In Situ Structural Analysis of Microsomal UDP-glucuronyltransferases by Radiation Inactivation

The Human UGT2B7 Nanodisc

The 20 uridine diphosphate glycosyl-transferases (UGTs) encoded in the human genome form an essential homeostatic network of overlapping catalytic functions that surveil and regulate the activity and clearance of scores of small molecule metabolites. Biochemical and biophysical UGT studies have been hampered by the inability to purify these membrane-bound proteins. Here, using cell-free expression and nanodisc technology, we assemble and purify to homogeneity the first UGT nanodisc-the human UGT2B7•nanodisc. The complex is readily isolated in milligram quantities. It is stable and its initial-rate parameters are identical within error to those associated with UGT2B7 in microsomal preparations (i.e., Supersomes). The high purity of the nanodisc preparation simplifies UGT assays, which allows complexities traditionally associated with microsomal assays (latency and the albumin effect) to be circumvented. Each nanodisc is shown to harbor a single UGT2B7 monomer. The methods described herein should be widely applicable to UGTs, and these findings are expected to set the stage for experimentalists to more freely explore the structure, function, and biology of this important area of phase II metabolism. SIGNIFICANCE STATEMENT: Lack of access to pure, catalytically competent human uridine diphosphate glucuronosyl-transferases (UGTs) has long been an impediment to biochemical and biophysical studies of this disease-relevant enzyme family. Here, we demonstrate this barrier can be removed using nanodisc technology-a human UGT2B7•nanodisc is assembled, purified to homogeneity, and shown to have activity comparable to microsomal UGT2B7.

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.

Glucuronidation: a dual control

1. Glucuronidation is a major detoxication process catalyzed by uridine diphosphate glucuronosyltransferases. 2. The amount of enzyme can be modulated by numerous foreign compounds, such as common chemical inducers already implicated in the induction of other detoxication enzymes. 3. Hormones such as thyroid hormones or growth hormone also are implicated in the control of glucuronidation. 4. Because glucuronidation enzymes (isozymes) are anchored in the endoplasmic reticulum membrane, with their active site likely being located on the lumenal side of the membrane, the membrane environment of these enzymes was shown to modulate their functional state as evaluated by the conjugating activity per enzymatic molecular unit. 5. In accord with a first, previously proposed model, it seems that this modulation can be attributed to different conformational states of the enzymes, depending on the physicochemical state of the membrane. 6. In accord with a second model, the membrane may act as a barrier between the enzymes and the cosubstrate UDP-glucuronic acid, which is a polar and charged molecule synthesized in the cytosol. This would imply a transporting process for this molecule through the reticulum membrane, which has been characterized in vitro and could be of importance in vivo. 7. Glucuronidation is under the control of a dual regulation, by means of a specific isozyme expression level and by the modulation of their functional state.

Novel properties of hepatic canalicular reduced glutathione transport revealed by radiation inactivation

Transport of GSH at the canalicular pole of hepatocytes occurs by a facilitative carrier and can account for approximately 50% of total hepatocyte GSH efflux. A low-affinity unit with sigmoidal kinetics accounts for 90% of canalicular transport at physiological GSH concentrations. A low-capacity transporter with high affinity for GSH has also been reported. It is not known whether the same or different proteins mediate low- and high-affinity GSH transport, although they do differ in inhibitor specificity. The bile of rats with a mutation in the canalicular multispecific organic anion transporter (cMOAT or MRP-2, a 170-kDa protein) is deficient in GSH, implying that cMOAT may transport GSH. However, transport of GSH in canalicular membrane vesicles (CMV) from these mutant rats remains intact. We examined the functional size of the two kinetic components of GSH transport by radiation inactivation of GSH uptake in rat hepatic CMV. High-affinity transport of GSH was inactivated as a single exponential function of radiation dose, yielding a functional size of approximately 70 kDa. In contrast, low-affinity canalicular GSH transport exhibited a complex biexponential response to irradiation, characterized by an initial increase followed by a decrease in GSH transport. Inactivation analysis yielded a approximately 76-kDa size for the low-affinity transporter. The complex inactivation indicated that the low-affinity transporter is associated with a larger protein of approximately 141 kDa, which masked approximately 80% of the potential transport activity in CMV. Additional studies, using inactivation of leukotriene C4 transport, yielded a functional size of approximately 302 kDa for cMOAT, indicating that it functions as a dimer.

Protein-protein interactions between UDP-glucuronosyltransferase isozymes in rat hepatic microsomes

The interactions between UDP-glucuronosyltransferase (UGT) isozymes, UGT1s and UGT2B1, in rat hepatic microsomes were investigated using an immunopurification technique with anti-peptide antibodies and a chemical cross-linking strategy. A 50 kDa protein coimmunopurified with UGT1s was identified as UGT2B1 by amino-terminal sequencing and immunodetection with anti-peptide antibody against UGT2B1. Evidence for direct interaction of UGT2B1 with UGT1s was obtained by the loss of UGT2B1 adsorption to immunoaffinity column in Gunn rat hepatic microsomes, which lack all UGT1 isozymes. When the microsomes were treated with the chemical cross-linking reagent 1,6-bis(maleimido)-hexane, a cross-linked product with an apparent molecular mass of 120-130 kDa was obtained that immunostained with antibodies against UGT1s and UGT2B1, indicating the formation of a heterodimer containing one of the UGT1 isozymes and UGT2B1. The effects of UGT complex formation on the stimulation of glucuronidation of testosterone and uptake of UDP-glucuronic acid (UDP-GlcUA) by UDP-N-acetylglucosamine (UDP-GlcNAc) were examined. Alkaline pH-induced dissociation of the complexes was associated with the loss of UDP-GlcNAc-dependent stimulation of glucuronidation, suggesting that two functional states of UGTs with different kinetic parameters correspond to the monomer and oligomer form of UGTs in the membranes. The UDP-GlcNAc-dependent stimulation of UDP-GlcUA uptake into the microsomal vesicles also was affected by the extent of complex formation. These results suggest that complex formation of the UGT isozymes affects the UDP-GlcNAc-dependent stimulation of glucuronidation via stimulation of UDP-GlcUA uptake.

Biochemical basis for deficient paracetamol glucuronidation in cats: an interspecies comparison of enzyme constraint in liver microsomes

Unlike most other mammalian species, domestic cats glucuronidate phenolic compounds poorly and are therefore highly susceptible to the toxic side effects of many drugs, including paracetamol. In this study, we evaluated the role of enzyme constraint, a characteristic that limits the activity of all uridine 5'-diphosphoglucuronosyltransferase (UGT) enzymes, in the aetiology of this species-dependent defect of drug metabolism. Detergent activation experiments were performed using hepatic microsomes from cats (4), dogs (4), man (4), and 6 other mammalian species (1 liver each). In addition, we used microsomes from Gunn rats which are sensitive to paracetamol toxicity because of a genetic defect affecting all family 1 UGTs. Increase in paracetamol-UGT activity at optimum concentrations of detergent was used as an index of enzyme constraint. Native activity (measured in the absence of detergent) was less than one-sixth in cats compared with other species. Optimum detergent treatment tended to enhance rather than abolish this difference, however, indicating relatively lower levels of constraint of paracetamol-UGT in cats compared with other species. Similarly, detergent treatment failed to reduce the native activity difference between homozygous mutant and normal Gunn rats. Initially CHAPS (3-(3-cholamidopropyl)-dimethylammonio-1-propanesulphonic acid) was used as the detergent activator; in 3 of 4 microsomal preparations from man, however, inhibition rather than activation was observed at all detergent concentrations used. Studies were repeated using the non-ionic detergent, Brij 58 (polyoxyethylene 20-cetyl ether), which resulted in similar although more profound activation and no inhibition. We conclude that deficient paracetamol glucuronidation in cats does not result from increased paracetamol-UGT constraint in this species compared with other mammalian species. Other causes, such as differences in enzyme protein concentration or substrate affinity might be responsible.

The mathematics of radiation target analyses

Radiation target theory has been extended to complex biochemical systems. Mathematical analyses are presented for multiple forms of biological active proteins, for the presence of large inhibitors or activators, for compounds which regulate rate or affinity and for multiple-step reactions. Several predictions of these models have been verified experimentally.

Gender-related differences in the amount and functional state of rat liver UDP-glucuronosyltransferase

The basis for gender-dependent differences in rates of glucuronidation of xenobiotics is uncertain. To clarify this issue, the glucuronidation of p-nitrophenol was compared in liver microsomes from adult male and female rats. The activity of native UDP-glucuronosyltransferase was 47% higher in microsomes from male than from female rats. Immunoblotting of microsomal protein with anti-UDP-glucuronosyltransferase antiserum revealed 66% more immunoreactive protein in male microsomes. A kinetic method for measuring glucuronidating enzyme content confirmed the result of the immunoblot. Responses of UDP-glucuronosyltransferase to activation by palmitoyllysophosphatidylcholine or high pressure indicated that the activity of the enzyme was more latent in male than in female microsomes. Differences in enzyme latency could be due to differences in membrane structure. A comparison of microsomal fatty acid composition revealed significantly higher levels of oleic and linoleic acids and lower levels of stearic and docosahexaenoic acids in male than in female microsomes. The phospholipid composition, ratio of cholesterol:phospholipid, and membrane fluidity were similar in male and female microsomes. These results indicate that gender-dependent differences in UDP-glucuronosyltransferase activity are due to differences in both the amount and functional state of the enzyme.

Radiation inactivation analysis of microsomal UDP-glucuronosyltransferases catalysing mono- and diglucuronide formation of 3,6-dihydroxybenzo(a)pyrene and 3,6-dihydroxychrysene

Indirect evidence has suggested that multiple subunits of microsomal UDP-glucuronosyltransferases (UGTs) are involved in diglucuronide formation of diphenols of polycyclic aromatic hydrocarbons (Bock et al., Mol Pharmacol 42: 613-618, 1992). To substantiate this suggestion functional target sizes of UGTs catalysing these reactions were determined in microsomes in situ by radiation inactivation analysis. Target sizes of UGTs catalysing the glucuronidation of 1-naphthol and 6-hydroxychrysene were found to be 91 +/- 29 and 120 +/- 27 kDa, respectively. However, target sizes for mono- and diglucuronide formation of 3,6-dihydroxybenzo(a)pyrene were 118 +/- 33 and 218 +/- 24 kDa, respectively. Similarly, using 3,6-dihydroxychrysene as substrate target sizes of 109 +/- 21 and 101 +/- 23 kDa were found for 6-O-monoglucuronide and 3-O-monoglucuronide formation and a target size of 192 +/- 34 kDa observed for diglucuronide formation. Based on subunit molecular masses of 50-60 kDa for UGTs, these results suggest that UGTs involved in monoglucuronide formation of phenols may function as dimers. In contrast, UGTs involved in diglucuronide formation of diphenols of polycyclic aromatic hydrocarbons may function as tetramers in microsomes in situ.

Developmental changes in the amount and functional state of UDP-glucuronosyltransferase

The effect of postnatal development on the activity of liver microsomal UDP-glucuronosyltransferase was determined in male Wistar rats between 25 and 200 days of age using p-nitrophenol as aglycone. Enzyme activity (measured at 1.0 mM UDP-glucuronic acid, 0.05 mM p-nitrophenol) decreased 55% between 25 and 88 days of age and was constant thereafter. Treatment of microsomes with palmitoyl-lysophosphatidylcholine, which allows for an estimation of the amount of enzyme, showed approximately a four-fold decrease in enzyme concentration during the same period. This decrease was confirmed by Western blotting of microsomes with anti-UDP-glucuronosyltransferase antiserum. The fact that a nearly four-fold decline in enzyme concentration led to only a 55% decrease in activity indicates that there was an increase in activity per molecule of UDP-glucuronosyltransferase as the concentration of enzyme decreased. Treatment of microsomes with high pressure or detergent caused a greater extent of enzyme activation in microsomes prepared from 25 than 200 day old rats, suggesting that a fraction of the enzyme in older rats was activated in untreated microsomes. Fatty acid analysis of liver microsomal lipids during postnatal development revealed changes in docosahexaenoic acid (22:6) which correlated with levels of UDP-glucuronosyltransferase activity.

Effect of brief treatment at alkaline pH on the properties of UDP-glucuronosyltransferase

The kinetic properties of UDP-glucuronosyltransferase were measured after brief treatment of liver microsomes at alkaline pH, followed by assay with p-nitro-phenol as aglycone, at pH 7.5. Enzyme activity increased in a graded fashion as the pH of pretreatment was increased above 8.0, with apparent maximal activation of eight-fold for a pretreatment pH of 11.1. The pH for half maximal activation was 10.6. Brief treatment at alkaline pH prior to assay at pH 7.5 was associated too with a graded conversion of the kinetics of the enzyme from non-Michaelis-Menten to Michaelis-Menten at pH 11.7. Sensitivity to the allosteric modulator, UDP-N-acetylglucosamine decreased as the pH increased. A fifty percent loss of sensitivity to UDP-N-acetylglucosamine-induced activation occurred at pH 10.6. Thus, pretreatment at alkaline pH had irreversible effects on the properties of UDP-glucuronosyltransferase in microsomes. In order to establish the cause for the irreversibility of the changes induced by alkaline pH, microsomes were treated at pH 11.6 prior to purifying UDP-glucuronosyltransferase. Enzyme purified from alkali-treated and untreated microsomes had approximately the same specific activity. More importantly, responses to activation by lipids, and regeneration of allosteric properties were the same for both purified enzymes (from alkali-treated and control microsomes). Pure enzyme was not activated by pretreatment at alkaline pH. We interpret these data to mean that the irreversible effects of alkaline pH on the properties of UDP-glucuronosyltransferase in microsomes were not due to direct effects on the enzyme, but to how the enzyme interacted normally with molecules within the plane of the membrane.