N-oxidative metabolism of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in humans: Excretion of the N-2-glucuronide conjugate of 2-hydroxyamino-MeIQx in urine

2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), a major heterocyclic aromatic amine (HAA) formed in cooked meats, is metabolically transformed to mutagenic/carcinogenic Intermediates, Cytochrome P4501A2 (CYP1A2)-mediat ed N-hydroxylation followed by phase II O-esterification by N-acetyltransfe rase (NAT2) are generally regarded as activation processes in which MeIQx a nd other HAAs are converted to genotoxic species, In this study, we determi ned the relationship between the activities of these two enzymes and the ur inary excretion level of the N-2-glucuronide conjugate of 2-hydroxyamino-Me IQx-N-2-(beta-1-glucosiduronyl)-2-hydroxyamino-3,8- dimethylimidazo[4,5-f]q uinoxaline (N-OH-MeIQx-N-2-glucuronide)-among healthy subjects fed a unifor m diet containing high-temperature cooked meat, The individuals (n = 66) in the study ate meat containing known amounts of MeIQx, and urine was collec ted from 0 to 12 h after the meal. After addition of the deuterium-labeled internal standard to urine, N-OH-MeIQx-N-2-glucuronide was isolated using s olid-phase extraction and immunoaffinity separation, The isolated conjugate was converted to the deaminated product 2-hydroxy-3,8 dimethylimidazo[4,5- f]quinoxaline (2-OH-MeIQx) by heating with acetic acid, 2-OH-MeIQx and its deuterated analogue were derivatized to form the corresponding 3,5-bis(trif luoromethyl)benzyl ether derivatives and analyzed by capillary gas chromato graphy-negative ion chemical ionization mass spectrometry using selected io n monitoring procedures, The subjects in the study excreted an average of 9 .4 +/- 3.0% (+/-SD) of an ingested dose of MeIQx as N-OH-MeIQx-N-2-glucuron ide in urine; the range varied from 22 to 17.1%. A significant correlation was found between the level of N-OH-MeIQx-N-2-glucuronide in urine and the amount of MeIQx ingested (r(s) = 0.44; P = 0.0002), The excretion level of N-OH-MeIQx-N-2-glucuronide in urine was not associated with the enzyme acti vities of NAT2 or CYP1A2. This is expected with the latter enzyme because t he metabolism of MeIQx is first order and very rapid at the amounts ingeste d, The amount of N-OH-MeIQx-N-2-glucuronide in urine was not correlated wit h the age or sex of the individuals, Our results indicate that biotransform ation of MeIQx via CYP1A2 oxidation to form the N-hydroxylamine followed by N-2-glucuronidation Is a general pathway of MeIQx metabolism in humans; th e variability in the excreted levels of N-OH-MeIQx-N-2-glucuronide is proba bly due to interindividual differences in UDP-glucuronosyltransferase activ ity and/or excretion pathways.