Glucuronidation of N-hydroxy heterocyclic amines by human and rat liver microsomes

Abstract

The food-borne carcinogenic and mutagenic heterocyclic aromatic amines undergo bioactivation to the corresponding N-hydroxy (OH)-arylamines and the subsequent N-glucuronidation of these metabolites is regarded as an important detoxification reaction. In this study, the rates of glucuronidation for the N-OH derivatives of 2-amino-3-methylimidazo[4,5-f]-quinoline (IQ), 2-amino-1-methyl-6-phenylimidazo[4,5-b]-pyridine (PhIP), 2-amino-6-methyl-dipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) and 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) by liver microsomal glucuronosyltransferase were compared to that of the proximate human urinary bladder carcinogen, N-OH-aminobiphenyl (N-OH-ABP) and the proximate rat colon carcinogen N-OH-3,2'-dimethyl-4-amino-biphenyl (N-OH-DMABP). Human liver microsomes catalyzed the uridine 5'-diphosphoglucuronic acid (UDPGA)-dependent glucuroidation of N-OH-IQ, N-OH-PhIP, N-OH-Glu-P-1 and N-OH-MeIQx at rates of 59%, 42%, 35% and 27%, respectively, of that measured for N-OH-ABP (11.5 nmol/min/mg). Rat liver microsomes also catalyzed the UDPGA-dependent glucuronidation of N-OH-PhIP, N-OH-Glu-P-1 and N-OH-IQ at rates of 30%, 20% and 10%, respectively of that measured for N-OH-DMABP (11.2 nmol/min/mg); activity towards N-OH-MeIQx was not detected. Two glucuronide(s) of N-OH-PhIP, designated I and II, were separated by HPLC. Conjugate II was found to be chromatographically and spectrally identical with a previously reported major biliary metabolite of PhIP in the rat, while conjugate I was identical with a major urinary metabolite of PhIP in the dog. Hepatic microsomes from rat, dog and human were found to catalyze the formation of both conjugates. The rat preferentially formed conjugate II (I to II ratio of 1:15), while the dog and human formed higher relative amounts of conjugate I (I to II ratio of 2.5:1.0 and 1.3:1.0 respectively). Fast atom bombardment mass spectrometry of conjugates I and II gave the corresponding molecular ions and showed nearly identical primary spectra. However, collision-induced spectra were distinct and were consistent with the identity of conjugates I and II as structural isomers. Moreover, the UV spectrum of conjugate I exhibited a lambda max at 317 nm and was essentially identical to that of N-OH-PhIP, while conjugate II was markedly different with a lambda max of 331 nm. Both conjugates were stable in 0.1 N HCl and were resistant to hydrolysis by rat, dog and human liver microsomal beta-glucuronidases.(ABSTRACT TRUNCATED AT 400 WORDS)

UGT1A1 polymorphisms are important determinants of dietary carcinogen detoxification in the liver†‡

“…37,38 Glucuronidation by UGT enzymes is one of the few phase II drug metabolism pathways that significantly contribute to the detoxification of HCAs in liver and extrahepatic tissues. 8,10,11,19,39,40 Large interindividual differences in the rates of PhIP N-oxidation producing the carcinogenic intermediate N-OH-PhIP have previously been observed. 30,41 However, we report here for the first time that its hepatic glucuronidation is also highly variable.…”

Host-microbial interactions in the metabolism of therapeutic and diet-derived xenobiotics

“…Enterohepatic cycling may also exacerbate harm from HCAs that are detoxified through hepatic glucuronidation, including abundant diet-derived compounds such as PhIP, IQ, and MeIQx (56). Studies of the carcinogenic and mutagenic compound IQ have repeatedly observed more DNA adducts and DNA damage in conventional mice versus their germ-free counterparts (57,58).…”

Joint Effects between UDP-Glucuronosyltransferase 1A7 Genotype and Dietary Carcinogen Exposure on Risk of Colon Cancer

“…[10][11][12]. Results from a human exposure study identified N 2 -OH-PhIP-N 2 -glucuronide as the predominant urinary metabolite (13), suggesting that a large proportion of ingested PhIP is converted into N-OH-PhIP and subsequently conjugated with glucuronic acid by UGTs (14,15). In contrast, the extent of the in vivo role of the glucuronidation pathway for other HCAs, such as 2-amino-3,8-dimethylimidazo [4,5-f]quinoxaline (MeIQx) and 2-amino-3,4,8-trimethylimidazo [4,5-f]quinoxaline (DiMeIQx) remains to be shown but most likely involved UGT1A enzyme family (16).…”