Urinary Excretion of Unmetabolized and Phase II Conjugates of 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline in Humans: Relationship to Cytochrome P4501A2 and N-Acetyltransferase Activity

Abstract

Cooking meat, fish, or poultry at high temperature gives rise to heterocyclic aromatic amines (HAAs), which may be metabolically activated to mutagenic or carcinogenic intermediates. The enzymes cytochrome P4501A2 (CYP1A2) and N-acetyltransferase (NAT2) are principally implicated in such biotransformations. We have determined the relationship between the activity of these two enzymes and the urinary excretion of unmetabolized and Phase II conjugates of the two HAAs MeIQx (2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline) and PhIP (2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) in individuals fed a uniform diet containing high-temperature cooked meat. The subjects in the study ate meat containing known amounts of MeIQx and PhIP, and urine collections were made 0–12 and 12–24 h after a meal. MeIQx and PhIP were measured in urine after acid treatment that quantitatively hydrolyzes the Phase II conjugates to the respective parent amine. The extracts containing the HAAs were purified by immunoaffinity chromatography and analyzed by liquid chromatography using electrospray ionization-tandem mass spectrometry. The MeIQx content in the 0–12 h urine increased after acid hydrolysis by a factor of 3–21-fold. After acid treatment, the total amount of MeIQx (unmetabolized plus the N²-glucuronide and sulfamate metabolites) excreted in the 0–12 h urine was 10.5 ± 3.5% (mean ± SD) of the dose, whereas the total amount of PhIP [unmetabolized plus acid-labile conjugate(s)] in the 0–12 h period was 4.3 ± 1.7% (mean ± SD) of the dose. The total amount of PhIP in the 12–24 h urine after acid treatment was 0.9 ± 0.4% (mean ± SD) of the dose. Linear regression analysis of the amounts of MeIQx and PhIP excreted in the 0–12 h period expressed as a percentage of the ingested dose, for all subjects, gave a low but significant correlation (r = 0.37, P = 0.005). Linear regression analyses showed that lower total MeIQx (unmetabolized plus the N²-glucuronide and sulfamate metabolites) in urine was associated with higher CYP1A2 activity, whereas total PhIP (unmetabolized plus conjugated) in urine showed no association to CYP1A2 activity. These results indicate that in humans, MeIQx metabolism and disposition are more strongly influenced by CYP1A2 activity than are those of PhIP. Linear regression analysis found no association between NAT2 activity and the levels (unmetabolized plus acid-labile conjugates) of MeIQx or PhIP excreted in urine.