Hydroxy Metabolites of Methyl-n-amylnitrosamine Produced by Esophagus, Stomach, Liver, and Other Tissues of the Neonatal to Adult Rat and Hamster

Abstract

We measured the ability of neonatal to adult MRC-Wistar rat and Syrian hamster tissues to convert the esophageal carcinogen methyl-n-amylnitrosamine (MNAN) into the stable metabolites 2- to 5-hydroxy-MNAN and 3- and 4-oxo-MNAN. Slices or pieces of freshly removed tissues were incubated for 3 h with 23 µm MNAN and dichloromethane extracts were analyzed by gas chromatography-thermal energy analysis. The sum of the metabolites was expressed as percent metabolism of MNAN/100 mg tissue (“percent metabolism”). Tissues of animals from 1 day before birth to 56–70 days of age were examined. Metabolites in rat esophagus reached 12.6% at 6 days of age, three times the adult level, and that in hamster esophagus reached 13.1% at birth, 22 times the adult level. Forestomach metabolism was 1.9% in 3-day rats and 5.7% in 3-day hamsters, though the adult levels were <0.5%. Metabolism in rat, but not hamster, liver showed a peak at 9 days that was 3.6 times the adult level. Hamster, but not rat, skin showed about 1% metabolism. Total metabolism by glandular stomach, lung, and trachea of both species also showed changes with age. Ratios between 2-, 3-, 4-, and 5-hydroxy-MNAN were of three types: considerable 2-, 3-, and 4-hydroxy-MNAN, typical of esophagus; mainly 4-hydroxy-MNAN, typical of liver; and mainly 5- with some 4-hydroxy-MNAN, typical of rat lung. Incubation of adult rat liver and esophagus with varied MNAN concentrations showed apparent K_m values of 150 (esophagus) and 300 (liver) µm. Metabolite yields after young and adult rat esophagus and liver were incubated with 23 µm MNAN for 1, 2, or 3 h indicated that differing in vitro stability of enzyme activities did not explain the age differences. The 2.9- to 3.6-fold differences in total metabolite yield between young and adult rat esophagus and liver, observed when these tissues were incubated with 23 µm MNAN, was in contrast to the 1.3- to 1.6-fold difference when these tissues were incubated with 300 or 600 µm MNAN, suggesting that much of the observed age difference was specific to low MNAN concentrations. MNAN hydroxylation could be used to indicate tissue susceptibility to MNAN carcinogenesis and the presence of enzymes (probably cytochrome P-450 isozymes) that catalyze each of the three types of MNAN metabolism.