Mechanism of Dimethylnitrosamine Metabolism and Activation in Rats2

Abstract

Dimethylnitrosamine (DMN) was metabolized by liver tissue slices to CO₂, and reactive metabolites were formed that bound to nucleic acids. SKF 525-A [2-(diethylamlno)ethyl- 2,2-diphenylvalerate hydrochloride], 1 mm or 0.5 mm In the incubation mixture, strongly inhibited both CO₂ production and binding. Liver slices derived from Sprague-Dawley rats pretreated with phenobarbital sodium (PB) metabolized less DMN to CO₂ but, like the control liver slices, did bind to nucleic acids. Liver slices from rats treated with CoCl₂ metabolized less DMN to CO₂ and bound less ¹⁴C from DMN to nucleic acids than did the control silces. We also observed an NADPH-dependent, heatsensitive DMN activation process leading to reactive metabolites that bound covalently to proteins in both microsomes and 9,000×g supernatants but not In 100,000×g supernatants. Treatment with 1 mM SKF 525-A markedly Inhibited the activation process in microsomes and only slightly inhibited it in 9,000×g supernatants. Microsomes or 9,000×g supernatants from PB-pretreated animals were less able to activate DMN than those from controls. The 9,000×g supernatant preparations from CoCl₂- pretreated animals had a markedly decreased capacity for DM~ activation, but the mlcrosomes from these animals had only a moderately decreased capacity. When activation of DMN was studied in an 80% CO atmosphere, the ability for DMN activation was decreased to about 35.6% In microsomes and was completely abolished in 9,000×g supernatants. When an N₂ atmosphere was employed during incubation, the ability for DMN activation in microsomes remained unchanged and that in 9,000×g supernatants only partially decreased. The results suggested the presence of multiple pathways for DMN metabolism and activation, only some of them being dependent on cytochrome P₄₅₀.