Reconstitution of rabbit liver microsomal N-nitrosopyrrolidine alpha-hydroxylase activity

The in vitro alpha-hydroxylation of N-nitrosopyrrolidine (NPYR) by both isolated rabbit liver microsomes and purified cytochrome P-450 isozymes was investigated. Microsomes from untreated rabbits catalyzed the alpha-hydroxylation of NPYR at rates similar to those reported previously for rats, mice, and hamsters. The effect of established inducers of microsomal P-450 caused complex changes in apparent rates of alpha-hydroxylation of NPYR which made interpretation of responses to inducer pretreatment difficult and suggested the participation of multiple cytochrome P-450 isozymes in the metabolism of NPYR. Partial inhibition of alpha-hydroxylase activity by antibodies against rabbit isozymes 2, 3a, and 5 indicated the participation of at least these three isozymes in microsomal catalysis. Reconstitution studies using purified rabbit isozymes 2, 3a, 3b, 3c, 4, and 6 indicated that isozymes 2, 3a, 4, and 6 possessed significant alpha-hydroxylase activity with isozymes 3a and 6 exhibiting the highest activity when assayed at 20 mM NPYR. As NPYR concentrations were decreased, the rates of catalysis for the reconstituted systems were differentially decreased such that isozyme 3a exhibited the highest activity at low NPYR concentrations. These data indicate that isozyme 3a is the preferred catalyst for the alpha-hydroxylation of NPYR at low substrate concentrations and suggest that conditions such as chronic ethanol consumption which lead to the induction of isozyme 3a in rabbits or its orthologue in other species can account for enhanced rates of alpha-hydroxylation and metabolic activation of NPYR.