An investigation of the transverse topology of bilirubin UDP-glucuronosyltransferase in rat hepatic endoplasmic reticulum.
ABSTRACT: Bilirubin UDP-glucuronosyltransferase (UDPGT) activity in sealed hepatic microsomes from clofibrate-treated rats was highly latent and was fully expressed by disruption of vesicles with detergents. Antibodies raised against purified bilirubin UDPGT were used to study the transmembrane orientation of the protein to provide a molecular understanding of the UDPGT latency. Immunoblot analysis of sealed microsomes, and microsomes after treatment with proteinases, showed that only a small portion of the protein resides on the cytoplasmic side of the microsomal vesicles. Treatment of microsomes with sodium deoxycholate allowed subtilisin and proteinase K to cleave the transferase, causing loss of activity and the release of smaller immunodetectable peptides. Treatment of the purified bilirubin UDPGT with peptide N-glycosidase F indicated that the enzyme was a glycoprotein. A working model of the transmembrane topology of bilirubin UDPGT is described.
Project description:The aim of this work was to determine if a non-mammalian species had multiple UDP-glucuronosyltransferase (UDPGT) isoforms. At least six highly purified UDPGT isoenzymes were partially resolved by anion-exchange chromatography and UDP-hexanolamine-Sepharose 4B affinity chromatography from liver microsomes of a fish, the plaice. Q-Sepharose FF, chromatofocusing and affinity-chromatographic procedures were employed to separate and purify the phenol UDPGT isoform to apparent homogeneity. The purified enzyme conjugated 1-naphthol, but not bilirubin or steroids, and displayed a pI of 7.0 and a subunit molecular mass of 55 kDa. Bilirubin and testosterone UDPGT activities were more labile and, although purified over 200-fold, these preparations also contained the phenol UDPGT and had multiple polypeptides with molecular masses of 52-57 kDa. Antisera to rat bilirubin/phenol UDPGT and testosterone/phenol UDPGT isoforms cross-reacted strongly with the partially purified plaice UDPGT isoforms of molecular masses 52, 53 and 57 kDa and less strongly with phenol UDPGT 54 kDa and 56 kDa isoforms. Fish and mammalian UDPGTs therefore apparently possess a high degree of evolutionary conservation.
Project description:When [14C]arachidonoyl-CoA was incubated with crude extracts of rat liver microsomes, [14C]arachidonic acid was incorporated into many proteins, suggesting that modification of these proteins with fatty acid, i.e. acylation, occurred. Using a [14C]arachidonyl-CoA labelling assay, 50 and 53 kDa proteins were purified from rat liver microsomes to near homogeneity by sequential chromatography on Red-Toyopearl, hydroxyapatite, heparin-Toyopearl, Blue-Toyopearl and UDP-hexanolamine-agarose. Acylation of the 50 and 53 kDa proteins occurred in the absence of any other protein, suggesting that these molecules catalyse autoacylation. The acylation was dependent on the length of the incubation period and the concentration of [14C]arachidonoyl-CoA. The 50 and 53 kDa proteins also had acyl-CoA-binding activity; initial rates of acyl-CoA binding and acylation were 0.25 and 0.004 min-1 respectively. The proteins also had weak but distinct acyl-CoA-hydrolysing activity (0.006 min-1). These results suggest that the proteins catalysed the sequential reactions of binding to acyl-CoA, autoacylation, and hydrolysis of fatty acid. N-terminal amino acid sequencing analysis showed these proteins to be UDP-glucuronosyltransferase (UDPGT) isoforms. UDPGT activity was inhibited by arachidonoyl-CoA. These results suggest that binding of acyl-CoA and acylation of UDPGT isoforms regulate the enzyme activities, implying a possible novel function for fatty acyl-CoA in glucuronidation, which is involved in the metabolism of drugs, steroids and bilirubin.
Project description:UDP-glucuronosyltransferases (EC 188.8.131.52) is an isoenzyme family located primarily in the hepatic endoplasmic reticulum (ER) that displays latency of activity both in vitro and in vivo, as assessed respectively in microsomes and in isolated liver. The postulated luminal location of the active site of UDP-glucuronosyltransferases (UGTs) creates a permeability barrier to aglycone and UDP-GlcA access to the enzyme and implies a requirement for the transport of substrates across the ER membrane. The present study shows that the recently demonstrated carrier-mediated transport of UDP-GlcA across the ER membrane is required and rate-limiting for glucuronidation in sealed microsomal vesicles as well as in the intact ER of permeabilized hepatocytes. We found that in both microsomes and permeabilized hepatocytes a gradual inhibition by N-ethylmaleimide (NEM) of UDP-GlcA transport into the ER produced a correspondingly increasing inhibition of 4-methylumbelliferone glucuronidation. That NEM selectively inhibited the UDP-GlcA transporter, without affecting intrinsic UGT activity, was demonstrated by showing that NEM had no effect on glucuronidation in microsomes or hepatocytes with permeabilized ER membrane. Additional evidence that UDP-GlcA transport is rate-limiting for glucuronidation in sealed microsomal vesicles as well as in the intact ER of permeabilized hepatocytes was obtained by showing that gradual selective trans-stimulation of UDP-GlcA transport by UDP-GlcNAc, UDP-Xyl or UDP-Glc in each case produced correspondingly enhanced glucuronidation. Such stimulation of transport and glucuronidation was inhibited completely by NEM, which selectively inhibited UDP-GlcA transport.
Project description:Antibodies raised against purified components of glucose-6-phosphatase were used to study the transmembrane orientation of the complex. Measurements of glucose-6-phosphatase activities and immunoblot analysis of sealed microsomes and detergent-solubilized microsomes after treatment with proteases suggested that most of the catalytic subunit resides within the lumen of the endoplasmic reticulum. In contrast, other components of glucose-6-phosphatase are accessible to the cytoplasm. Treatment of the partially purified glucose-6-phosphatase enzyme with glycopeptide N-glycosidase indicated that the catalytic subunit of the enzyme was a glycoprotein.
Project description:We examined regulatory properties of bilirubin UDP-glucuronyltransferase in sealed RER (rough endoplasmic reticulum)- and SER (smooth endoplasmic reticulum)-enriched microsomes (microsomal fractions), as well as in nuclear envelope from rat liver. Purity of membrane fractions was verified by electron microscopy and marker studies. Intactness of RER and SER vesicles was ascertained by a high degree of latency of the lumenal marker mannose-6-phosphatase. No major differences in the stimulation of UDP-glucuronyltransferase by detergent or by the presumed physiological activator, UDPGlcNAc, were observed between total microsomes and RER- or SER-enriched microsomes. Isolated nuclear envelopes were present as a partially disrupted membrane system, with approx. 50% loss of mannose-6-phosphatase latency. The nuclear transferase had lost its latency to a similar extent, and the enzyme failed to respond to UDPGlcNAc. Our results underscore the necessity to include data on the integrity of the membrane permeability barrier when reporting regulatory properties of UDP-glucuronyltransferase in different membrane preparations.
Project description:1. Reconstitution of purified bilirubin UDP-glucuronyltransferase from Wistar-rat liver into Gunn-rat liver microsomes provides a better environment than phosphatidylcholine liposomes, such that the final specific activity of the Wistar-rat liver enzyme was increased up to 85 units/mg of protein. 2. Gunn- and Wistar-rat liver microsomes were equally effective for reconstitution of the purified enzyme. 3. The transferase activity does not appear to be fully expressed in the more rigid environment of foetal Wistar-rat liver microsomes. 4. These reconstitution experiments reveal a final specific activity for the purified bilirubin UDP-glucuronyltransferase consistent with the capacity of the whole rat liver to glucuronidate bilirubin and indicate that the absence of this enzyme activity in Gunn-rat liver microsomes is not due to an abnormal microenvironment.
Project description:Highly purified bilirubin UDP-glucuronyltransferase from Wistar-rat liver, when reconstituted with Gunn-rat liver microsomes (microsomal fraction), was able to catalyse the conversion of unesterified bilirubin into both bilirubin monoglucuronide and diglucuronide. Under zero-order kinetic conditions for monoglucuronide formation, the fraction of bilirubin diglucuronide formed by incubation of bilirubin with the reconstituted highly purified transferase accounted for 18% of total bilirubin glucuronides, which was only slightly lower than the fraction of diglucuronides (23% of total bilirubin glucuronides) formed by incubation with hepatic microsomes in the presence of UDP-N-acetylglucosamine or Lubrol. The reconstituted purified enzyme also catalysed the UDP-glucuronic acid-dependent conversion of bilirubin monoglucuronide into diglucuronide and, when bilirubin was incubated with UDP-glucose or UDP-xylose, the formation of bilirubin glucosides and xylosides respectively. These results suggest that a single microsomal bilirubin UDP-glycosyltransferase may be responsible for the formation of bilirubin mono- and di-glycosides.
Project description:Saturable bilirubin binding to human erythrocyte membranes was measured before and after digestion with neuraminidase and phospholipases. Neuraminidase-treated erythrocyte membranes did not show any change in their binding properties, indicating that gangliosides could be excluded as candidates for saturable bilirubin-binding sites on erythrocyte membranes. Although bilirubin-binding properties of the membranes did not change after phospholipase D digestion, either, phospholipase C treatment greatly enhanced bilirubin binding. Thus it is suggested that a negatively charged phosphoric acid moiety of phospholipids on the membrane surface may play a role to prevent a large amount of bilirubin from binding to the membranes. Further saturable bilirubin binding to inside-out sealed erythrocyte membrane vesicles showed values comparable with those of the right-side-out sealed membranes, suggesting that the bilirubin-binding sites may be distributed on both outer and inner surfaces of the membranes, or may exist in the membranes where bilirubin may be accessible from either side.
Project description:The activity of UDP-glucuronosyltransferase (UDPGT, EC 184.108.40.206) in human foetal liver cells in culture was measured with two acceptor substrates, namely harmol and 1-naphthol. There was a dose-dependent increase of about 10-400% in UDPGT activity when the cells were exposed to 1-30 microM-HgCl2. Above a critical concentration of 30 microM-HgCl2, the heavy metal ion was toxic to the cells. Kinetic studies of the glucuronidation reaction with harmol and 1-naphthol showed that Hg2+ ions seemed to induce the expression of a high-affinity form of UDPGT, which was absent from the normal controls. The dramatic increase in specific activity in UDPGT was accompanied by a parallel increase in Vmax. measured with harmol and UDP-glucuronic acid. The significance of a possible induction of UDPGT in human foetal liver cells by HgCl2 is discussed.
Project description:Bilirubin glucuronidation, catalysed by UGT1A1 [UGT (UDP glucuronosyltransferase) isoform 1A1, EC 220.127.116.11], is critical for biliary elimination of bilirubin. UGT1A1 deficiency causes CN-1 (Crigler-Najjar syndrome type 1), which is characterized by potentially lethal unconjugated hyperbilirubinaemia. Nucleotide sequence analysis of UGT1A1 in two CN-1 patients revealed that patient A was homozygous for a nt 530 G-->A (where nt 530 G-->A means guanine to adenine transition at nucleotide 530) mutation, predicting a C177Y substitution, and patient B had a nt 466 T-->C mutation on one allele and a nt 1070 A-->G mutation on the other, predicting a C156R and a Q357R substitution respectively. All 11 cysteine residues of mature human UGT1A1 are highly conserved in other human UGT isoforms and in rat, mouse and Rhesus monkey UGT1A1, suggesting their functional importance. Expression of mutagenized UGT1A1 plasmids showed that substitution of any of the seven cysteine residues located within the endoplasmic reticulum cisternae (including those mutated in patients A and B) abolished UGT1A1 activity or markedly increased its apparent K(m) for bilirubin. Substitution of the three cysteine residues within the C-terminal cytosolic tail had minimal effect on basal UGT1A1 activity, but prevented UGT1A1 activation by UDP-GlcNAc. N-Ethylmaleimide did not inhibit UGT1A1 activity in native microsomes, but prevented UGT1A1 activation by UDP-GlcNAc and inhibited the activity in digitonin-permeabilized microsomes. Dithiothreitol did not affect UGT1A1 activity in human liver microsomes. Together, the results suggested that free thiol groups, but not disulphide bonding, of seven cysteine residues within the intracisternal region of human UGT1A1 are important for its catalytic activity, while cysteine residues in the cytosolic domain may be involved in its physiological activation by UDP-GlcNAc.