Metabolism of the pancreatic carcinogen n-nitroso-2,6-dimethylmorpholine by hamster liver and component cells of pancreas

N-Nitroso-2,6-dimethylmorpholine (NNDM) is an indirect nitrosamine carcinogen which induces ductal adenocarcinoma of the pancreas in the Syrian golden hamster. NNDM is oxidized by normal male hamster pancreas postmitochondrial supernatants (S-9) in the presence of reduced nicotinamide adenine dinucleotide phosphate and oxygen to give three major identifiable metabolites as measured by high-pressure liquid chromatography. The major products are N-nitrosobis(2-hydroxypropyl)amine N-nitroso(2-hydroxypropyl) (2-oxopropyl)amine, and N-nitrosobis(2-oxopropyl)amine. Two other major metabolites have been detected, one which elutes near the solvent front and is referred to as X₁ and one which is more hydrophobic than N-nitrosobis(2-oxopropyl)amine and is called X₂. The rates of formation of N-nitrosobis(2-hydroxypropyl)amine N-nitroso(2-hydroxypropyl) (2-oxopropyl)amine, N-nitrosobis- (2-oxopropyl)amine X₁ and X₂ were linear with protein up to a final concentration of 15 mg of pancreatic S-9 per ml. The in vitro metabolism of NNDM by liver and pancreas S-9 and microsomes was compared. Although the liver subcellular fractions only produced two major metabolites, N-nitroso(2-hydroxypropyl) (2-oxopropyl)amine and X₁ under initial rate conditions the liver enzymes were greater than 40 times more active than the pancreatic enzymes with respect to the overall metabolism of NNDM. The enzyme(s) for the initial oxidative metabolism of NNDM are associated with the microsomes in the liver and the pancreas. However, the metabolite profiles obtained with pancreatic S-9 preparations are different from those observed with pancreatic microsomes, suggesting the involvement of cytosolic proteins in modulating the pancreatic metabolism of NNDM. Acinar and islet cell S-9 fractions metabolize NNDM at similar rates and exhibit a 2-fold increase in NNDM metabolism over whole pancreas S-9. However, the metabolite profiles for the two cell types are very different. Pretreatment of hamsters with 2,3,7,8-tetrachlorodibenzo-p-dioxin results in a marked increase in the rate of NNDM metabolism by both acinar and islet cells. However, the two cell types exhibit different patterns of induction as measured by changes in the metabolite profiles. Pretreatment of hamsters with 2,3,7,8-tetrachlorodibenzo-p-dioxin results in a greater than 20-fold induction of aryl hydrocarbon hydroxylase activity in acinar cell S-9, whereas there is no induction of aryl hydrocarbon hydroxylase activity in the islet cell S-9. These results suggest the presence of different forms of the microsomal.