded that aminium radicals are a branch point in Noxygenation and N-dealkylation reactions catalyzed by P450, and P450s use specific base catalysis (by the FeO²⁺ entity) to remove protons from aminium radicals, in contrast to the other hemeproteins such as horse radish peroxidase.