Interaction of uridine 5'-diphosphoglucuronic acid with microsomal UDP-glucuronosyltransferase in primate liver: the facilitating role of uridine 5'-diphospho-N-acetylglucosamine

Posttranscriptional regulation of uridine diphosphate glucuronosyltransferases

INTRODUCTION

The uridine diphosphate (UDP)-glucuronosyltransferase (UGT) superfamily of enzymes (EC 2.4.1.17) conjugates glucuronic acid to an aglycone substrate to make them more polar and readily excreted. In general, this reaction terminates the activities of chemicals, drugs and toxins, although occasionally a more active or toxic species is produced.

AREAS COVERED

In addition to their well-known transcriptional responsiveness, UGTs are also regulated by posttranscriptional mechanisms. Here, the authors review these mechanisms, including latency, modulation of co-substrate accessibility and binding, dimerization and oligomerization, protein-protein interactions, allosteric inhibition and activation, posttranslational structural and functional modifications and developmental switching for UGTs.

EXPERT OPINION

Posttranscriptional regulation of UGTs has traditionally received less attention than nuclear regulation, in part because mechanisms involving ribosomes and endoplasmic reticula are challenging to investigate. Most promising of the posttranscriptional mechanisms reviewed are likely to be effects on co-substrate (UDP-glucuronic acid) transport and availability and structure-function changes to UGT proteins through, for example, glycosylation and phosphorylation. Although classical biochemistry continues to illuminate many aspects of UGT function, advances in proteomics and structural biology are beginning to assist in the determination of posttranscriptional regulation mechanisms for UGTs.

Modulation of UDP-glucuronosyltransferase activity by endogenous compounds

Glucuronidation is one of the major pathways of metabolism of endo- and xenobiotics. UDP-Glucuronosyltransferase (UGT)-catalyzed glucuronidation accounts for up to 35% of phase II reactions. The expression and function of UGT is modulated by gene regulation, post-translational modifications and protein-protein association. Many studies have focused on drug-drug interactions involving UGT, and there are a number of reports describing the inhibition of UGT by xenobiotics. However, studies about the role of endogenous compounds as an inhibitor or activator of UGT are limited, and it is important to understand any change in the function and regulation of UGT by endogenous compounds. Recent studies in our laboratory have shown that fatty acyl-CoAs are endogenous activators of UGT, although fatty acyl-CoAs had been considered as inhibitors of UGT. Further, we have also suggested that adenine and related compounds are endogenous allosteric inhibitors of UGT. In this review, we summarize the endogenous modulators of UGT and discuss their relevance to UGT function.

Hepatic synthesis and urinary elimination of acetaminophen glucuronide are exacerbated in bile duct-ligated rats

Renal and intestinal disposition of acetaminophen glucuronide (APAP-GLU), a common substrate for multidrug resistance-associated proteins 2 and 3 (Mrp2 and Mrp3), was assessed in bile duct-ligated rats (BDL) 7 days after surgery using an in vivo perfused jejunum model with simultaneous urine collection. Doses of 150 mg/kg b.w. (i.v.) or 1 g/kg b.w. (i.p.) of acetaminophen (APAP) were administered, and its glucuronide was determined in bile (only Shams), urine, and intestinal perfusate throughout a 150-min period. Intestinal excretion of APAP-GLU was unchanged or decreased (-58%) by BDL for the 150 mg and 1 g/kg b.w. doses of APAP, respectively. In contrast, renal excretion was increased by 200 and 320%, respectively. Western studies revealed decreased levels of apical Mrp2 in liver and jejunum but increased levels in renal cortex from BDL animals, whereas Mrp3 was substantially increased in liver and not affected in kidney or intestine. The global synthesis of APAP-GLU, determined as the sum of cumulative excretions, was higher in BDL rats (+51 and +110%) for these same doses of APAP as a consequence of a significant increase in functional liver mass, with no changes in specific glucuronidating activity. Expression of apical breast cancer resistance protein, which also transports nontoxic metabolites of APAP, was decreased by BDL in liver and renal cortex, suggesting a minor participation of this route. We demonstrate a more efficient hepatic synthesis and basolateral excretion of APAP-GLU followed by its urinary elimination in BDL group, the latter two processes consistent with up-regulation of liver Mrp3 and renal Mrp2.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Chemical modification of rat hepatic microsomes with N-ethylmaleimide results in inactivation of both UDP-N-acetylglucosamine-dependent stimulation of glucuronidation and UDP-glucuronic acid uptake

Chemical modification of rat hepatic microsomes with N-ethylmaleimide (NEM) resulted in inactivation of UDP-N-acetylglucosamine (UDP-GlcNAc)-dependent stimulation of glucuronidation of p-nitrophenol. Inactivation kinetics and pH dependence were in agreement with the modification of a single sulfhydryl group. NEM also inactivated the uptake of UDP-glucuronic acid (UDP-GlcUA) but not UDP-glucose. With various sulfhydryl-modifying reagents, the inactivation of UDP-GlcUA uptake was linked to that of glucuronidation. UDP-GlcUA protected against NEM-sensitive inactivation of both UDP-GlcNAc-dependent stimulation of glucuronidation and UDP-GlcUA uptake, suggesting that the sulfhydryl group is located within or near the UDP-GlcUA binding site of the microsomal protein involved in the stimulation. Using microsomes labeled with biotin-conjugated maleimide and immunopurification with anti-peptide antibody against UDP-glucuronosyltransferase family 1 (UGT1) isozymes, immunopurified UGT1s were found to be labeled with the maleimide and UDP-GlcUA protected against the labeling as it did with the NEM-sensitive inactivation. These data suggest the involvement of a sulfhydryl residue of microsomal protein in the UDP-GlcNAc-dependent stimulation mechanism via the stimulation of UDP-GlcUA uptake into microsomal vesicles.

Structure and function of uridine diphosphate glucuronosyltransferases

1. The uridine diphosphate (UDP)-glucuronosyltransferases (UGT) are a family of enzymes that catalyse the covalent addition of glucuronic acid to a wide range of lipophilic chemicals. They play a major role in the detoxification of many exogenous and endogenous compounds by generating products that are more polar and, thus, more readily excreted in bile or urine. 2. Inherited deficiencies in UGT forms are deleterious, as exemplified by the debilitating effects of hyperbilirubinaemia and neurotoxicity in subjects with mutations in the enzyme that converts bilirubin to its more polar glucuronide. 3. The UGT protein can be conceptually divided into two domains with the amino-terminal half of the protein demonstrating greater sequence divergence between isoforms. This region apparently determines aglycone specificity. The aglycone binding site is presumed to be a 'loose' fit, as many structurally diverse substrates can be bound by the same UGT isoform. The carboxyl-terminal half, which is more conserved in sequence between different isoforms, is believed to contain a binding site for the cosubstrate UDP glucuronic acid (UDPGA). 4. Uridine diphosphate glucuronosyltransferase is localized to the endoplasmic reticulum (ER) and spans the membrane with a type I topology. The putative transmembrane domain is located near the carboxyl terminus of the protein such that only a small portion of the protein resides in the cytosol. This cytosolic tail is believed to contain an ER-targeting signal. The major portion of the protein is located in the ER lumen, including the proposed substrate-binding domains and the catalytic site. 5. The microsomal membrane impedes the access of UDPGA to the active site, resulting in latency of UGT activity in intact ER-derived microsomes. Active transport of UDPGA is believed to occur in hepatocytes, but the transport system has not been fully characterized. Uridine diphosphate glucuronosyltransferase activity is also highly lipid dependent and the enzyme may contain regions of membrane association in addition to the transmembrane domain.

Evidence for glucuronide (small molecule) sorting by human hepatic endoplasmic reticulum

The entry of substrates into, and the export of glururonides from, the lumen of hepatic endoplasmic reticulum (ER) in vitro (sealed microsomes) has been measured using radioactivity-labelled materials and a rapid filtration assay. Analysis of liver microsomes from a jaundiced patient showed the accumulation of bilirubin glucuronides within the lumen of the ER. Further analysis of these hepatic microsomes revealed that newly synthesized 1-naphthol glucuronide could exit from the microsomes whereas bilirubin glucuronide was accumulated within the microsomes. These results suggest the existence of mechanisms for the sorting of small molecules, destined for export through bile canalicular or basolateral plasma membranes, by ER. Furthermore, these sorting processes may be regulated by specific transporters within the ER.

Membrane translocation and regulation of uridine diphosphate-glucuronic acid uptake in rat liver microsomal vesicles

BACKGROUND/AIMS

Hepatic glucuronidation is quantitatively the most important conjugation reaction by which an array of endogenous compounds and xenobiotics undergo biotransformation and detoxification. The active site of the uridine diphosphate (UDP) glucuronosyltransferases, which catalyze glucuronidation reactions, has been postulated to reside in the lumen of the endoplasmic reticulum. The aim of this study was to characterize the process whereby UDP glucuronic acid (UDP-GlcUA), the cosubstrate for all glucuronidation reactions, is transported into microsomal vesicles.

METHODS

The uptake process was analyzed using rapid filtration techniques, radiolabeled UDP-GlcUA, and rat liver microsomes.

RESULTS

Uptake was saturable with respect to time and concentration, inhibited by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothio-cyanatostilbene-2-2'-disulfonic acid, and was osmotically sensitive. Transport was stimulated by Mg2+ and guanosine triphosphate (50 mumol/L) but not guanosine 5'-O-(3-thiotriphosphate) or adenosine triphosphate. Luminal UDP-N-acetylglucosamine (1 mmol/L) produced enhanced uptake of UDP-GlcUA (trans stimulation). In contrast to nucleotide sugar transport in the Golgi apparatus, trans uridine monophosphate and UDP did not alter UDP-GlcUA transport in microsomes, indicating distinct processes.

CONCLUSIONS

These data provide unambiguous evidence for the existence of a unique, substrate-specific, regulated, carrier-mediated process that transports UDP-GlcUA into the lumen of hepatocyte microsomes. This transporter may regulate glucuronidation in vivo.

Characterization of UDP-glucuronic acid transport in rat liver microsomal vesicles with photoaffinity analogs

The endoplasmic reticulum (ER) of rat liver contains several well characterized UDP-glucuronosyltransferases (UGTs), membrane-bound proteins of 50-54 kDa, and also less well identified UDP-glucosyltransferases, with nucleotide binding sites located on the lumenal surface. There is evidence that the substrates for these enzymes, UDP-glucuronic acid (UDP-GlcUA) and UDP-glucose (UDP-Glc), biosynthesized in the cytosol, are transported into the lumen of the ER via unknown mechanisms, the characteristics of which are poorly defined. A new approach for the study of the transport process has been devised using two active-site directed photoaffinity analogs, [beta-32P]5-azido-UDP-GlcUA and [beta-32P]5-azido-UDP-Glc. Photoincorporation of these probes into the lumenally oriented UGTs of intact rat liver microsomal vesicles was used as an indicator of transport. In intact vesicles, [32P]5N3UDP-GlcUA was efficiently incorporated into UGTs in a time, temperature and concentration dependent manner. In contrast, [32P]5N3UDP-Glc apparently was not transported effectively; maximal photolabeling of the 50-54 kDa proteins by this probe was dependent on detergent disruption of the vesicles. Vesicular uptake of and subsequent photolabeling of the 50-54 kDa proteins by [32P]5N3UDP-GlcUA were inhibited by UDP-GlcUA and 5N3UDP-GlcUA while UDP-Glc, 5N3UDP-Glc, UDP-xylose and UDP-N-acetylglucosamine were less inhibitory, suggesting a high degree of specificity for the uptake/photolabeling process. The anionic transport inhibitors DIDS and SITS inhibited [32P]5N3UDP-GlcUA photoincorporation into UGTs in intact vesicles, but also inhibited photolabeling of these and other enzymes in detergent disrupted vesicles. These data suggest the presence in rat liver microsomal vesicles of a specific, carrier-mediated transport process for UDP-GlcUA which is distinct from the mechanism of UDP-Glc transport.

Analysis of p-nitrophenol glucuronidation in hepatic microsomes from lactating rats

In the present study, hepatic p-nitrophenol glucuronidation was analyzed comparatively in virgin female, lactating mother and nonlactating mother rats (the last two groups 19-21 days post-partum). Enzyme assays were performed in native and activated microsomal suspensions. Activation was achieved either by including UDP-N-acetylglucosamine in the incubation mixtures or by preincubating native microsomes with optimal concentrations of Triton X-100 or palmitoyl-lysophosphatidylcholine. When UDP-N-acetylglucosamine was used as activator, enzyme activity increased in both lactating (about 80% increment) and nonlactating mothers (about 30% increment) as compared with virgin females. From an analysis of the degree of activation by Triton X-100 and palmitoyl-lysophosphatidylcholine, it can be inferred that the pregnancy-delivery event decreased the latency of UDP-glucuronosyltransferase activity that was detectable even 3 weeks post-partum, irrespective of whether suckling newborns were or were not kept with their mothers (lactating and nonlactating mothers, respectively). The estimation of apparent Vmax toward UDP-glucuronic acid in palmitoyl-lysophosphatidylcholine-activated microsomes, which allows an estimation of the amount of the enzyme, showed that lactation increased the number of catalytic units (about 40%). Hepatic UDP-glucuronic acid content was 70% higher in lactating rats than in other groups. The lipid composition and membrane fluidity (using 1,6-diphenyl-1,3,5-hexatriene as probe) were also analyzed in microsomes from all groups. A significant decrease in the unsaturation index that correlated with the rigidization of microsomal membranes was consistent with the changes in the degree of enzyme latency observed in lactating and nonlactating mothers. In conclusion, lactating rats exhibited enhanced p-nitrophenol UDP-glucuronosyltransferase activity as well as an increase in the hepatic content of UDP-glucuronic acid. These findings and the fact that lactation increased the liver to body weight ratio emphasize the role of the liver in the metabolism of planar phenolic derivatives in these circumstances.

Kinetic properties of bilirubin UDP-glucuronyltransferase in squirrel monkeys exhibiting fasting hyperbilirubinemia

1. Bolivian squirrel monkeys (BoSM), unlike Brazilian squirrel monkeys (BrSM), exhibit a marked fasting hyperbilirubinemia (FH) and serve as animal models for Gilbert's syndrome type I. 2. Compared to BrSM, BoSM possess a higher apparent UDPGAKm (0.51 vs 0.29 mM) and lower Vm (0.36 vs 0.48 nmol BR conjugated/min per mg microsomal protein) for hepatic bilirubin (BR) UDP-glucuronyl-transferase (BR UDPG-T). 3. Lineweaver-Burk plots are linear and obey Michaelis-Menten kinetics when UDP-acetylglucosamine is used as activator and UDPGA substrate concentrations are within the physiologic range present in the liver during the fed and fasted state (0.10-0.71 mM); above these concentrations, there is a discontinuity of kinetic plots as noted in other species. 4. There is no effect of fasting on the Km of BR conjugation (i.e. sum of mono- and diglucuronides) in either monkey; however, fasting is associated with lower Vm values (15-20%) in each subspecies. 5. By calculating the potential BR flux (nmol BR conjugated/min per kg) using known hepatic UDPGA concentrations, liver weights and in vitro Km and Vm, a markedly lower BR flux is observed in BoSM (58.4 nmol/min per kg) than in BrSM (91.6 nmol/min per kg). 6. Significantly higher apparent UDPGAKm and lower Vm of BR UDPG-T for conjugation of BR to BR monoglucuronide appears responsible in part for the four- to five-fold elevations in unconjugated BR in the liver and plasma in the fasted BoSM.