Several novel UDP glycosyltransferase (UGT) genes, mainly UDP glucuronosyltransferases, have been identified in the human, mouse and rat genomes and in other mammalian species. This review provides an update of the UGT nomenclature to include these new genes and prevent the confusion that arises when the same gene is given different names. The new genes are named following previously established recommendations, taking into consideration evolutionary relatedness and the names already in general usage in the literature. The mammalian UGT gene superfamily currently has 117 members that can be divided into four families, UGT1, UGT2, UGT3 and UGT8. The 5-exon genes of the UGT1 family each contain a unique first exon, plus four exons that are shared between the genes; the exons 1 appear to have evolved by a process of duplication, leading to the synthesis of proteins with identical carboxyl-terminal and variable amino-terminal domains. Exon-sharing is also seen with the 6-exon UGT2A1 and UGT2A2 genes. However, UGT2A3 and those of the UGT2B (six exons), UGT3 (seven exons) and UGT8 gene families (five or six exons) do not share exons and most likely were derived by a process of duplication of all exons in the gene. Most UGT1 and UGT8 enzymes have been characterized in detail; however, the catalytic functions of the UGT3A enzymes and several UGT2 enzymes remain to be characterized.

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

The glucuronidation kinetics of the prototypic substrates 4-methylumbelliferone (4MU) and 1-naphthol (1NP) by human UDP-glucuronosyltransferases (UGT) 1A1, 1A3, 1A4, 1A6, 1A7, 1A8, 1A9, 1A10, 2B7, 2B15, and 2B17 were investigated. Where activity was demonstrated, inhibitory effects of diclofenac, probenecid, and the solvents acetone, acetonitrile, dimethyl sulfoxide, ethanol, and methanol were characterized. All isoforms except UGT1A4 glucuronidated 4MU, whereas all but UGT 1A4, 2B15, and 2B17 metabolized 1NP. However, kinetic models varied with substrate (for the same isoform) and from isoform to isoform (with the same substrate). Hyperbolic (Michaelis-Menten), substrate inhibition, and sigmoidal kinetics were variably observed for both 4MU and 1NP glucuronidation by the various UGTs. K_m or S₅₀ (sigmoidal kinetics) and V_max values varied 525- (8–4204 μM) and 5535-fold, respectively, for 4MU glucuronidation, and 1360- (1.3–1768 μM) and 560-fold, respectively, for 1NP glucuronidation. The use of a two-site model proved useful for those reactions exhibiting non-Michaelis-Menten glucuronidation kinetics. The organic solvents generally had a relatively minor effect on UGT isoform activity. UGT 1A6, 2B15, and 2B17 were most susceptible to the presence of solvent, although solvent-selective inhibition was occasionally observed with other isoforms. Diclofenac and probenecid inhibited all isoforms, precluding the use of these compounds for the reaction phenotyping of xenobiotic glucuronidation pathways in human tissues. Diclofenac and probenecid K_i values, determined for selected isoforms, ranged from 11 to 60 μM and 96 to 2790 μM, respectively. Overall, the results emphasize the need for the careful design and interpretation of kinetic and inhibition studies with human UGTs.

This article is an updated report of a symposium held at the June 2000 annual meeting of the American Society for Pharmacology and Experimental Therapeutics in Boston. The symposium was sponsored by the ASPET Divisions for Drug Metabolism and Molecular Pharmacology. The report covers research from the authors9 laboratories on the structure and regulation of UDP-glucuronosyltransferase (UGT) genes, glucuronidation of xenobiotics and endobiotics, the toxicological relevance of UGTs, the role of UGT polymorphisms in cancer susceptibility, and gene therapy for UGT deficiencies.

Relatively few selective substrate and inhibitor probes have been identified for human UDP-glucuronosyltransferases (UGTs). This work investigated the selectivity of trifluoperazine (TFP), as a substrate, and amitriptyline, androsterone, canrenoic acid, hecogenin, phenylbutazone, quinidine, quinine, and sulfinpyrazone, as inhibitors, for human UGTs. Selectivity was assessed using UGTs 1A1, 1A3, 1A4, 1A6, 1A7, 1A8, 1A9, 1A10, 2B7, and 2B15 expressed in HEK293 cells. TFP was confirmed as a highly selective substrate for UGT1A4. However, TFP bound extensively to both HEK293 lysate and human liver microsomes in a concentration-dependent manner (fuinc 0.20-0.59). When corrected for nonspecific binding, Km values for TFP glucuronidation were similar for both UGT1A4 (4.1 microM) and human liver microsomes (6.1+/-1.2 microM) as the enzyme sources. Of the compounds screened as inhibitors, hecogenin, alone, was selective; significant inhibition was observed only for UGT1A4 (IC50 1.5 microM). Using phenylbutazone and quinine as "models," inhibition kinetics were variously described by competitive and noncompetitive mechanisms. Inhibition of UGT2B7 by quinidine was also investigated further, because the effects of this compound on morphine pharmacokinetics (a known UGT2B7 substrate) have been ascribed to inhibition of P-glycoprotein. Quinidine inhibited human liver microsomal and recombinant UGT2B7, with respective Ki values of 335+/-128 microM and 186 microM. In conclusion, TFP and hecogenin represent selective substrate and inhibitor probes for UGT1A4, although the extensive nonselective binding of the former should be taken into account in kinetic studies. Amitriptyline, androsterone, canrenoic acid, hecogenin, phenylbutazone, quinidine, quinine, and sulfinpyrazone are nonselective UGT inhibitors.