In vertebrates, the glucuronidation of small lipophilic agents is catalyzed by the endoplasmic reticulum UDP-glucuronosyltransferases (UGTs). This metabolic pathway leads to the formation of water-soluble metabolites originating from normal dietary processes, cellular catabolism, or exposure to drugs and xenobiotics. This classic detoxification process, which led to the discovery nearly 50 years ago of the cosubstrate UDP-glucuronic acid (19), is now known to be carried out by 15 human UGTs. Characterization of the individual gene products using cDNA expression experiments has led to the identification of over 350 individual compounds that serve as substrates for this superfamily of proteins. This data, coupled with the introduction of sophisticated RNA detection techniques designed to elucidate patterns of gene expression of the UGT superfamily in human liver and extrahepatic tissues of the gastrointestinal tract, has aided in understanding the contribution of glucuronidation toward epithelial first-pass metabolism. In addition, characterization of the UGT1A locus and genetic studies directed at understanding the role of bilirubin glucuronidation and the biochemical basis of the clinical symptoms found in unconjugated hyperbilirubinemia have uncovered the structural gene polymorphisms associated with Crigler-Najjar's and Gilbert's syndrome. The role of the UGTs in metabolism and different disease states in humans is the topic of this review.

Human UDP-Glucuronosyltransferases: Metabolism, Expression, and Disease

UDP-Glucuronosyltransferases (UGTs) are glycoproteins localized in the endoplasmic reticulum (ER) which catalyze the conjugation of a broad variety of lipophilic aglycon substrates with glucuronic acid using UDP-glucuronic acid (UDP-GIcUA) as the sugar donor. Glucuronidation is a major factor in the elimination of lipophilic compounds from the body. In this review, current information on the substrate specificities of UGT1A and 2B family isoforms is discussed. Recent findings with regard to UGT structure and topology are presented, including a dynamic topological model of UGTs in the ER. Evidence from experiments on UGT interactions with inhibitors directed at specific amino acids, photoaffinity labeling, and analysis of amino acid alignments suggest that UDP-GIcUA interacts with residues in both the N- and C-terminal domains, whereas aglycon binding sites are localized in the N-terminal domain. The amino acids identified so far as crucial for substrate binding and catalysis are arginine, lysine, histidine, proline, and residues containing carboxylic acid. Site-directed mutagenesis experiments are critical for unambiguous identification of the active-site architecture.

Structural and functional studies of udp-glucuronosyltransferases*

Although the liver plays the major role in drug metabolism [e.g. by oxidative cytochrome P450 (CYP)-dependent phase I and conjugation or phase II reactions], drug metabolising enzymes are also present at other sites. Depending on the particular drug and enzymes involved, these extrahepatic organs and/or tissues can contribute to the elimination of drugs and, thus, should be considered in any discussion of drug disposition. By the use of relatively new techniques in molecular biology, e.g. immunoblotting with antibodies directed to various CYP isoenzymes, the tissue and organ distribution of these drug metabolising enzymes can be determined. In addition, microsomal and cytosolic enzyme activity and capacity can be directly assessed in vitro by incubation of the enzymes with the drugs of interest. Both approaches have demonstrated the presence of 3 CYP families at different extrahepatic sites, such as the mucosa of the gastrointestinal tract, kidney, lung, brain or skin. Enzymes including epoxide hydrolases, hydrolysing enzymes, glutathione S-transferases, UDP-glucuronosyltransferases, sulphotransferases, N-acetyltransferases, and methyltransferases are discussed. Indirect evidence of extrahepatic drug metabolism can be generated from pharmacokinetic studies whenever total body clearance exceeds liver blood flow, or when severe liver dysfunction or anhepatic conditions do not affect metabolic clearance. Indeed, extrahepatic metabolism has been demonstrated for numerous drugs. Therefore, the metabolic profile and sites of enzymatic reactions for each drug should be determined.

Extrahepatic Metabolism of Drugs in Humans

Bilirubin UDP-glucuronosyltransferase 1 is the only relevant bilirubin glucuronidating isoform in man.

The primary function of the small intestine is to absorb nutrients and water. This is achieved by mixing food with digestive enzymes to increase the contact of foodstuffs with the absorptive cells of the mucosa. In humans, the small intestine is about 5 to 6 m in length and comprises approximately 1

Is the Role of the Small Intestine in First-Pass Metabolism Overemphasized?

https://www.gastrojournal.org/article/0016-5085(87)90329-5/pdf

Characterization and tissue specificity of a monoclonal antibody against human uridine 5′-diphosphate-glucuronosyltransferase

Immunocharacterization of UDP-glucuronyltransferase isoenzymes in human liver, intestine and kidney.

Influence of phenobarbital treatment on p-nitrophenol and bilirubin glucuronidation in Wistar rat, Gunn rat and cat.

beta-Glucuronidase-resistant bilirubin glucuronide isomers in cholestatic liver disease--determination of bilirubin metabolites in serum by means of high-pressure liquid chromatography.

Peter L.M. Jansen

Papers

Characterization and tissue specificity of a monoclonal antibody against human uridine 5′-diphosphate-glucuronosyltransferase

Immunocharacterization of UDP-glucuronyltransferase isoenzymes in human liver, intestine and kidney.

Influence of phenobarbital treatment on p-nitrophenol and bilirubin glucuronidation in Wistar rat, Gunn rat and cat.

beta-Glucuronidase-resistant bilirubin glucuronide isomers in cholestatic liver disease--determination of bilirubin metabolites in serum by means of high-pressure liquid chromatography.

The isomerisation of bilirubin monoglucuronide.