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.