THE CAPABILITY OF RAT COLON TISSUE-SLICES TO METABOLIZE THE COOKED-FOOD CARCINOGEN 2-AMINO-1-METHYL-6-PHENYLIMIDAZO[4,5-B]PYRIDINE

A major target tissue for carcinogenesis from the cooked-food carcinog en 2-amino-1-methyl-6-phenglimidazo[4,5-b]pyridine (PhIP) in rodents i s the colon, yet the role of colon metabolism on the carcinogenicity o f PhIP is not clearly understood, The mutagenic potency of PhIP is hig hly dependent upon cytochrome P450 N-hydroxylation. In the present stu dy, the ability of rat colon tissue to activate PhIP to a mutagen was investigated in Salmonella typhimurium (strains TA98 and YG1024) and r at colon tissue slices, In the Ames/Salmonella assay, using rat colon S9 as the activating system, no mutations mere evident from bacteria e xposed to PhIP at any concentration tested, However, mutations were ob served when bacteria were exposed to 2-aminoanthracene (2AA) and colon S9, indicating sufficient P150 activity in the S9 to activate 2AA but not PhIP, In rat colon slice preparations, the sulfotransferase and a cetyltransferase inhibitors pentachlorophenol (PCP) and 2,6-dichloro-4 -nitrophenol (DCNP) were used to modulate DNA adduct and metabolite fo rmation, Incubations of 3-methylcholanthrene-induced colon slices dose d with 50 mu m [H-3]PhIP produced no detectable metabolites, However, incubations of uninduced slices exposed to 10 mu M of the reactive int ermediate, [H-3]2-(hydroxyamino)- 1-methyl-6-phenylimidazo[4,5-b]pyrid ine (N-hydroxy-PhIP), produced a single detectable metabolite, a glucu ronide conjugate of N-hydroxy-PhIP, This metabolite decreased when PCP or DCNP was added to the incubation medium, DNA adducts were detected in colon slices exposed to N-hydroxy-PhIP at approximately 33 adducts /10(7) nucleotides, Interestingly, when PCP was added to the incubatio n mixture, an increase in DNA adduct levels was detected, whereas DCNP produced a decrease in adducts, Because these inhibitors are thought to have similar mechanisms with regard to sulfotransferase inhibition, the inverse relationship in DNA adduct levels due to PCP or DCNP trea tment is at present unexplainable, The formation of DNA adducts and me tabolites from colon slices exposed to N-hydroxy-PhIP but not PhIP imp lies that there is insufficient P450 activity in the rat colon to acti vate PhIP to hydroxylated metabolites, suggesting that the rat colon i s a site of Phase II metabolism for PhIP and that the liver is the pri mary source for hydroxglation.