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*

At present, the methods and enzymology of the UDP-glucuronosyltransferases (UGTs) lag behind that of the cytochromes P450 (CYPs). About 15 human UGTs have been identified, and knowledge about their regulation, substrate selectivity, and tissue distribution has progressed recently. Alamethicin has been characterized as a treatment to remove the latency of microsomal glucuronidations. Most UGT isoforms appear to have a distinct hepatic and/or extrahepatic expression, resulting in significant expression in kidney, intestine, and steroid target tissues. The gastrointestinal tract possesses a complex expression pattern largely containing members of the UGT1A subfamily. Thus, these forms are poised to participate in the first pass metabolism of oral drugs. The authors and others have identified a significant expression of UGT1A1 in human small intestine, an enzyme possessing considerable allelic variability and a polymorphic expression pattern in intestine. Intestinal glucuronidation therefore plays a major role not only in first pass metabolism, but also in the degree of interindividual variation in overall oral bioavailability. Due to issues such as significant genetic variability and tissue localization in first-pass organs, clearance due to UGT1A1 should be minimized for new drugs.

The role of hepatic and extrahepatic udp-glucuronosyltransferases in human drug metabolism *,†

Structural aspects of the membrane of the endoplasmic reticulum.

### A. History

The kidneys have important physiological functions including maintenance of water and electrolyte balance, synthesis, metabolism and secretion of hormones, and excretion of the waste products from metabolism. In addition, the kidneys play a major role in the excretion of drugs,

Renal Drug Metabolism

Effects of phenobarbital and 3-methylcholanthrene on substrate specificity of rat liver microsomal UDP-glucuronyltransferase.

The rate of o-aminophenol glucuronide synthesis in the small intestine of the rat was found to decrease when going from the oral to the aboral end, where it was 15 per cent of the duodenal level. In the glandular stomach, caecum, and colon, the activity was 34, 15 and 30 per cent, respectively, of the duodenal value. The decrease in UDP glucuronyltransferase (p-nitrophenol) activity in the mucosal extracts was almost linear and amounted to 66 per cent from the oral to the aboral end of the small intestine. In the glandular stomach, caecum, and colon, the activities were 63, 32, and 83 per cent respectively, of the duodenal value. The β-glucuronidase activity increased linearly by 41 per cent when going from the oral to the aboral end of the small intestine. In the glandular stomach, caecum, and colon, activities of 82, 221, and 162 per cent respectively of the duodenal ones were observed.These results indicate that the distribution pattern of glucuronide synthesis, observed in tissue slices, and the UDP g...

Gastrointestinal distribution of glucuronide synthesis and the relevant enzymes in the rat.

1. Acetone (0.8 M) enhanced the p-hydroxylation of aniline about 3-4 fold when added to the incubation mixture. Only inhibition in hydroxylation of 3,4-benzpyrene or N-demethylation of ethylmorphine was observed.2. Phospholipase C treatment of rat liver microsomes decreased aniline p-hydroxylation about 20%. The enhancing action of acetone vanished after such a treatment.3. Acetone diminished the type II binding of aniline. In the presence of acetone, low concn. of aniline produced a type I difference spectrum with rat hepatic microsomes. With higher concn. of aniline a type II spectral change was obtained. This indicates that aniline, a well-known type II substrate, can also interact with the type I binding site of cytochrome P-450. Correlation between the type I spectral change and activation of aniline p-hydroxylation is suggested. Acetone itself elicited a spectral change with rat liver microsomes resembling a type II difference spectrum.4. Acetone partly reversed the inhibition produced by aniline on...

Osmo Hänninen

Papers

Gastrointestinal distribution of glucuronide synthesis and the relevant enzymes in the rat.

Enhancement of aniline p-hydroxylation by acetone in rat liver microsomes.

The competitive inhibition of p-nitrophenyl-β-d-glucopyranosiduronic acid synthesis by aliphatic alcohols in vitro

Enhanced glucuronide formation in different tissues following drug administration

Activation of microsomal UDP glucuronyltransferase by phospholipases