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

The food-borne carcinogenic and mutagenic heterocyclic aromaticamines undergo bioactivation to the corresponding N-hydroxy(OH)-arylamines and the subsequent N-glucuronidation of thesemetabolites is regarded as an important detoxification reaction.In this study, the rates of glucuronidation for the N-OH derivativesof 2-amino-3-methylimidazo4,5-f]-quinoline (IQ), 2-amino-l-methyl-6-phenylimidazo4,5-b]-pyridine (PhIP), 2-amino-6-methyi-dipyridol,2-a:3',2'-d]imidazole(Glu-P-1) and 2-amino-3,8-dimethylimidazo4,5-f]quinoxaline(MelQx) by liver microsomal glucuronosyltransferase were comparedto 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-aminobiphenyl(N-OH-DMABP). Human liver microsomes catalyzed the uridine 5'-diphosphoglucuronicacid (UDPGA)-dependent glucuronidation 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 glucuronidationof 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 (11.2nmol/min/mg); activity towards N-OH-MelQx was not detected.Two glucuronide(s) of N-OH-PhIP, designated I and II, were separatedby HPLC. Conjugate II was found to be chromatographically andspectrally identical with a previously reported major biliarymetabolite of PhlP in the rat, while conjugate I was identicalwith a major urinary metabolite of PhIP in the dog. Hepaticmicrosomes from rat, dog and human were found to catalyze theformation of both conjugates. The rat preferentially formedconjugate II (I to II ratio of 1:15), while the dog and humanformed higher relative amounts of conjugate I (I to II ratioof 2.5:1.0 and 1.3:1.0 respectively). Fast atom bombardmentmass spectrometry of conjugates I and II gave the correspondingmolecular ions and showed nearly identical primary spectra.However, collision-induced spectra were distinct and were consistentwith the identity of conjugates I and II as structural isomers.Moreover, the UV spectrum of conjugate I exhibited a