Metabolism of 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by Human CYP1B1 Genetic Variants

• Published in: Drug metabolism and disposition Vol. 36; no. 4; pp. 745 - 752
• Main Authors: HAN, Jing-Fen, HE, Xiao-Yang, HERRINGTON, Jason S, WHITE, Lori A, ZHANG, Jun-Feng, HONG, Jun-Yan
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Pharmacology and Experimental Therapeutics 01.04.2008
• Subjects: Animals; Aryl Hydrocarbon Hydroxylases; Biological and medical sciences; Cell Line; Cytochrome P-450 CYP1B1; Cytochrome P-450 Enzyme System - genetics; Cytochrome P-450 Enzyme System - metabolism; Genetic Variation - genetics; Humans; Imidazoles - chemistry; Imidazoles - metabolism; Insecta; Medical sciences; Metabolic Networks and Pathways - physiology; Microsomes - metabolism; Pharmacology. Drug treatments; Rats; Species Specificity; Human; Cytochrome P450 1B1; Enzyme; Isozyme; Cytochrome P450; Genetic variant; Pharmacokinetics; Metabolism
• ISSN: 0090-9556; 1521-009X
• DOI: 10.1124/dmd.107.016824

Human cytochrome P450 1B1 (CYP1B1) plays a critical role in the metabolic activation of a variety of procarcinogens, including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The existence of human CYP1B1 missense genetic variants has been demonstrated, but their activities in metabolizing PhIP are unknown. In this study, we expressed 15 naturally occurring CYP1B1 variants (with either single or multiple amino acid substitutions) and determined their activity changes in metabolizing PhIP to its two major metabolites, 2-hydroxyamino-PhIP and 4′-hydroxy-PhIP. Although the PhIP-metabolizing activities of four variants (Ala 119 Ser, Pro 379 Leu, Ala 443 Gly, Arg 48 Gly/Leu 432 Val) were comparable with that of the expressed wild-type CYP1B1, five variants (Trp 57 Cys, Gly 61 Glu, Arg 48 Gly/Ala 119 Ser, Arg 48 Gly/Ala 119 Ser/Leu 432 Val, Arg 48 Gly/Ala 119 Ser/Leu 432 Val/Ala 443 Gly) exhibited more than 2-fold decrease in activity and a reduction in the catalytic efficiency ( V max / K m ) for both N - and 4-hydroxylation of PhIP. Six variants (Gly 365 Trp, Glu 387 Lys, Arg 390 His, Pro 437 Leu, Asn 453 Ser, Arg 469 Trp) showed little activity in PhIP metabolism, but the molecular mechanisms involved are apparently different. The microsomal CYP1B1 protein level was significantly decreased for the Trp 365 , Lys 387 , and His 390 variants and was not detectable for the Ser 453 variant. In contrast, there was no difference between the Trp 469 variant and the wild-type in the microsomal CYP1B1 protein level and P450 content but the Trp 469 variant totally lost its metabolic activity toward PhIP. The Leu 437 variant also had a substantial amount of CYP1B1 protein in the microsomes, but there was a lack of detectable P450 peak and activity. Our results should be useful in selecting appropriate CYP1B1 variants as cancer susceptibility biomarkers for human population studies related to PhIP exposure.