Studies on the in vitro metabolism of dimethylnitrosamine by rat liver

Abstract

The biotransformation of dimethylnitrosamine (DMN) to formaldehyde, generally attributed to the mediation of a demethylase enzyme associated with the microsomal mixed function oxidase system, has been investigated in rat liver preparations. All of the enzyme activity was found in the postmitochondrial fraction and the microsomes contained approximately 50% of this activity. The restoration of total activity resulting from the addition of the cytosol to the microsomal fraction was found to be due to the presence of diffusible, heat-labile constituents in the cytosol. Enzyme kinetic studies revealed that DMN was metabolized to formaldehyde by either a multistep or a multicomponent process. DMN demethylase was found to be relatively stable to storage in contrast to cytochrome P-450 and a number of mixed function oxidase enzyme activities. In spectral interaction studies DMN was found to form an atypical interaction spectrum with either control, phenobarbitone-pretreated or phospholipid-depleted microsomal preparations. DMN had little effect on the Type II spectral interaction of aniline, but non-competitively inhibited the Type I spectral interactions of benzphetamine and biphenyl. Whilst the mixed function oxidase enzyme inhibitors SKF 525A and metyrapone markedly reduced the metabolism of ethylmorphine and aniline, DMN demethylase was little affected by the former compound and appreciably enhanced by metyrapone. Moreover, DMN demethylase was strongly inhibited by a number of metal chelators. Finally, DMN, in vitro, did not inhibit a number of hepatic mixed function oxidases, but significantly reduced anaerobic nitroreductase activity. The results of these studies reveal important differences between the properties of the enzymatic systems which metabolize DMN and mixed function oxidase substrates, and are consistent with the conclusion that the degradation of DMN to formaldehyde by rat liver preparations is a multicomponent system not rate limiting with respect to cytochrome P-450.