Metabolism of N-nitrosodimethyl- and N-nitrosodiethylamine by rat hepatocytes: effects of pretreatment with ethanol

Abstract

Hepatocytes were isolated from the livers of ethanol-pretreated rats, and the relationship between the generation of CO2 and the loss of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) from the incubation mixtures was examined. The evolution of CO2 by hepatocytes isolated from untreated, control rats was compared with the evolution of CO2 by hepatocytes isolated from rats treated with 10% EtOH in their drinking water. The CO2 generated from either NDMA or NDEA represented only a fraction of the parent compound that was metabolized during the incubation period. Therefore, the measurement of CO2 evolution as an indication of the metabolism of these simple dialkylnitrosamines is inadequate, and the actual loss of the parent compound must be measured directly when utilizing isolated hepatocytes as a model system to study the metabolism of nitrosamines. The liver microsomal metabolism of NDMA and NDEA was also examined. Pretreatment of the rats with ethanol resulted in a marked increase in the microsomal metabolism of NDMA but had a relatively small effect on NDEA metabolism. Phenobarbital pretreatment did not result in any increase in NDMA metabolism whereas there was a very significant (6-fold) increase in NDEA metabolism. These results suggest that different isozymes of cytochrome P-450 may be primarily responsible for the metabolism of the two nitrosamines. The inhibition patterns observed when an antibody inhibitory to cytochrome P-450j was added to microsomes derived from control and ethanol- and phenobarbital-pretreated rats conclusively demonstrate that NDMA and NDEA are preferentially metabolized by distinct isozymes of cytochrome P-450.