Activation of Nitrosamines to Mutagens by Postmitochondrial Fraction of Hamster Pancreas

The nitrosamines 2,6-dimethylnitrosomorpholine (DMNM) and β-nitrosobis(2-oxopropyl)amine (NBOP) are carcinogens which induce adenocarcinoma of the pancreatic ducts in the Syrian golden hamster. Since both compounds are indirect carcinogens and must be metabolically activated to their carcinogenic form(s) by the enzyme system(s) of the host, the capacity of the mixed-function oxidases in hamster pancreas 9000 ˣ g (S-9) fraction to activate these compounds to mu-tagens was studied. The postmitochondrial fraction (S-9) prepared from pancreas was capable of converting both compounds to forms mutagenic for Salmonella typhimurium TA-1535. The mutagenic activity of DMNM with hamster pancreas S-9 was dose dependent, with 22 ± 7 (S.E.) revertants formed per mg protein of S-9 with 1 mg DMNM and 109 ± 9 revertants formed per mg with 10 mg DMNM. Although NBOP is the more potent carcinogen, it was consistently less mutagenic, with 12 ± 4 revertants being formed per mg of S-9 protein with 1 mg of NBOP and 39 ± 8 revertants formed when 10 mg of carcinogen were used. The capacity of the hamster pancreatic S-9 to convert both carcinogens to mutagens was increased significantly following pretreatment of the hamster with a single dose of β-naphthoflavone (140 ± 12/10 mg DMNM), or 2,3,7,8-tetrachlorodibenzo-p-dioxin (182 ± 11/10 mg DMNM). A concomitant increase in the aryl hydrocarbon hydroxylase activity of pancreas S-9 isolated from hamsters treated with β-naphthoflavone (4 times control) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (15 times control) was observed, suggesting a relationship between the levels of aryl hydrocarbon hydroxylase activity and the metabolic activation of DMNM and NBOP in the pancreas. In contrast to 2,3,7,8-tetrachloro-dibenzo-p-dioxin and β-naphthoflavone, 3-methyl-cholan-threne induction did not result in an increased capacity of the S-9 to activate either DMNM or NBOP to mutagenic metabolites. This suggests that a different form of the enzyme may be induced by 3-methylcholanthrene and that it metabolizes DMNM and NBOP along predominantly detoxification pathways leading largely to noncarcinogenic metabolites. The absolute requirement by these pancreatic enzymes for reduced nicotinamide adenine dinucleotide phosphate, and their inhibition by α-naphthoflavone and 2,5-diphenyloxazole are strong points of evidence that they are mixed-function oxidases.