UDP-glucuronyltransferase activation

Raijmakers, M.T., Jansen, P.L., Steegers, E.A., et al. (2000) Association of human liver bilirubin UDP-glucuronyltransferase activity with a polymorphism in the promoter region of the UGTIAI gene. J. Hepatol. 33, 348-351. 

As yet it cannot be excluded that the observed latency is a preparation artifact resulting, e.g., from vesiclization (E2, Dl). If so, artificial activation would yield values closer to the native situation. Support for this hypothesis is found in the observation that maximum bilirubin excretion rates correlated well with the UDP-glucuronyltransferase activity of fully activated liver homogenate (H2, HIO). 

In discussing the basic approaches used to assay bilirubin UDP-glucuronyltransferase activity, potential extension to the UDP-glucosyl-and UDP-xylosyltransferases will be outlined. Determination of the rates of synthesis of diconjugates and of nonglycosidic conjugates will be dealt with in Section 6. 

WlO. Winsnes, A., Variable effect of phenobarbital treatment of mice on hepatic UDP-glucuronyltransferase activity when judged by slightly different enzyme-assay techniques. Biochem. Pharmacol. 20, 1853-1857 (1971). 

Hepatic microsomes of several animal species possess UDP glucuronyltransferase activity and with p-nitrophenol as a substrate, a 12-fold difference in activity due to species variation is evident. Phospholipase-A activates the enzyme and results of activation experiments indicate that the amount of constraint on the activity of this enzyme is variable in different animal species. 

Andersson, T., M. Pesonen, and C. Johansson. 1985. Differential induction of cytochrome P-450-dependent monoxygenase, epoxide hydrolase, glutathione transferase and UDP glucuronyltransferase activities in the liver of the rainbow trout by 3-naphthoflavone or Clophen A50. Biochem. Pharmacol. 34 3309-3314. 

Glutathione S -transferase activity (substrate CDNB) was present in the starfish Psilaster andromeda, Pseudoarcaster parelii and A, rubens, the sea urchin Echinus esculentus and the sea cucumber Parastichopus tremulus viz. 10.7 to 51.6 nmol min mg protein no activity was detected with DCNB as substrate and only low activity with ETHA (Stenersen et al. 1987). Low UDP-glucuronyltransferase activity has been indicated in the sea urchin Erechinus chloroticus towards substituted phenols and in other echinoderm species towards 2-aminophenol, but only in sexually mature animals (Dutton 1980). 

The present review covers a description of methodology and properties of UDP-glucuronyltransferase and of related UDP-glycosyltransferase activities (assayed with bilirubin as the acceptor substrate) and attempts to delineate applications to human disease. Studies with other hydro-phobic acceptor substances will be discussed as far as relevant to the subject matter. 

Studies of UDP-glucuronyltransferase with bilirubin as the acceptor substrate are technically difficult. This is indicated by frequent modification of the initial assay systems (A8, G9, L4, S4) and by the wide range of reported enzyme activities (Table 1). Possible causes of these discrepancies, which are manyfold, will be discussed in some detail below. The conclusions drawn should be helpful in the design of assays of conjugate formation of bilirubin and of the synthesis of mono- and diconjugates. 

With albumin-solubilized bilirubin, pH optima of microsomal bilirubin UDP-glucuronyltransferase were 7.4-8.0 for rat (H2, HIO, SIO) and 7.4 for guinea pig (M13) and rabbit (T8). Above pH 8 the enzyme activity decreased abruptly (HIO). In absence of carrier protein, optima were at pH 8 and 8.2 with preparations from liver of guinea pig (P3) and rat (W12), respectively. The activity-pH curve with optimum at pH 8.2 (W12) showed pronounced skewing, with a steady and rather rapid increase of enzyme activity from pH 7.4 to 8.2. One may wonder whether such measurements were influenced by the rapid increase of solubility of the acceptor substrate occurring over the same pH range (B25). 

The effects of Mg-+ on UDP-glucuronyltransferase depend on preparations and substrates (D9, L14). Bilirubin UDP-glucuronyltransferase in untreated (F17, W12) and detergent-activated microsomal preparations from rat liver (HIO, Y2) and in purified fractions (A2, H2), is stimulated by Mg +. Employing purified enzyme (probably still linked to a piece of... 

With bilirubin UDP-glucuronyltransferase from rat liver, Mn-+ was more (HIO), and Ca + less, stimulatory than Mg + (A2, F17, HIO). The behavior was similar when either UDP-glucose or UDP-xylose was used as the glycosyl donor (F3). Enzyme activities were also stimulated by Fe and Co (F3, HIO) Pb + activated glucuronyl transfer but was inhibitory with the other UDP-sugars. The effects of Mg +, Mn +, and Co are in accordance with work of Lucier et al. (L14) on the catalysis of glucuronyl transfer to p-nitrophenol and 1-naphthol by Triton X-100-activated and untreated microsomal material from rat liver. 

With bilirubin as the acceptor substance, UDP-glucuronyltransferase is activated by aging (SIO, W7), by alkaline dialysis (H2), or by treatment with Triton X-100 (M16, P5, W8, W9, WIO), deoxycholate (V2) or digitonin (HIO, W7). Comparable maximum activities were found. Studies with xenobiotic acceptor substrates yielded the same conclusion (H13, L14, L15, V4, W 7). Very rapid and maximum activation of p-nitrophenol UDP-glucuronyltransferase was obtained at pH 9.8-10.5 (V4). Digitonin-activation of UDP-glucosyl- and UDP-xylosyltransfer-ase activities (both assayed with bilirubin as the acceptor substrate) has also been reported (F3). 

A close dependence of the activity of UDP-glucuronyltransferase (A12, G3), glucose-6-phosphatase and enzymes related to microsomal electron transport to NADPH (W6), on the structural integrity of the... 

Partial or total deficiency, or inhibition of bilirubin UDP-glucuronyltransferase may cause unconjugated hyperbilirubinemia. Increased production (hemolysis, ineffective erythropoiesis) should be excluded by investigating hematologic parameters. Determination in vitro of bilirubin UDP-glycosyltransferase activities can contribute to a differential diagnosis. To minimize the effect of cytoplasmic carrier proteins, in in vitro... 

F17. Frei, J., Multiplicity and specificity of UDP-glucuronyltransferase. I. Effect of divalent cations and EDTA on the activity of UDP-glucuronyltransferase assayed with bilirubin, 4-methylumbelliferone and p-nitrophenol. Emymol. Biol. Clin. 11, 385-401 (1970). 

L14. Lucier, G. W., McDaniel, O. S., and Matthews, H. B., Microsomal rat liver UDP glucuronyltransferase. Effects of piperonyl butoxide and other factors on enzyme activity. Arch. Biochem. Biophys. 145, 520-530 (1971). 

One needs to keep in mind that the use of drugs by the mother will sometimes lead to impairment of the activity of bilirubin-UDP-glucuronyltransferase. Phenothiazines are an example of this kind of interaction. The use of drugs in the neonatal intensive care unit also can contribute to hyperbilirubinemia. Usually, the medications that compete for binding sites on albumin are the culprits in this case (see section on Bilburin Transport). An example of this type of interaction is the use of furosimide, which is a diuretic used to decrease fluid retention and improve cardiac function and renal output. 

A number of genetic diseases are associated with defects in one or more of the enzymes involved in the metabolism of bilirubin. Three of these diseases result in elevated levels of unconjugated bilirubin, and two of these result in elevated levels of conjugated bilirubin. All three of the diseases that produce elevated levels of unconjugated bilirubin are related to defects in the level of expression or in the inherent activity of bilirubin-UDP-glucuronyltransferase (also called UGT1 Al).The mildest and most common of these disease is Gilbert syndrome, which is present in about 10% of the Caucasian population. 

The activation of another membrane-bound enzyme of interest in biochemical toxicology, UDP glucuronyltransferase, is probably due to effects on the membrane. In this case, significant stimulation is brought about by aging the enzyme preparation, by sonication, and by such agents as dilute detergents and proteolytic enzymes. 

The plasma concentration of retinoyl glucuronide is between 5 and 14 mnol per L, and the activity of retinoic acid UDP-glucuronyltransferase increases in vitamin A deficiency, suggesting that glucuronidation may be important... 

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