N-Glucuronidation of 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP) and N-hydroxy-PhIP by specific human UDP-glucuronosyltransferases

Abstract

Glucuronidation is a major metabolic pathway in the biotransformation of many xenobiotics. Recent studies have shown that in humans, UDP-glucuronosyltransferase (UGT)-mediated glucuronidation plays a critical role in the detoxification of food-borne carcinogenic heterocyclic amines. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), the most abundant carcinogenic heterocyclic amine found in well-cooked meats, has been shown to be extensively glucuronidated in humans. To determine which UGT isozymes are involved in the biotransformation of PhIP and the cytochrome P4501A2-mediated reactive intermediate N-hydroxy-PhIP, microsomes expressing human UGT1A1, -1A4, -1A6 or -1A9 were incubated with PhIP and N-hydroxy-PhIP and the reaction products analyzed by HPLC and ESI-MS. Incubations containing N-hydroxy-PhIP and UGT1A1 expressing microsomes, with an apparent Km of 4.58 microM and a Vmax of 4.18 pmol/min/mg protein, had the highest capacity to convert N-hydroxy-PhIP to N-hydroxy-PhIP-N2-glucuronide. Microsomes expressing UGT1A9 produced N-hydroxy-PhIP-N3-glucuronide at the highest rate with an apparent Km and Vmax of 3.73 microM and 4.07 pmol/min/mg, respectively. A third previously undefined glucuronide accounted for 31% of the total glucuronides formed from the UGT1A4 expressing microsomes. No glucuronide conjugates were detected from microsomes expressing UGT1A6. Incubations containing PhIP as substrate formed direct PhIP-glucuronides in microsomes expressing UGT1A1, UGT1A4 and UGT1A9 but at levels averaging 53-fold lower than when N-hydroxy-PhIP was used as the substrate. Knowing the glucuronidation capacity of the specific UGT isozymes involved in PhIP and N-hydroxy-PhIP glucuronidation should help in determining the individual susceptibility to the potential cancer risk from exposure to PhIP.

Control of steroid, heme, and carcinogen metabolism by nuclear pregnane X receptor and constitutive androstane receptor

“…Moreover, down-regulation of UGT1 mRNA is observed in the early stages of cancer but not in benign tumorogenesis (25). Environmental mutagens, such as PhIP and benzo(a)pyrenes, have been identified as substrates for several UGT1 proteins (15,20,26). Indeed, PhIP is glucuronidated at a higher rate in VP-hPXR mice (Fig.…”

Uridine diphosphoglucuronosyltransferase pharmacogenetics and cancer

“…These substrates include the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), the bladder cancer -causing agent benzidine, compounds present in tobacco including nicotine, cotinine and NNAL, and the anticancer drug tamoxifen (Malfatti and Felton, 2001;Zenser et al, 2002;Kuehl and Murphy, 2003;Kaku et al, 2004;Wiener et al, 2004a). A recent study examined UGT1A4 genotype-phenotype correlations between UGT1A4 polymorphisms and NNAL N-glucuronidation (Wiener et al, 2004b).…”

Expression of UDP-glucuronosyltransferase 1A, nuclear factor erythroid-E2-related factor 2 and Kelch-like ECH-associated protein 1 in colonic mucosa, adenoma and adenocarcinoma tissue

“…It has been reported that heterocyclic amines can cause DNA damage in intestinal epithelial cells in patients with colon cancer (1). Uridine diphosphate glucuronyl transferase (UGT), one of the most important phase II metabolic enzymes for biotransformation, catalyzes the binding of N-hydroxy compounds and glucuronic acid, which in turn prevents the DNA mutagenesis caused by heterocyclic amines (2). The UGT family, mainly composed of the UTG1A and UTG2B subfamilies, is expressed in the liver, gastrointestinal tract, gall bladder and breast (3)(4)(5).…”