The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Reactivation of phospholipase A-inactivated enzyme by phospholipids and detergents

Specific modifications of the membrane fatty acid composition of human myotubes and their effects on the muscular sodium channels

The fatty acid (FA) composition of human myotube primary cultures was varied by modifications of the contents of FA in the culture medium. An incubation time of 18 h with a defined FA mixture resulted in the most effective alteration of the original FA pattern of the cells. The increases reached for the relative amounts of palmitic acid (16:0), linoleic acid (18:2) or arachidonic acid (20:4) were 3-5-fold. More than 50% of the extra FA were incorporated in the phospholipid fraction, the remaining share in the triglyceride fraction. Shorter incubation times resulted in less FA incorporation, longer incubation times raised the uptake of FA into the triacylglycerol fraction. For a study of the influence of the membrane modification on the function of the sodium channels, the myotubes were converted into myoballs. The sodium channel properties were then determined using the whole-cell clamp technique. The modified cultures showed no significant alterations in the time constants of activation and inactivation, in the voltage dependence of inactivation (h infinity curves) or in the average amplitudes of the sodium currents.

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.

Ox liver butyryl-coenzyme A synthetase--a subunit enzyme?

Butyryl-CoA synthetase extracted from untreated or freeze-dried liver mitochondria consisted of two fractions of Mr 40,000 and 46,000 as determined by gel permeation chromatography, the higher value being confirmed by sedimentation equilibrium. A red pigment having the characteristic of haem was associated with the enzyme. This appeared to be an artifact of isolation. A number of active bands were obtained on polyacrylamide gel electrophoresis and isoelectric focusing, and on treatment with sodium dodecylsulphate or 6 M guanidine hydrochloride both dissociation and aggregation of the enzyme fractions occurred. As no evidence of protease contamination or proteolytic action could be detected, it is suggested that the active enzymes contain more than one polypeptide chain.

The phospholipid-dependence of UDP glucuronosyltransferase. Purification, delipidation and reconstitution of microsomal enzyme from guinea-pig liver

In order to study the interaction of liver microsomal UDPglucuronosyltransferase and microsomal phospholipids under closely defined conditions, guinea-pig enzyme was purified to homogeneity (as judged by sodium dodecyl sulphate gel electrophoresis) by detergent-solubilisation, salt precipitation, chromatography on DEAE-cellulose and DEAE-Sephadex, and affinity chromatography on UDPglucuronosyl-diaminohexanyl--Sepharose 4B. The purified transferase, which catalysed the glucuronidation of p-nitrophenol with high specific activity, was associated with microsomal phospholipids, and phosphatidylcholine was the major species present; the transferase protein had a subunit molecular weight of about 55 000. The enzyme was almost completely inactivated by delipidation of the protein by hydroxyapatite chromatography and efficient reconstitution of high activity was observed only with fluid (microsomal and egg-yolk) phosphatidylcholines. These results confirm that microsomal UDPglucuronosyltransferase is phospholipid-dependent with a specific requirement for phosphatidylcholine.

Associative properties of butyryl-coenzyme A synthetase from ox liver mitochondria

Butyryl-coenzyme A synthetase (butyrate:CoA ligase (AMP-forming), EC 6.2.1.2) from an acetone-dried powder of ox liver mitochondria was found to have a molecular weight of approx. 40 000. The sedimentation equilibrium analysis suggested the presence in solution of higher molecular weight forms of the enzyme and these could also be obtained by extracting the enzyme from the mitochondrial powder in non-reducing conditions. The enzyme was inhibited by sulphydryl reagents, and was found to have at least one available thiol group/molecule. The relationship between enzymic activity and concentration was non-linear, and suggested that an inactive monomer-active dimer equilibrium was present. The 5--6-fold activation by bovine serum albumin required the presence of free thiol groups in the albumin and involved association of albumin with the enzyme.

Phospholipid-dependence of oestrone UDP-glucuronyltransferase and p-nitrophenol UDP-glucuronyltransferase

Hepatic UDP-glucuronyltransferase activity was resolved into two fractions, one exhibiting oestrone glucuronyltransferase activity and the other exhibiting p-nitrophenol glucuronyltransferase activity. Hydroxyapatite-column chromatography removed greater than 95% of the phospholipids from both preparations. The partially purified delipidated enzymes were essentially devoid of catalytic activity, but activities were restored by the addition of phospholipids or phosphatidylcholine mixtures containing various saturated and unsaturated fatty acids. Both oestrone and p-nitrophenol glucuronyl-transferase activities were reconstituted to similar degrees with the phosphatidylcholine mixtures. When purified phospholipids were tested, phosphatidylcholine and lysophosphatidylcholine were most effective in restoring activity, whereas phosphatidylethanolamine was the least effective. These results further suggest that oestrone and p-nitrophenol UDP-glucuronyltransferases are dependent on phospholipids for their activity.

The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Temperature-dependence of microsomal enzyme activity and thermotropic changes in membrane structure

Arrhenius plots of the non-latent UDP-glucuronyltransferase (p-nitrophenol acceptor) activity of guinea-pig microsomal membranes prepared with 154 mM-KCl were linear from 5 to 40 degrees C. Arrhenius plots for other microsomal preparations from guinea pig and rat liver that show various degrees of transferase latency, exhibited two linear regions intersecting at a sharp transition point near 20-25 degrees C. This discontinuity was abolished or greatly decreased when transferase latency was removed by treating the membranes with perturbants of phospholipid bilayer strucutre. The fluorescent probe N-phenyl-1-naphthyl-amine detected a thermotropic change in the fluidity of the phospholipid acyl chains of all the microsomal membrane preparations studied, at temperatures close to those of the Arrhenius-plot transitions. It is concluded that the thermotropic change in the structure of the membrane bilayer probably is a 'phase separation' or clustering of phospholipids, which affects a permeability barrier that restricts access of substrate to the transferase molecules.

Phospholipid dependence of UDP-glucuronyltransferase

Very extensive hydrolysis of phospholipids with pure Bacillus cereus phospholipase C at 5 degrees C greatly inhibited the maximum demonstrable rate of glucuronidation of p-nitrophenol by UDPglucuronyltransferase in guinea pig liver microsomes. Lysophosphatidylcholine restored much of the inhibited activity but non-phospholipid surfactants or hydrolysis of diglycerides failed to reactivate. Phospholipid depletion likewise inhibited o-aminophenol glucuronidation and phospholipids restored activity. It is concluded that glucuronyltransferase specifically requires phospholipids for optimal activity. It seems unlikely that these phospholipids only serve to dissolve aglycones, or that they are direct physiological regulators of the transferase. Instead, a permissive role is ascribed to phospholipids, allowing glucuronyltransferase to be regulated by other means.

The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Phospholipid depletion and re-activation of guinea-pig liver microsomal enzyme

More than 80% of the phospholipid component of guinea-pig liver microsomal membranes (prepared with 154mM-KCl) was removed by treatment with phospholipase A followed by extraction of the lysophosphatides and fatty acids produced with albumin. Delipidation strongly inactivated the highly active UDP-glucuronyltransferase of these preparations and activity was restored by mixtures of phosphatidylcholine and lysophosphatidylchlone. However, small quantities of lysophosphatides were still associated with the delipidated fractions after extraction with albumin and might have influenced the inactivation and re-activation observed. To eliminate these uncertainties, microsomal proteins and phospholipids were separated by gel filtration on Sephadex G-150 in the presence of cholate. This technique also strongly inactivated the enzyme but did not generate membrane-active phospholipid degradation products. High transferase activity was again restored to the delipidated protein by choline glycerophosphatides. These results confirm the view that the fully active form of microsomal UDP-glucuronyltransferase is phospholipid-dependent.

Observations on the accessibility of acceptor substrates to the active centre of UDP-glucuronosyltransferase in vitro

The partition coefficients between octanol and pH 7.4 buffer for eleven substrates of UDP-glucuronosyltransferase (EC 2.4.1.17) have been determined. They range between 1.1 and 690 in the order p-aminophenol less than phenol less than (o-aminobenzoic acid = o-aminophenol = p-aminobenzoic acid) less than p-nitrophenol less than 4-methylumbelliferone less than mercaptobenzothiazole less than harmol less than phenolphthalein less than 1-naphthol. The effect of Triton X-100, used as a model membrane pertubant, on the enzyme activity of UDP-glucuronosyltransferase in rat liver homogenates towards these substrates was determined and compared with the partition coefficients. Enzyme activities towards p-aminophenol and phenol were decreased by Triton X-100, the enzyme activities towards other acceptor substrates were enhanced maximally with 0.025% (w/v) Triton. "Native" enzyme activity (except for amino containing compounds) and activation could be related to partition coefficient of the substrate. An increase in lipid solubility resulted in reduced enzyme activity in untreated homogenates and greater activation. These results suggest UDP-glucuronosyltransferase lies behind a partially lipid-impenetrable abrrier and it is suggested that this barrier is broken up by membrane perturbants to permit free access of the more lipid-soluble substrates. In addition, the formation in vitro of a glucuronide from mercaptobenzothiazole was demonstrated.

Effect on uridine diphosphate glucuronyltransferase activity of depleting and restoring phospholipids to guinea-pig liver microsomal preparations

Phospholipid depletion substantially inhibited the maximum demonstrable activities of the forward (glucuronidation) and reverse reactions of UDP-glucuronyltransferase towards p-nitrophenol in guinea-pig liver microsomal preparations. Dispersions of liver phospholipids restored activity, whereas non-phospholipid amphipaths failed to do so effectively. These results suggest that the system is probably phospholipid-dependent rather than conformationally constrained by phospholipids.

The phospholipid-dependence of uridine diphosphate glucuronyltransferase. Effect of protein deficiency on the phospholipid composition and enzyme activity of rat liver microsomal fraction

After force-feeding a protein-free diet to male rats for 5-7 days a substantial (2.4-fold) increase in the specific activity of the liver microsomal enzyme UDP-glucuronyltransferase (EC 2.4.1.17) was observed. A similar activation of the enzyme occurred when rats were fed on a low-protein (5%, w/w, casein) diet for 60 days. Although both the short- and long-term protein-deficient diets decreased the contents of microsomal protein and phospholipid in liver tissue they did not significantly alter the ratio of these major membrane components. Protein deficiency profoundly altered the phospholipid composition of microsomal membranes. The most striking difference in microsomal phospholipid composition between control and protein-deficient rats was their content of lysophosphatides. Whereas microsomal membranes from protein-deficient rats contained significant proportions of lysophosphatidylcholine and lysophosphatidylethanolamine very little or no lysophosphatides were detected in control preparations. Pretreatment of microsomal fractions from normal rats with phospholipase A markedly increased their UDP-glucuronyltransferase activity as did their pretreatment with lysophosphatidylcholine. It is concluded that the quantities of lysophosphatides present in microsomal membranes from protein-deficient rats were sufficient to have caused the increased UDP-glucuronyltransferase activities of these preparations. Evidence is presented suggesting that these changes in microsomal phospholipid composition and UDP-glucuronyltransferase activity caused by protein deficiency reflect changes that occur in vivo. The possible physiological significance of these findings is discussed.

Properties of a 5'-nucleotidase purified from mouse liver plasma membranes

1. Extraction of a mouse liver plasma-membrane fraction with a detergent buffer, N-dodecylsarcosinate-Tris buffer (sarcosyl-Tris buffer), solubilized 90% of the protein and 70% of the 5'-nucleotidase activity. 2. The proteins of the sarcosyl-Tris buffer extract were fractionated by a rate-zonal centrifugation in a sucrose-detergent gradient. The major protein peak sedimented ahead of phospholipids, which mainly remained in the overlay. Glycoproteins were separated ahead of the protein peak. 3. The 5'-nucleotidase activity peak was associated with 5% of the protein applied to the gradient, and contained relatively few protein bands. 4. The 5'-nucleotidase was purified further by gel filtration on Sepharose and Sephadex columns equilibrated with sarcosyl-Tris buffer, to give a single glycoprotein band on sodium dodecyl sulphate-polyacrylamide-gel electrophoresis. The purified enzyme was lipid-free. 5. Electrophoresis in polyacrylamide gels in sarcosyl-Tris buffers showed that the enzymic activity was coincident with the protein band. 6. The molecular weight suggested for the enzyme activity by gel filtration or centrifugation in sucrose gradients was 140000-150000. Sometimes, a minor enzyme peak of lower molecular weight was obtained. 7. Polyacrylamide-gel electrophoresis in sodium dodecyl sulphate indicated that as the polyacrylamide concentration was increased from 5 to 15%, the apparent molecular weight of the enzyme decreased from 130000 to 90000. 8. The evidence that 5'-nucleotidase is composed of two active and similar, if not identical, glycoprotein subunits and the role of detergent in effecting the separation of membrane proteins and glycoproteins are discussed. 9. Substrate requirements, pH optima and the nature of inhibition by an analogue of adenosine diphosphate are reported.

Assay and properties of dititonin-activated bilirubin uridine diphosphate glucuronyltransferase from rat liver

1. The bilirubin UDP-glucuronyltransferase assay described by Van Roy & Heirwegh (1968) has been improved. 2. Extraction of final azo-derivatives is rendered more simple and efficient by thorough emulsification and by cooling. 3. Pretreatment of homogenates and cell fractions with digitonin increases the sensitivity of the assays and gives less variable results than those with untreated preparations. The activation procedure is flexible. 4. Blank values (obtained from incubation mixtures from which activating bivalent metal ion and UDP-glucuronic acid were omitted) are low. No endogenous conjugate formation could be detected except with untreated, fresh liver homogenates. Control incubation mixtures containing the latter preparations are preferably kept at 0 degrees C. 5. With activated microsomal preparations, rates of breakdown of UDP-glucuronic acid (as monitored by release of P(i)) were low. Little if any increase in enzyme activity was found when UDP-N-acetylglucosamine was included in the incubation mixtures. 6. Slight deviation from Michaelis-Menten kinetics with respect to bilirubin observed at low substrate concentrations is probably related to the use of binding protein in the assay mixtures. Michaelis-Menten kinetics were followed with respect to UDP-glucuronic acid. Part of the enzyme in microsomal preparations from rat liver functioned independently of added bivalent metal ions. Mn(2+) was slightly more, and Ca(2+) somewhat less, stimulatory than Mg(2+). The Mg(2+)-dependent fraction showed Michaelis-Menten kinetics with respect to the added Mg(2+). 7. The enzyme activities found were higher than values reported in the literature for untreated or purified preparations from rat liver. They were above reported values of the maximal biliary excretion rate of bilirubin.

Enzymic transfer of glucose and xylose from uridine diphosphate glucose and uridine diphosphate xylose to bilirubin by untreated and digitonin-activated preparations from rat liver

1. Digitonin-treated and untreated homogenates, cell extracts and washed microsomal preparations from liver of Wistar R rats are capable of transferring sugar from UDP-glucose or UDP-xylose to bilirubin. No formation of bilirubin glycosides occurred with UDP-galactose or d-glucose, d-xylose or d-glucuronic acid as the sources of sugar. 2. Procedures to assay digitonin-activated and unactivated bilirubin UDP-glucosyltransferase and bilirubin UDP-xylosyltransferase were developed. 3. In digitonin-activated microsomal preparations the transferring enzymes had the following properties. Both enzyme activities were increased 2.5-fold by pretreatment with digitonin. They were optimum at pH6.6-7.2. Michaelis-Menten kinetics were followed with respect to UDP-glucose. In contrast, double-reciprocal plots of enzyme activity against the concentration of UDP-xylose showed two intersecting straight-line sections corresponding to concentration ranges where either bilirubin monoxyloside was formed (at low UDP-xylose concentrations) or where mixtures of both the mono- and di-xyloside were synthesized (at high UDP-xylose concentrations). Both enzyme activities were stimulated by Mg(2+); Ca(2+) was slightly less, and Mn(2+) slightly more, stimulatory than Mg(2+). Of the activities found in standard assay systems containing Mg(2+), 58-78% (substrate UDP-glucose) and 0-38% (substrate UDP-xylose) were independent of added bivalent metal ion. Double-reciprocal plots of the Mg(2+)-dependent activities against the concentration of added Mg(2+) were linear. 4. In comparative experiments the relative activities of liver homogenates obtained with UDP-glucuronic acid, UDP-glucose and UDP-xylose were 1:1.5:2.7 for untreated preparations and 1:0.29:0.44 after activation with digitonin. 5. Bilirubin UDP-glucuronyltransferase was protected against denaturation by human serum albumin, whereas bilirubin UDP-xylosyltransferase was not. 6. Digitonin-treated and untreated liver homogenates from Gunn rats were inactive in transferring sugar to bilirubin from UDP-glucuronic acid (in agreement with the work of others), UDP-glucose or UDP-xylose.