Metabolism of the Cis and Trans Isomers of N-Nitroso-2,6-dimethylmorpholine in Rats, Hamsters and Guinea Pigs

Abstract

The in vivo metabolism of [3,5-³H]N-nitroso-2,6-dimethylmorpholine (NDMM) was studied in female Fisher rats, Syrian golden hamsters, and guinea pigs by analysis of urinary metabolites. Animals were treated by gavage with 12 mg/kg body weight of NDMM composed of both isomers and 12 μCi/kg body weight of either of the separated radioactive isomers (cis or trans). Control animals received 12 mg, 12 μCi/kg body weight NDMM with both isomers labeled in their natural proportion. High pressure liquid chromatography (HPLC) analysis of urine collected 24 hr after administration of [³H]NDMM to control animals revealed 12 metabolites, which appeared similiar in all three species. One primary difference observed was the presence of a major urinary metabolite (C) in the hamster urine, which was absent or present in small quantities in rat and guinea pig urine. Also the guinea pig urine had relatively more radioactivity chromatographing in a major peak (B) than that of the hamster or rat. Earlier studies indicated that the trans isomer was a more potent carcinogen in the rat, while deuterium substituted for hydrogen in the alpha position greatly reduced the carcinogenicity of NDMM in the rat. The trans isomer produced increased amounts of the A metabolite in all three species. The A metabolite was identified as 2-(2-hydroxyl-lmethyl)ethoxy propanoic acid, a product of alpha oxidation of NDMM. Metabolite K was identified as 2,6-dimethylmorpholine-3-one, possibly derived from the first product of alpha oxidation. The identification of these metabolites strongly suggested that alpha oxidation is an important metabolic pathway leading to carcinogenesis in the rat. In contrast, hamsters and guinea pigs treated with the cis isomer, which was more carcinogenic in these species, excreted twice as much of the C or B metabolites respectively and also produced increased amounts of N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP) and N-nitroso-bis(2-hydroxypropyl)amine (BHP), products of beta oxidation, than those treated with the trans isomer. These results were again in agreement with the experiments in which deuterium substitution in the alpha carbon atoms (3 and 5) to the nitroso function increased carcinogenesis, but substitution in the beta position decreased carcinogenesis in the hamster. Possibly alpha oxidation is not as relevant a metabolic pathway in the hamster or guinea pig as B-oxidation in relation to the carcinogenesis of NDMM. The trans isomer of NDMM appeared to be preferentially metabolized by alpha oxidation, while the cis isomer may be preferentially metabolized by beta oxidation.