Bilirubin-UDP-glucuronyltransferase

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. 

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 the exception of a recent bisubstrate kinetic analysis of bilirubin UDP-glucuronyltransferase (P5), saturation with either one of the substrates was investigated at some rather arbitrarily fixed concentration of the other substrate. The results, therefore, have to be interpreted with caution. 

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. 

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... 

H2. Halac, E., and Reff, A., Studies on bilirubin UDP-glucuronyltransferase. Biochim. Biophys. Acta 139, 328-343 (1967). 

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. 

Figure 22-1. Production of bilirubin (BR). The degradation of I e l 3-hcme by molecular oxygen and NAD PH, catalyzed by microsomal heme oxygenase, produces biUverdin, CO, and Fe+2. Subsequent reduction of biliverdin by NADPH, catalyzed by biliverdin reductase, produces bilirubin. Bilirubin that is produced in phagocytes from degradation of senescent erythrocytes is transported to liver for conjugation with glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. In some cells, the bilirubin is used as an antioxidant, where it recycles through the biliverdin reductase reaction.

Within hepatocytes, bilirubin forms a complex with an abundant cytosolic family of enzymes, the glutathione 5-transferases, that have bilirubinbinding sites and function to transport bilirubin to the surface of the endoplasmic reticulum, where one or two glucuronic acid moieties are added to the propionic acid side chains of bilirubin (Fig. 22-1).This reaction, catalyzed by bilirubin-UDP-glucuronyltransferase, involves transfer of glucuronic acid from UDP-glucuronide to bilirubin. The effect of attachment of one or... 

Figure 22-3. Transport and hepatic metabolism of bilirubin. Bilirubin that is produced in phagocytes is transported to liver as an albumin-bilirubin complex. Uptake into the hepatocytes takes place in liver sinusoids. Within the hepatocyte, bilirubin is transported to the endoplasmic reticulum (microsomes) bound to glutathione S-transferase (GST). Bilirubin is made water soluble by addition of one or two glucuronic acid moieties obtained from UPD-glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. The product, conjugated bilirubin, is transported across the bile canalicular membrane for secretion into the biliary system, with subsequent movement into the intestines.

Bilirubin-UDP-glucuronyltransferase is one member of a family of glucuronyl transferases that participate in the metabolism of xenobi-otics (foreign compounds) by increasing their... 

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. 

Unfortunately, visual assessment has not been correlated with actual bilirubin levels. Therefore, laboratory measurement of the actual total serum bilirubin levels is mandatory. The timing of these measurements becomes important because of the bilirubin load and level of maturation of bilirubin-UDP-glucuronyltransferase. 

Vanstapel F, Blanckaert N. Topology and regulation of bilirubin UDP-glucuronyltransferase in sealed native microsomes from rat liver. Arch. Biochem. Biophys. 1988 263 216-225. 

In Crigler-Najjar syndrome type /activity of hepatic bilirubin UDP-glucuronyltransferase is undetectable and bilirubin conjugates are absent from the serum, bile, and urine, but biliary secretion of sulfobromophthalein and indocyanine green is normal. The disease is apparent shortly after birth, kemicterus develops, and death commonly occurs during the neonatal period. The effectiveness of phototherapy is often transient. The enzyme is not inducible by phenobarbital. This autosomal recessive defect occurs in all races. The Gunn strain of Wister rats has a similar genetic defect and has been used to study the syndrome. 

Crigler-Najjar syndrome type II (Arias syndrome) is milder, usually benign, and caused by partial deficiency of bilirubin UDP-glucuronyltransferase. Jaundice may not appear until the second or third decade of life. The monoglucuronide is the predominant pigment in bile. Phenobarbital induces the enzyme. Dominant and recessive inheritance patterns have been described. An accurate diagnosis of type 1, as opposed to type 2 Crigler-Najjar syndrome, is essential since orthotopic liver transplantation is an important therapy for type 1 patients. 

Y. Mamo, H. Sato, T. Yamano, et al. Gilbert syndrome caused by a homozygous missense mutation (Tyr486Asp) of bilirubin UDP-glucuronyltransferase gene. Journal of Pediatrics 132, 1045 (1998). 

Koga Y, Tsuda M, Ariyoshi N, et al. 1994. Induction of bilirubin UDP-glucuronyltransferase and CYP4A1 P450 by co-planar PCBs Different responsiveness of guinea pigs and rats. Chemosphere 28 639-645. 

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