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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.

The original novel UGT1 complex locus previously shown to encode six different UDP-glucuronosyltransferase (transferase) genes has been extended and demonstrated to specify a total of 13 isoforms. The genes are designated UGT1A1 through UGT1A13p with four pseudo ones. UGT1A2p and UGT1A11p through UGT1A13p have either nucleotide deletions or flawed TATA boxes and are therefore pseudo. In the 5' region of the locus, the 13 unique exons 1 are arranged in a tandem array with each having its own proximal TATA box element and, in turn, are linked to four common exons to allow for the independent transcriptional initiation to generate overlapping primary transcripts. Only the lead exon in the nine viable primary transcripts is predicted to undergo splicing to the four common exons generating mRNAs with identical 3' ends and transferase isozymes with an identical carboxyl terminus. The unique amino terminus specifies acceptor-substrate selection, and the common carboxyl terminus apparently specifies the interaction with the common donor substrate, UDP-glucuronic acid. In the extended region, the viable TATA boxes are either A(A)TgA(AA)T or AT14AT; in the original locus the element for UGT1A1 is A(TA)7A and TAATT/CAA(A) for all of the other genes. UGT1A1 specifies the critically important bilirubin transferase isoform. The relationships of the exons 1 to each other are as follows: UGT1A2p through UGT1A5 comprises a cluster A that is 87-92% identical, and UGT1A7 through UGT1A13p comprises a cluster B that is 67-91% identical. For the two not included in a cluster, UGT1A1 is more identical to cluster A at 60-63%, whereas UGT1A6 is identical by between 48% and 56% to all other unique exons. The locus was expanded from 95 kb to 218 kb. Extensive probing of clones beyond 218 kb with coding nucleotides for a highly conserved amino acid sequence present in all transferases was unable to detect other exons 1. The mRNAs are differentially expressed in hepatic and extrahepatic tissues. This locus is indeed novel, indicating the least usage of exon sequences in specifying different transferase isozymes that have an expansive substrate range.

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UDP-glucuronosyltransferase (UGT) isozymes detoxify metabolites, drugs, toxins, and environmental chemicals via conjugation to glucuronic acid. Based on the extended UGT1 locus combined with Northern blot analysis and in situ hybridization, we determined the distribution of UGT1A1 and UGT1A7 through UGT1A10 mRNAs and found them for the first time segmentally distributed in the mucosal epithelia layer of the gastrointestinal tract. Biochemically, recombinant isozymes exhibited pH optima of 5.5, 6.4, 7.6, 8.5, and/or a broad pH range, and activities were found to be unaffected or progressively inhibited by increasing substrate concentrations after attaining V max for certain chemicals. Under different optimal conditions, all exhibited wide substrate selections for dietary and environmentally associated chemicals. Evidence also suggests tandem effects of isozymes in the time for completion of reactions when comparing short-and long-term incubations. Moreover, treatment of colon cells with certain dietassociated constituents, curcumin and nordihydroguaiaretic acid, reversibly targets UGTs causing inhibition without affecting protein levels; there is no direct inhibition of control UGT using curcumin as substrate in the in vitro assay. In summary, we demonstrate that UGTs are located in gastrointestinal mucosa, have vast overlapping activities under differential optimal conditions, and exhibit marked sensitivity to certain dietary substrates/constituents, representing a first comprehensive study of critical properties concerning glucuronidating isozymes in alimentary tissues. Additionally, the highly dynamic, complex, and variable properties necessarily impact absorption of ingested chemicals and therapeutic drugs.The gastrointestinal (GI) 1 system has the critical function of absorbing nutrients from the diet and, simultaneously, acting as a barrier to ingested toxins and unwanted chemicals. To this end, the human UGT1 complex locus, which encodes multiple UDP-glucuronosyltransferase (UGT) isozymes distributed in the GI tract, participates in the broad and critical function of detoxifying lipid-soluble phenols derived metabolically or ingested as part of the diet, its contaminants, and as medications. UGT conjugates glucuronic acid to the acceptor substrate, converting the lipophile to an inactive glucuronide that undergoes facilitated excretion. The isozymes metabolize endogenous substrates such as bilirubin, steroids, bile acids, retinoic acid, and thyroid hormones. Equally important, isozymes also metabolize four categories of dietary phytochemicals that could pose risks for humans and animals when absorbed in excess. Highly abundant dietary flavonoids found in plants are shown to inhibit aromatase (1), and less abundant anthraquinones and two types of phytoestrogens (2-4) can also cause cellular damage. Particular anthraquinones were mutagenic when bioactivated (3) and were carcinogens in long-term feeding experiments in mice (5). Injurious effects of diets were demonstrated by extensive lesions in the reprodu...

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