Substrate specificity and alkyl group selectivity in the metabolism of N-nitrosodialkylamines

Metabolic activation may be a key step in determining the tissue specificity of carcinogenic nitrosamines. In previous work, we characterized P450IIE1 (an acetone/ethanol-inducible form of cytochrome P-450) as the major enzyme for the metabolic activation of N-nitrosodimethylamine. In this work, we investigated the metabolism of other N-nitrosodialkylamines in rat liver microsomes and in reconstituted monooxygenase systems containing purified cytochrome P-450 isozymes. The enzyme specificities in the metabolism of N-nitrosoethylmethylamine and N-nitrosodiethylamine were similar to those of N-nitrosodimethylamine; i.e., these substrates were more efficiently metabolized by acetone- or ethanol-induced microsomes than by other types of microsomes. However, substituting one methyl group with a benzyl or butyl group, as in N-nitrosobenzylmethylamine or N-nitrosobutylmethylamine (NBMA), substantially changed the enzyme specificity. P450IIE1 efficiently catalyzed the demethylation but not the debutylation of NBMA, whereas P450IIB1 (a phenobarbital-inducible form) efficiently catalyzed both the debutylation and demethylation reactions. In the demethylation of NBMA by P450IIE1, the addition of cytochrome b5 markedly increased the activity at low but not at high substrate concentrations, suggesting a decrease in Km value. This effect, however, was not observed in the debutylation of NBMA by P450IIE1 or P450IIB1, and in the demethylation of NBMA by P450IIB1. These studies demonstrate the substrate specificity and alkyl group selectivity in the metabolism of nitrosamines by cytochrome P-450 isozymes.