Evidence for cytosolic glutathione transferase-mediated denitrosation of nitrosocimetidine and 1-methyl-2-nitro-1-nitrosoguanidine.

Nitrosocimetidine (NC) and 1-methyl-2-nitro-1-nitrosoguanidine (MNNG) are closely related N-nitrosamidines. NC is the nitrosated derivative of cimetidine (Tagamet), an orally administered compound used extensively in the treatment of gastric ulcers. MNNG is a potent carcinogen capable of initiating tumors close to the site of administration and used experimentally to produce stomach cancer. It has become evident that the primary metabolic fate of both of these agents is denitrosation. We have discovered an activity in the cytosol fraction of hamster liver which is capable of denitrosating these nitrosamidines with an efficiency approaching 100%. The activity is heat sensitive and requires reduced glutathione as a cofactor. Inhibition of the denitrosating activity with compounds which inhibit in parallel the conjugation of glutathione with 1-chloro-2,4-dinitrobenzene (CDNB) provides evidence that the activity is glutathione transferase. One molecule of reduced glutathione is consumed in each denitrosation event. Nitrite is formed as denitrosation proceeds with a yield equivalent to 25-50% of the denitrosated product produced. Glutathione disulfide is a minor reaction product, representing 3% of the denitrosation product yield in the MNNG case and 12% in the NC case. Thus far in our survey of N-nitrosamines, N-nitrosamides and N-nitrosamidines, only the nitrosamidines appear to be vulnerable to the cytosolic denitrosating activity. In an attempt to evaluate the importance of the glutathione-dependent reaction in the intact hamster, we have depleted glutathione by pretreatment with the commonly used agents diethyl maleate (DEM) and L-buthionine-S,R-sulfoximine (L-BSO). Nitroso compound was administered i.v. and the circulating blood levels of intact and denitrosated compound 5 min after dosing quantified. NC- and MNNG-derived methylation of organ DNA was also monitored. Pretreatment had no effect on the cytosolic denitrosating or CDNB-conjugating activities. L-BSO pretreatment had no apparent effect on the denitrosative metabolism of NC or MNNG. With DEM pretreatment we obtained clear indications of a decreased rate of denitrosation and observed a 10-fold increase in MNNG-derived liver DNA methylation. The differential effects of these pretreatments are taken as an indication that DEM-sensitive processes other than those requiring glutathione dominate N-nitrosamidine denitrosation in the hamster.