Enzymatic phase II activation of the N-hydroxylamines of IQ, MelQx and PhIP by various organs of monkeys and rats

2-Amino-3-methylimidazo4,5-f]quinoline (IQ), 2-amino-3,8-dimethylimidazo4,5-f]quinoxaline(MelQx) and 2-amino-1-methy1-6-phenylimidazo4,5-b]pyridine(PhIP) are mutagenic and carcinogenic heterocyclic amines producedduring the ordinary cooking of meat. These compounds undergometabolic activation via both cytochrome P450-mediated N-oxidationand phase II esterification in order to exert their genotoxicity.In the current study, we examined the in vitro phase II activationof N-hydroxy-IQ, N-hydroxy-PhIP and N-hydroxy-MelQx by cytosolicacetyltransferase, sulfotransferase, aminoacyl-tRNA synthetaseand phosphatase from a number of tissues including liver, kidney,colon and heart. These tissues were chosen for study becauseeach is either a target organ for carcinogenicity or has displayedhigh levels of DNA adducts in in vivo studies with the heterocyclicamines. Cytosol from various tissues of both monkeys and ratswas incubated with and without the respective cofactors, andcarcinogen binding to calf thymus DNA was measured by ³²P-postlabelinganalysis. Our results show that all four phase II enzymes mayparticipate in the activation of the N-hydroxylamines. However,the degree of activation depends on the substrate, tissue andanimal species. For example, in both monkeys and rats, the highestacetyl CoA-enhanced binding was observed with N-hydroxy-IQ andthe lowest acetyl CoA-enhanced binding was observed with N-hydroxy-MelQx.In contrast, no significant adenosine 3'-phosphate 5'-phosphosulfate-dependentactivation of N-hydroxy-IQ was observed with monkey cytosolfrom liver, kidney, heart or colon but the sulfotransferase-mediatedactivation of N-hydroxy-PhIP was at least 10 times higher inall four tissues of monkeys than in rats. Prolylation appearsimportant in the activation of all three N-hydroxylamines byrat liver and heart cytosol, whereas in monkeys, prolylationappears important in kidney cytosol. The differences observedin the phase II activation of heterocyclic amines may have implicationsfor DNA adduct formation, toxicity and carcinogenicity.