Functional and immunochemical comparison of hepatic UDP-glucuronosyltransferases in a piscine and a mammalian species

Characterization of the zebrafish Ugt repertoire reveals a new class of drug-metabolizing UDP glucuronosyltransferases

The zebrafish genome contains a gene superfamily of 40 Ugt genes that can be divided into Ugt1, Ugt2, and Ugt5 families. Because the encoded zebrafish UDP glucuronosyltransferase (UGT) proteins do not display orthologous relationships to any of the mammalian and avian UGT enzymes based on molecular phylogeny, it is difficult to predict their substrate specificity. Here, we mapped their tissue-specific expression patterns. We showed that the zebrafish UGT enzymes can be glycosylated. We determined their substrate specificity and catalytic activity toward diverse aglycone substrates. Specifically, we measured mRNA levels of each of the 40 zebrafish Ugt genes in 11 adult tissues and found that they are expressed in a tissue-specific manner. Moreover, functional analyses with the donor of UDP glucuronic acid (UDPGA) for each of the 40 zebrafish UGT proteins revealed their substrate specificity toward 10 important aglycones. In particular, UGT1A1, UGT1A7, and UGT1B1 displayed good glucuronidation activities toward most phenolic aglycones (4-methylumbelliferone, 4-nitrophenol, 1-naphthol, bisphenol A, and mycophenolic acid) and the two carboxylic acids (bilirubin and diclofenac). Importantly, some members of the UGT5, a novel UGT family identified recently, are capable of glucuronidating multiple aglycones with the donor cofactor of UDPGA. In particular, UGT5A5, UGT5B2, and UGT5E1 glucuronidate phenols and steroids with high specificity toward steroid hormones of estradiol and testosterone and estrogenic alkylphenols 4-tert-octylphenol. These results shed new insights into the mechanisms by which fish species defend themselves against vast numbers of xenobiotics via glucuronidation conjugations and may facilitate the establishment of zebrafish as a model vertebrate in toxicological, developmental, and pathologic studies.

Steroid catabolism in marine and freshwater fish

Steroids play important roles in regulating many physiological functions in marine and freshwater fish. Levels of active steroid in blood and tissues are determined by the balance between synthetic and catabolic processes. This review examines what is known about pathways of catabolism of steroids, primarily sex steroids, in marine and freshwater fish. Cytochrome P450 (P450) isoforms present in hepatic microsomes catalyze steroid hydroxylation to metabolites with lower or no activity at estrogen or androgen receptors. Important pathways of steroid catabolism to readily excreted metabolites are glucuronidation and sulfonation of hydroxyl groups. Estradiol, testosterone, DHEA and hydroxylated metabolites of these and other steroids readily form glucuronide and sulfate conjugates in those fish species where these pathways have been examined. Little is known, however, of the structure and function of the UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) enzymes involved in steroid conjugation in fish. Glucuronide and sulfate conjugates of steroids may be transported into and out of cells by organic anion transporter proteins and multi-drug resistance proteins, and there is growing evidence that these proteins play important roles in steroid conjugate transport and elimination. Induction or inhibition of any of these pathways by environmental chemicals can result in alteration of the natural balance of steroid hormones and could lead to disruption of the endocrine system. Recent studies in this area are presented, with particular focus on phase II (conjugative) pathways.