Metabolism of the food-derived carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by lactating Fischer 344 rats and their nursing pups.

Abstract

An important class of dietary mutagens and carcinogens are the heterocyclic arylamine compounds that have been identified in a variety of cooked, protein-containing foods. Among these heterocyclic amines, 2-amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP) is potentially the most important carcinogen for human cancer risk. We have recently observed that PhIP-derived radioactivity is excreted into the breast milk of lactating rats administered [3H]PhIP. To better assess the significance of breast milk as a route of exposure of the newborn to dietary heterocyclic amines, we examined the metabolites of PhIP in breast milk and in urine of nursing pups. Lactating Fischer 344 rats with 5-day-old pups were given a single oral dose of 10 mg/kg of [3H]PhIP. We collected milk from the dams and urine from the pups and then analyzed the samples for metabolites of PhIP, using high-pressure liquid chromatography (HPLC). PhIP-DNA adduct levels in the tissues of the pups were determined by 32P-postlabeling analysis. Three radioactive peaks were observed by HPLC separation of milk samples: an unidentified early eluting peak, 4'-hydroxy-PhIP, and PhIP. Four metabolites and the parent compound were found in urine of the pups nursed by dams given radiolabeled PhIP: PhIP-4'-O-glucuronide, PhIP-4'-sulfate, 4'-hydroxy-PhIP, and N2-hydroxy-PhIP-N3-glucuronide. 4'-Hydroxy-PhIP and its conjugates contributed approximately 60% of the radioactivity found in the urine. By 32P-postlabeling analysis, PhIP-DNA adducts were detected in spleen, lung, heart, kidney, liver, and stomach of pups at mean levels ranging from 0.06 to 0.55 adducts/10(7) nucleotides. The large percentage of 4-hydroxy-PhIP and its conjugates in the urine indicates that 5-day-old pups detoxify PhIP and further metabolize 4'-hydroxy-PhIP obtained from the breast milk. The presence of the glucuronide conjugate of N-hydroxy-PhIP in the urine of pups and the lack of detectable conjugate or N-hydroxylamine itself in breast milk suggest that PhIP from breast milk undergoes metabolic activation via N-hydroxylation in 5-day-old rat pups. This conclusion was further supported by the observation that hepatic S9 fractions from the pups activated PhIP to a mutagen in the Ames Salmonella mutagenicity assay and by the presence of PhIP-DNA adducts in the tissues of the pups. The findings reported here may have carcinogenic and toxicologic implications for the offspring of women who breast-feed and consume a diet rich in cooked meat.