ABSTRACT
Polycyclic aromatic hydrocarbon (PAH)-type compounds induce at least two rat UDP-glucuronosyltransferase isoforms, UGT1A6 and UGT1A7. Among the glucuronidation reactions of PAH metabolites studied, mono- and diglucuronide formation of benzo[a]pyrene and chrysene-3,6-diphenol showed the highest induction factors in rat liver microsomes. Availability of AHH-1 cells stably expressing UGT1A7 allowed us to study whether this PAH-inducible isoform could catalyze benzo[a]pyrene and chrysene-3,6-diphenol glucuronidation. It was found that UGT1A7 indeed catalyzed mono- and diglucuronide formation of both benzo[a]pyrene and chrysene 3,6-diphenols. V79 cell-expressed rat UGT1A6 also catalyzed these reactions, except for chrysene diphenol diglucronide formation (Bock et al., Mol Pharmacol 42: 613-618, 1992). Enzyme kinetic studies of the glucuronidation of 6-hydroxychrysene (used as a stable PAH phenol) indicated that UGT1A7 conjugated this compound with a lower apparent Km value (0.1 microM) than UGT1A6 (10 microM). The results suggest that the two PAH-inducible UGTs may cooperate in conjugating PAH metabolites, but that UGT1A7 is more efficient.
Subject(s)
Benzo(a)pyrene/metabolism , Chrysenes/metabolism , Glucuronates/metabolism , Glucuronosyltransferase/metabolism , Cell Line , Glucuronates/analysis , Glucuronosyltransferase/biosynthesis , Hymecromone/metabolism , Kinetics , Phenols/metabolism , Substrate Specificity , TransfectionABSTRACT
Transcriptional regulation and function of rat and human PAH-inducible UDP-glucuronosyltransferase (UGT) isoforms have been studied. 1. At least two PAH-inducible UGT isoforms are expressed in a variety of tissues, the rat isoforms UGT1A6 and UGT1A7, and the human isoforms UGT1A6 and UGT1A9. 2. For the rat and human UGT1A6 isoforms two modes of tissue- and cell-specific regulation were found, PAH-inducible and constitutive expression. 3. Transient transfection studies, using human UGT1A6/CAT fusion constructs and colon carcinoma Caco-2 cells, revealed that PAH induction of human UGT1A6 is mediated by the Ah receptor. 4. Cell-expressed UGT isoforms were used to study their function in PAH metabolism. Rat UGT1A7 and human UGT1A9 appear to be more efficient than the corresponding UGT1A6 isoforms in catalyzing glucuronide formation of PAH phenols and diphenols. Several isoforms may act together in the formation of benzo(a)pyrene-3.6-diol diglucuronide, the major glucuronide found in rat bile. The results suggest complex modes of transcriptional regulation of PAH-inducible UGTs. They also suggest a major role of these UGT isoforms in detoxication of PAHs.
