Kinetic properties of the rat intestinal microsomal 1-naphthol:UDP-glucuronosyl transferase. Inhibition by UDP and UDP-N-acetylglucosamine

Xyloglucan Xylosyltransferase 1 Displays Promiscuity Toward Donor Substrates During in Vitro Reactions

Glycosyltransferases (GTs) are a large family of enzymes that add sugars to a broad range of acceptor substrates, including polysaccharides, proteins and lipids, by utilizing a wide variety of donor substrates in the form of activated sugars. Individual GTs have generally been considered to exhibit a high level of substrate specificity, but this has not been thoroughly investigated across the extremely large set of GTs. Here we investigate xyloglucan xylosyltransferase 1 (XXT1), a GT involved in the synthesis of the plant cell wall polysaccharide, xyloglucan. Xyloglucan has a glucan backbone, with initial side chain substitutions exclusively composed of xylose from uridine diphosphate (UDP)-xylose. While this conserved substitution pattern suggests a high substrate specificity for XXT1, our in vitro kinetic studies elucidate a more complex set of behavior. Kinetic studies demonstrate comparable kcat values for reactions with UDP-xylose and UDP-glucose, while reactions with UDP-arabinose and UDP-galactose are over 10-fold slower. Using kcat/KM as a measure of efficiency, UDP-xylose is 8-fold more efficient as a substrate than the next best alternative, UDP-glucose. To the best of our knowledge, we are the first to demonstrate that not all plant XXTs are highly substrate specific and some do show significant promiscuity in their in vitro reactions. Kinetic parameters alone likely do not explain the high substrate selectivity in planta, suggesting that there are additional control mechanisms operating during polysaccharide biosynthesis. Improved understanding of substrate specificity of the GTs will aid in protein engineering, development of diagnostic tools, and understanding of biological systems.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Enzyme kinetics of uridine diphosphate glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bi-substrate reaction that requires the aglycone and a cofactor, UDPGA. Accumulating evidence suggests that the bi-substrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modelling of glucuronidation reactions in vitro, UDPGA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for in experimental design and data interpretation. Assessing drug-drug interactions (DDIs) involving UGT inhibition remains challenging. However, the increasing availability of UGT enzyme-specific substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of DDI potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often under-predicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation (IVIVE).

UDP-glucuronic acid binds first and the aglycone substrate binds second to form a ternary complex in UGT1A9-catalyzed reactions, in both the presence and absence of bovine serum albumin

The presence of bovine serum albumin (BSA) largely modulates the enzyme kinetics parameters of the human UDP-glucuronosyltransferase (UGT) 1A9, increasing both the apparent aglycone substrate affinity of the enzyme and its limiting reaction velocity (Drug Metab Dispos 39:2117-2129, 2011). For a better understanding of the BSA effects and an examination of whether its presence changes the catalytic mechanism, we have studied the enzyme kinetics of 4-methylumbelliferone glucuronidation by UGT1A9 in the presence and absence of 0.1% BSA, using bisubstrate enzyme kinetic experiments, in both the forward and reverse directions, as well as product and dead-end inhibition. The combined results strongly suggest that the reaction mechanism of UGT1A9, and presumably other human UGTs as well, involves the formation of a compulsory-order ternary-complex, with UDP-α-d-glucuronic acid (UDPGA) as the first binding substrate. Based on the enzyme kinetic parameters measured for the forward and reverse reactions, the equilibrium constant of the overall reaction was calculated (Keq = 574) and the relative magnitudes of the reaction rate constants were elucidated. The inclusion of BSA in the bisubstrate kinetic experiments quantitatively changed the apparent enzyme kinetic parameters, presumably by removing internal inhibitors that bind to the binary enzyme-UDPGA (E-UDPGA) complex, as well as to the ternary E-UDPGA-aglycone complex. Nevertheless, the underlying compulsory-order ternary-complex mechanism with UDPGA binding first is the same in both the absence and presence of BSA. The results offer a novel understanding of UGT enzyme kinetic mechanism and BSA effects.

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.

Product inhibition of UDP-glucuronosyltransferase (UGT) enzymes by UDP obfuscates the inhibitory effects of UGT substrates

Substrates that are specific for certain UDP-glucuronosyltransferase (UGT) isoforms are usually used as specific inhibitors to identify UGT isoforms responsible for the glucuronidation of drugs. 1-Naphthol and 4-nitrophenol are probe substrates for human UGT1A6. In the present study, we found that UGT1A1-catalyzed estradiol 3-O-glucuronide formation and UGT1A4-catalyzed imipramine N-glucuronide formation in human liver microsomes were prominently decreased in the presence of 1-naphthol, but those by recombinant human UGT1A1 and UGT1A4, respectively, were not. Interestingly, when recombinant UGT1A6 was added in the reaction mixture, these activities by recombinant UGT1A1 and UGT1A4 were diminished in the presence of 1-naphthol. To interpret this phenomenon, the inhibitory effects of 1-naphthol O-glucuronide and UDP, products of the glucuronidation of 1-naphthol, were investigated. We found that UDP strongly inhibited the UGT1A1 (K(i) = 7 microM) and UGT1A4 (K(i) = 47 microM) activities in a competitive manner for the 5'-diphosphoglucuronic acid binding. These results suggest that UDP produced by UGT1A6-catalyzed 1-naphthol glucuronidation, but not 1-naphthol O-glucuronide and 1-naphthol per se, is the actual inhibition substance. Next, we examined the inhibitory effects of 15 compounds that are substrates of UGTs on estradiol 3-O-glucuronide formation in human liver microsomes compared with those by recombinant UGT1A1. Among them, 4 compounds (1-naphthol, 2-naphthol, 4-nitrophenol, and 4-methylumbelliferone) with high turnover rates (V(max)/K(m) value >200 mul/min/mg) showed more potent inhibition of the activity in human liver microsomes compared with that by the recombinant UGT1A1. Thus, we should pay attention to the inhibitory effects of UDP on UGT, which may cause erroneous evaluations in inhibition studies using human liver microsomes.

KINETIC CHARACTERIZATION OF THE 1A SUBFAMILY OF RECOMBINANT HUMAN UDP-GLUCURONOSYLTRANSFERASES

The initial glucuronidation rates were determined for eight recombinant human UDP-glucuronosyltransferases (UGTs) of the 1A subfamily, and the bisubstrate kinetics and inhibition patterns were analyzed. At low substrate concentrations, the reactions followed general ternary complex kinetics, whereas at higher concentrations of both substrates, the reactions were mostly characterized by ternary complex kinetics with substrate inhibition. The glucuronidation of entacapone by UGT1A9 was inhibited by 1-naphthol in a competitive fashion, with respect to entacapone, and an uncompetitive fashion, with respect to UDP-glucuronic acid (UDPGA). Its inhibition by UDP, on the other hand, was noncompetitive with respect to entacapone and competitive with respect to UDPGA. These inhibition patterns are compatible with a compulsory ordered bi bi mechanism in which UDPGA is the first-binding substrate. Despite the identical primary structure of the C-terminal halves of the UGT1A isoforms, there were marked differences in the respective K(m) values for UDPGA, ranging from 52 microM for UGT1A6 to 1256 microM for UGT1A8. Relative specificity constants were calculated for the eight UGT1A isoforms with 1-hydroxypyrene, 4-nitrophenol, scopoletin, 4-methylumbelliferone, and entacapone as aglycone substrates. The results demonstrated that seven of the UGT1A isoforms are capable of conjugating phenolic substrates with similar highest k(cat) values, and UGT1A4 has a lower relative turnover rate. The highest specificity constants were obtained for 1-hydroxypyrene, even with UGT1A6, which has been regarded as a specific isoform for small planar phenols. A k(cat) value of 1.9 s(-1) was calculated for the glucuronidation of scopoletin by purified UGT1A9.

Structural and functional studies of UDP-glucuronosyltransferases

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.

Inhibitory effects of uridine diphosphate on UDP-glucuronosyltransferase

Inhibitory effects of uridine diphosphate on the enzymatic activity of UDP-glucuronosyltransferase (UGT) were investigated. Pyrimidine nucleotides such as UDP, UTP and cytidine diphosphate reduced the activity of rat purified UGT (phenol UGT) to about 10%, 48% and 46% of the control, respectively, at the same concentration as a donor substrate, UDP-glucuronic acid. Purine nucleotides, uridine monophosphate, glucuronic acid and some UDP-sugars were only slightly inhibitory toward the transferase. Similar effects were observed in the expressed UGT (UGT1A6; corresponding to phenol UGT) in yeast cells and rat liver microsomal membrane-binding UGT, indicating that uracil and diphosphate residues are essential for the UDP inhibition. Interestingly, 2'-deoxy UDP was found to be a less effective inhibitor (about 50% inhibition) than UDP on the purified, the expressed (UGT1A6 and UGT2B1) and microsomal membrane-binding UGTs. These results indicate that not only uracil and diphosphate residues but also 2'-hydroxyl residue of UDP ribose participates in the interactions between UDP and UDP-glucuronosyltransferase.

Purification and kinetic analysis of a baculovirus ecdysteroid UDP-glucosyltransferase

The baculovirus ecdysteroid UDP-glucosyltransferase (EGT) disrupts the hormonal balance of the insect host by catalysing the conjugation of ecdysteroids, the moulting hormones, with the sugar moiety from UDP-glucose or UDP-galactose. In this study, Autographa californica nucleopolyhedrovirus EGT has been overproduced and purified, and its kinetic properties determined. The enzyme was purified 1100-fold to near-homogeneity using only two major steps, ion-exchange and gel-filtration chromatography. EGT activity was eluted from the gel-filtration column as a single peak corresponding to a 260+/-50 kDa protein, suggesting that the enzyme is an oligomer of three to five subunits, as the subunit molecular mass is approximately 56 kDa. Kinetic analysis showed that EGT has broadly similar specificities for UDP-galactose and UDP-glucose (kcat/Km=1790.8 and 902.1 respectively) when ecdysone is used as the other substrate. On the other hand, it shows marked differences in specificity for the various ecdysteroids tested. Ecdysone seems to be the optimal substrate (kcat/Km=7101.1), whereas 3-dehydroecdysone, an ecdysone precursor in Lepidoptera, is seven times less favourable (kcat/Km=1085.7). Notably, 20-hydroxyecdysone, the active form of the hormone, is conjugated very poorly (kcat/Km=31.6). Analysis of the data revealed that the enzyme mechanism involves the formation of an ecdysteroid-UDP-sugar-enzyme ternary complex. This work represents the most detailed biochemical characterization of an EGT to date.

Mechanistic studies of uridine diphosphate glucuronosyltransferase

Bisubstrate reaction kinetics and product inhibition studies were used to characterize the kinetic mechanism of a partially purified uridine diphosphate glucuronosyltransferase (UDPGT). These studies indicate that the reaction most likely occurs via a random order sequential mechanism. The effect of electron withdrawing and donating groups on the rate of reaction was also determined. It was found that electron donating groups increased the rate of glucuronide conjugation. This result is consistent with nucleophilic attack of the C-1 carbon of the UDP-glucuronic acid (UDPGA) by an SN2 mechanism. This is the first direct evidence for a SN2 mechanism in UDPGT catalysis.

The activity of 1-naphthol-UDP-glucuronosyltransferase in the brain

Cerebral microsomes catalysed efficiently the glucuronidation of 1-naphthol, this formation of glucuronide being activated by treatment with Triton X-100 or digitonin. Activated microsomes from the brain of the rat conjugated 1-naphthol with an apparent Km of 95 microM and a Vmax of 5.47 nmol/hr mg protein at 30 degrees C. Microsomal uridine diphosphate (UDP)-glucuronosyltransferase activity in brain towards 1-naphthol was not significantly induced by pretreatment of animals with 3-methylcholanthrene or phenobarbital. These data suggest that UDP-glucuronosyltransferases in brain are different from the hepatic enzymes with regard to biochemical parameters and in response to inducers of drug metabolism. The hepatic UDP-glucuronosyltransferase deficiency in Gunn rats was also observed in the brain.