Mitochondrial formation of β-oxopropyl metabolites from bladder carcinogenic ω-carboxyalkylnitrosamines

Certain environmentally relevant nitrosamines specifically induce malignant tumors in the urinary bladder in several animal species. For this organotropic effect, formation of omega-carboxylated proximal metabolites has been found to be obligatory. The mechanism of action of these intermediates, however, is not yet clear. We investigated biotransformation of butyl-3-carboxypropylnitrosamine (CAS: 38252-74-3), methyl-3-carboxypropylnitrosamine (CAS: 61445-55-4) and methyl-5-carboxypentylnitrosamine by mitochondrial fractions from rat liver and renal cortex. On incubation with mitochondrial fractions, the respective beta-oxidized metabolites butyl-2-oxopropylnitrosamine (CAS: 51938-15-9) or methyl-2-oxopropylnitrosamine (CAS: 55984-51-51) were formed. This biotransformation was ATP dependent, associated with the presence of mitochondrial marker enzyme (cytochrome c oxidase) in 7000 x g subfractions and was inhibited by octanoic acid. Highest metabolic rates were observed with rat liver fractions. These results demonstrate that omega-carboxylated nitrosamines are substrates for mitochondrial enzymes of fatty acid degradation, most probably following the degradation pathway of medium-chain fatty acids. By this reaction, water-soluble carboxylated nitrosamines of low genotoxic potential are converted into rather lipophilic 2-oxopropyl metabolites with high genotoxic and carcinogenic potency. In contrast to carboxylated metabolites, 2-oxopropyl derivatives are good substrates for cytochrome P-450 dependent monooxygenases. Therefore, mitochondrial beta-oxidation appears to be an important step in metabolic activation of nitrosamines tumorigenic in the urinary bladder.