Evidence that UDP-glucuronyltransferase in liver microsomes at 37 degrees C is in a gel phase lipid environment

Characterization of a new class of inhibitors of the recombinant human liver UDP-glucuronosyltransferase, UGT1*6

The inhibitory effect of a series of novel structurally related compounds on the human UDP-glucuronosyltransferase UGT1*6 stably expressed in a V79 cell line was investigated. The inhibitors contain a lipophilic N-acyl phenylaminoalcohol residue and a uridine moiety connected by a spacer varying for each compound. The effects of these compounds on the glucuronidation reaction measured with 4-methylumbelliferone as substrate were determined. The best inhibitor of the series, D-DPMSU, had an IC50 of 39 microM in the assay conditions. Low Ki values were found toward both UDP-glucuronic acid and 4-methylumbelliferone (17 and 21 microM, respectively). The inhibition was competitive toward both substrates. A similar strong and competitive inhibitory effect was observed with two other inhibitors, DHPASU and DHPASiU. Another compound, D-DPASiU, showed a pure competitive inhibition towards UDP-glucuronic acid, but a non-competitive inhibition towards the acceptor substrate. These data and the optimization of the structures of the inhibitors by molecular modeling suggest that D-DPMSU and DHPASiU compounds may be transition state analog inhibitors of the recombinant UGT1*6 enzyme.

Lipid domains in model and biological membranes

Lipid domains that occur within biological of model membranes encompass a variety of structures with very different lifetimes. The separation of membrane lipids into compositional domains can be due to lateral phase separation, immiscibility within a single phase, or interaction of lipids with integral or peripheral proteins. Lipid domains can affect the extent and rate of reactions in the membrane and provide sites for the activity of specialized proteins. Domains are likely to be involved in the process of lipid sorting to various cellular membranes, as well as in other processes which involve membrane budding or invagination.

Analysis of the interaction uridin 5'-diphosphoglucuronic acid with intestinal bilirubin UDP-glucuronyltransferase

1. Bilirubin UDP-glucuronyltransferase Michaelis-Menten kinetic parameters for UDP-glucuronic acid were studied using native and digitonin activated microsomes obtained from rat intestinal mucosa. 2. The intestinal enzyme showed a lower apparent Vmax compared with the hepatic enzyme in both native and activated microsomes; digitonin pretreatment enhanced Vmax 4 times in the former enzyme and 2 times in the latter. 3. The affinity of UDP-glucuronic acid for the intestinal enzyme was about 2 times lower than that for the hepatic enzyme and it was not substantially modified by detergent neither in the intestine nor in the liver. 4. The lipid analysis of intestinal and hepatic microsomes showed that the former present a higher content of cholesterol and a lower phosphatidylcholine/sphingomyelin ratio than the latter, accordingly the estimation of membrane fluidity using the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene indicated that intestinal microsomes are more "rigid" than the hepatic ones. 5. These characteristics would provoke a restrictive milieu surrounding the enzyme that modifies its kinetic properties thus limiting its participation in the metabolism of bilirubin.

Selective induction of bilirubin UDP-glucuronosyl-transferase by perfluorodecanoic acid

Differential effects of perfluorodecanoic acid (PFDA) on rat liver UDP-glucuronosyltransferase isoenzymes have been observed after a single i.p. administration of the compound to young male Sprague-Dawley rats. (1) Bilirubin glucuronidation was induced 2-fold. The induced state was stable for at least 3 weeks. (2) Glucuronidation of 1-naphthol, morphine and testosterone was decreased to half of the control values. These decreases were maximal after 12 days but all three activities returned to normal levels after 3 weeks. (3) Immunoblotting experiments indicated that the differential effects of PFDA on UDP-glucuronosyltransferase activities were due to modulation of enzyme protein concentrations rather than activation/inactivation mechanisms. With respect to its influence on UDP-glucuronosyltransferase isoenzymes, PFDA may be classified as a clofibrate-type inducer. The persistence of the induction after a single application however is unique among peroxisome proliferators and therefore PFDA may be a useful, elective inducer of bilirubin glucuronidation.

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.

Cross-linking of phosphatidylethanolamine neighbors with dimethylsuberimidate is sensitive to the lipid phase

Dimethylsuberimidate was reacted with aqueous dispersions of dipalmitoylphosphatidylethanolamine, dimyristoylphosphatidylethanolamine, dilauroylphosphatidylethanolamine, and dielaidoylphosphatidylethanolamine at pH 10 and at pH 8. The amount of amidine dimer formation was about four times greater above the gel-to-fluid phase transition of each lipid than below the transition. The transition temperature of each phosphatidylethanolamine, measured by steady-state fluorescence anisotropy of cis-parinaric acid, was lower at pH 10 than at pH 8 or in water. The ability of dimethylsuberimidate to discriminate between phosphatidylethanolamines in the fluid and gel phases should allow use of this reagent to identify phosphatidylethanolamine species within the gel or fluid lipid phase.

Effect of phospholipids on the thermal stability of microsomal UDP-glucuronosyltransferase

The GT2P isoform of microsomal UDP-glucuronosyltransferase from pig liver is a lipid-dependent enzyme. The data in the present work indicate that, in addition to regulation of activity, the thermal stability of the enzyme also is modulated by the acyl chain composition of phosphatidylcholines (PC) used to reconstitute the activity of pure enzyme. There was a reversible, temperature-dependent change in the state of the pure enzyme to an inactive form with onset at T greater than 38 degrees C, depending on the environment of the enzyme. The midpoint for the transition shifted from 39.8 degrees C for enzyme in a bilayer of distearoylphosphatidylcholine (DSPC) to 47.5 degrees C for enzyme in a bilayer of 1-stearoyl-2-oleoylphosphatidylcholine (SOPC). For all lipids, the transition from a catalytically active to an inactive form of the enzyme was associated with large compensating changes in H and S. Lipid-induced stabilization of the active form of UDP-glucuronosyltransferase at T greater than 37 degrees C was associated with decreases in delta H and delta S, but the decreases in delta S were larger, indicating that lipid-induced stabilization of the active form of the enzyme was entropic. The transition between the active and inactive forms of the enzyme was too rapid in either direction to measure in a standard spectrophotometer. In addition to reversible inactivation of the enzyme, there was a slower irreversible, temperature-dependent inactivation. The rate of this process depended on the acyl chains of the phosphocholines interacting with the enzyme. However, there was no obvious correlation between the structures of lipids that stabilized the different inactivation reactions.

Glucosyltransferase activity in mitochondria. Biosynthesis of glucosyl-phosphoryl-dolichol in inner mitochondrial membranes

1. Inner mitochondrial membranes are able to transfer [14C]glucose from UDP-[14C]glucose onto dolichylmonophosphate. 2. Synthesis of dolichyl-phosphoryl-glucose takes place only in the presence of exogenous dolichyl-monophosphate loaded into phospholipid vesicles. 3. Neutral phospholipids interact preferentially with the membrane-bound enzyme. The effect of phospholipids is not related to the length of fatty acid chains but a correlation between the activation and the degree of unsaturation of fatty acid chains has been found. 4. This enzyme required divalent cations for activity. Such a requirement might be related to lipid-protein interactions which favour a suitable conformation of glycosyltransferase.

Effects of prochlorperazine on the function of integral membrane proteins

We have studied the effects of prochlorperazine on the activities of UDP-glucuronosyltransferase and glucose-6-phosphatase (glucose-6-P'ase) in rat liver microsomes. The activity of UDP-glucuronosyltransferase was increased in a graded fashion by addition of prochlorperazine. Maximal stimulation occurred at 1 mg prochlorperazine to 2 mg microsomal protein, which resulted in a 6-fold increase in activity. However, with smaller concentrations of drug, there was a time-dependent increase in the activity of UDP-glucuronosyltransferase. Sensitivity of UDP-glucuronosyltransferase to activation by UDP-N-acetylglucosamine was lost after treatment of microsomes with prochlorperazine. These results indicate that prochlorperazine causes a profound reorganization of the interactions between lipids and enzyme since the activity and allosteric properties of UDP-glucuronosyltransferase are known to depend on interactions with lipids in a gel phase. Glucose-6-P'ase also was activated in a graded fashion by prochlorperazine; 1 mg of drug/2 mg microsomal protein resulted in a 60% increase in activity. The temperature-dependent instability of glucose-6-P'ase was increased by treatment of microsomes with prochlorperazine and could be prevented only partially by substrate. We conclude that prochlorperazine disrupts the structural organization between lipids and proteins in microsomal membranes, altering thereby the activity and regulation of at least two different integral membrane proteins.

Alpha-lactalbumin acts as a bimodal regulator of rat parotid acinar cell growth

Isoproterenol, a beta-adrenergic receptor agonist, causes hypertrophy and hyperplasia of the rat parotid gland. The stimulation of parotid acinar cells to a growth phase is accompanied by a cell surface localization of the enzyme 4 beta-galactosyltransferase. Alpha-lactalbumin, a specific modifier protein for 4 beta-galactosyltransferase, when given subsequent to the initiation of isoproterenol treatment and the commencement of parotid enlargement, resulted in a termination of gland hypertrophy and DNA synthesis. Gland size did not, however, return to control levels with the continued injection of isoproterenol and alpha- lactalbumin. In contrast, the injection of alpha-lactalbumin in neonatal rats (7-14 days post-partum) stimulated parotid gland hypertrophy and DNA synthesis. This treatment also lead to the precocious expression of the major parotid gland salivary enzyme, amylase.

Cholesterol-dependent modification of microsomal dynamics and UDPglucuronyltransferase kinetics

The effect of both in vitro incorporation and removal of cholesterol in guinea pig liver microsomes on the lipid composition, dynamic properties of the membrane, and kinetic constants of UDPglucuronyltransferase was studied. No significant changes either in the fatty acid composition or in the distribution of phospholipid classes were observed upon cholesterol incorporation and removal. Lateral and rotational mobility measured by the efficiency of pyrene excimer formation and fluorescence of 1,6-diphenylhexatriene decreased with cholesterol incorporation and increased in parallel to cholesterol removal. These changes were associated with alterations in the kinetic properties of UDPglucuronyltransferase. Whereas Vmax increased, the Km of the different steps of the reaction decreased with cholesterol incorporation. The negative homotropic effect and apparent cooperativity of UDP-glucuronic acid decreased when cholesterol was incorporated and increased after cholesterol removal. Moreover, the UDP-N-acetylglucosamine-dependent activation of the enzyme decreased in correlation with an increase of cholesterol concentration in microsomes. It has been demonstrated that both the shift of the non-Michaelian kinetics of the enzyme to Michaelian and the decrease of the UDP-N-acetylglucosamine-dependent activation of the enzyme are evoked by a change of the physical state of the UDPglucuronyltransferase milieu from a gel phase to a liquid-crystalline phase. Therefore, we must admit that cholesterol incorporation in the microsomes while producing an increased packing of the bulk lipids would also cause the separation of more fluid phospholipids, which increase the proportion of molecules in the liquid-crystalline state within the enzyme environment.

Interface between membrane biology and clinical medicine

Many enzymes that are embedded within membranes of cells are sensitive to the chemical and physical properties of the lipid components of the membrane. Because of this, the functions of these integral membrane-bound enzymes can be regulated to some extent by changes within the lipid portions of biologic membranes. That the functions of membrane-bound proteins can be manipulated by modifications of their intimate environment is not surprising. It is well known, for example, that the properties of the surrounding aqueous phase can modulate the function of proteins that are soluble in the cytosol of cells. In contrast, whereas significant changes in the chemical composition and physical properties of the aqueous portion of the cell (e.g., ionic strength and pH) are not allowed, normally tolerable fluctuations of diet appear to influence significantly the composition and properties of the lipid portions of intracellular membranes to the extent of altering the function of some membrane-bound enzymes. In addition, it appears that changes of this type can be induced by diseases that alter dietary intake and/or intermediary metabolism. In other words, it is likely that the functions of at least some integral membrane proteins can be manipulated in patients. Such manipulations may prove to be efficacious. Alternatively, the importance of diet and processes of intermediary metabolism for altering the course of certain diseases may not be fully appreciated. It is worthwhile to review, therefore, current ideas of how the lipid portion of a membrane interacts with integral proteins. The property of the lipids that appears to be most important in this regard is their viscosity. The types of manipulations of function of membrane-bound enzymes that can be achieved are illustrated by in vitro effects secondary to varying the lipids used to reconstitute pure, delipidated forms of these enzymes. The functions of pure delipidated enzymes are discussed for the hepatic drug-metabolizing enzyme uridine diphosphoglucuronyltransferase. In addition, data are presented to indicate that the function of this enzyme can be modified extensively in intact animals by changing the diet.

Effect of dietary cholesterol on microsomal membrane composition, dynamics and kinetic properties of UDPglucuronyl transferase

The effect of cholesterol administration in vivo on the lipid composition, dynamic properties of the microsomal membrane of guinea pig livers and the kinetic properties of UDPglucuronyl transferase were studied. Cholesterol administration in the diet evoked an increase of microsomal cholesterol, but no significant changes in the fatty-acid composition of total lipids or of each phospholipid class. Instead, the phosphatidylethanolamine, phosphatidylcholine molar ratio of the membrane was markedly decreased from 0.57 to 0.38. This decline was not enough to counterbalance the overall 'ordering' effect of cholesterol and consequently, the fluorescence anisotropy of the membranes labeled with 1,6-diphenylhexatriene was increased. The lateral diffusion evaluated by measuring the pyrene excimer formation was decreased by the cholesterol incorporation. These physical changes were associated with changes in the kinetic properties of UDPglucuronyl transferase: Vmax increased, while the Km of the different steps of the reaction decreased in the modified microsomes. Furthermore, a shift of the non-michaelian kinetics to michaelian, equivalent to a decrease of a negative homotropic effect and apparent cooperativity of UDPglucuronic acid was observed since the Hill coefficient changed, approaching 1. A non-michaelian kinetics of this enzyme is an indication of boundary lipids in the gel phase and a shift to michaelian, a change of the surrounding lipids to a liquid-crystalline structure. In consequence, our results suggest that cholesterol incorporation in the microsomal membrane while producing a condensing effect of bulk lipids would produce an opposite effect on the UDPglucuronyl transferase boundary lipids.

Glycosyltransferase activities in liver mitochondria. Phospholipid-dependence of inner membrane mannosyltransferase

The role of phospholipids in the activity of inner mitochondrial mannosyltransferase was investigated. This enzyme catalyzes the direct transfer from GDP-mannose to lipidic acceptor. Inner mitochondrial membranes from purified mice liver mitochondria are prepared by digitonin treatment. Swelling of mitoplasts leads to the formation of inner membrane vesicles, which are then purified on a discontinuous sucrose gradient. The validity of this fractionation procedure is controlled by measurements of specific enzymatic activities and by electron microscopy. Measurement of mannosyltransferase activity in native inner mitochondrial membranes is unsuccessful, even in the presence of exogenous dolichyl monophosphate. Treatment of inner membranes with specific phospholipid liposomes in the presence of exogenous dolichyl monophosphate is essential in order to measure this enzymatic activity. Addition of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and cardiolipin in the presence of Mg2+ results in a high degree of activation of the mannosyltransferase system. Maximal enzymatic activity is obtained with an approximate 3:7 mass ratio of exogenous phospholipid to inner membrane proteins. These experiments establish that sensitivity to activation by phospholipids is an inherent property of inner membrane mannosyltransferase. Another approach to this problem was to reconstitute an in vivo lipidic environment of the inner membrane. The results of this procedure suggest that the activity of inner mitochondrial mannosyltransferase may be subject to modulation by outer membrane lipidic extract treatment.