Metabolism of carcinogenic and non-carcinogenic N-nitroso-N-methylaminopyridines. I. Investigations in vitro.

A comparative study of in vitro metabolism of isomeric N-nitroso-N-methylaminopyridines (NMPY) by rat liver microsomes revealed remarkable differences in the respective rates of potentially activating and deactivating processes. N-Nitroso-N-methyl-2-aminopyridine (2-NMPY), a potent carcinogen in rats and a mutagen in the Ames test was demethylated to a much greater extent than its non-carcinogenic and non-mutagenic isomers 3-NMPY and 4-NMPY. In contrast to the findings for 2-NMPY, enzymatic denitrosation and N-oxide formation play a substantial role in metabolism of 3-NMPY and 4-NMPY. The respective rates of presumed activating to deactivating metabolic processes were found to be -1.5 for 3-NMPY, 0.5 for 4-NMPY, but 3 for 2-NMPY at 10 mM substrate concentration. At 1 mM concentration (which is more close to the conditions prevailing in carcinogenicity experiments), however, the respective ratio for 2-NMPY was approximately 50. 2-Hydroxypyridine, the hydrolysis product of the putative pyridine-2-diazonium intermediate, generated after oxidative demethylation, was formed in high yields from 2-NMPY. Only traces of 3-hydroxypyridine were formed from 3-NMPY and 4-hydroxypyridine could not be detected at all. Experimental evidence suggests that generation of 2-aminopyridine, which was found in high yields after incubation of 2-NMPY, is explained best by a secondary metabolic process: reductive cleavage of azo coupling products formed by reaction of the pyridine-2-diazonium intermediate with appropriate nucleophiles in the incubation mixture.