On the binding of bilirubin and its structural analogues to hepatic microsomal bilirubin UDPglucuronyltransferase

Hepatic glucuronidation of the asymmetrical natural bilirubin molecule results in formation of two different positional isomers, bilirubin C-8 monoglucuronide and bilirubin C-12 monoglucuronide. In view of the existence of multiple isoforms of UDPglucuronyltransferase, which is the microsomal enzyme system responsible for bilirubin esterification, we performed kinetic analysis of microsomal glucuronidation of bilirubin and a number of its structural congeners to determine whether synthesis of the two monoglucuronide isomers involved two distinct substrate-binding sites or reflected two different modes of binding to a single catalytic site. Both isomers were found in all tested species (man, rat, guinea pig, sheep), but there were marked species differences in the C-8/C-12 ratio of monoglucuronide found in bile or formed by liver microsomes. Correspondence between in vivo and in vitro results for such regioselectivity of glucuronidation was excellent in each species. On the basis of our results of kinetic analysis of bilirubin esterification at variable pigment substrate concentrations and inhibition studies with alternative substrates, we postulate that both natural monoglucuronide isomers are synthesized at a single binding site. Possible mechanisms responsible for the markedly regioselective esterification of bilirubin by rat and sheep liver were investigated by study of glucuronidation of selected structural analogues of the pigment. Our results do not support explanations of regioselectivity of bilirubin glucuronidation in terms of (i) preferential binding of either the C-8- or C-12-containing dipyrrolic half of the asymmetrical bilirubin molecule or (ii) enantioselective complexation of bilirubin UDPglucuronyltransferase to one of the two chirality enantiomers of intramolecularly hydrogen-bonded bilirubin.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous esterification of bilirubin by liver microsomes. Evidence for an intramicrosomal pool of UDP-glucose and lumenal orientation of bilirubin UDP-glycosyltransferase

Conjugation of natural bilirubin (BR) depends on a hepatic microsomal UDP-glycosyltransferase using UDP-Glc, UDP-xylose, and predominantly UDP-GlcA. We found that esterification of BR occurred when washed intact microsomes derived from rat or guinea pig liver were incubated with BR in the absence of added UDP-sugar. This endogenous esterification was shown to lead predominantly to formation of the two positional isomers of BR monoglucoside and displayed the same regioselectivity as found for the BR monoglucosides formed by microsomes incubated with a saturating concentration of added UDP-Glc. This finding and absence of endogenous esterification in liver microsomes from mutant rats lacking BR UDP-glycosyltransferase activities demonstrated that endogenous esterification depended on UDP-glycosyltransferase and indicated, therefore, that UDP-Glc was present in the intact microsomal vesicles. With UDP-Glc added to the extramicrosomal incubation medium, BR glucosidation was markedly enhanced when the membrane permeability barrier was disrupted by pretreatment of the microsomes with detergent, sonication, or Staphylococcus aureus alpha-toxin. In contrast, such membrane disruption resulted in abolishment of endogenous esterification of BR, and a direct relationship was found between impairment of endogenous esterification and degree of vesicle disruption, suggesting that the UDP-Glc on which endogenous esterification depended was present in the lumenal space of the microsomes. Kinetic evidence and absence of an effect of increasing the microsomal concentration of dolichol-P-Glc (Dol-P-Glc) on endogenous esterification excluded direct or indirect involvement of Dol-P-Glc in the endogenous esterification reaction. Preincubation of intact microsomes with UDP-Glc or UDP-xylose at 37 degrees C, but not at 0 degrees C, led to expansion of the microsomal UDP-sugar pool on which endogenous esterification depended, suggesting that both UDP-sugars can enter the microsomal vesicles by a temperature-dependent mechanism. In contrast to these findings, no increase of BR esterification was detected when the microsomes had been preincubated at 37 degrees C with UDP-GlcA. We conclude that native, intact microsomes contain a lumenal pool of endogenous UDP-Glc and that BR UDP-glucosyltransferase and UDP-xylosyltransferase, by virtue of a lumenal orientation, have direct access to the postulated intramicrosomal pool of nucleotide sugar.

Assay of mannose-6-phosphatase in untreated and detergent-disrupted rat-liver microsomes for assessment of integrity of microsomal preparations

An accurate, precise, and convenient procedure was developed for measurement of the latency of the low-Km mannose-6-phosphatase activity for the purpose of assessment of the membrane permeability barrier in microsomes. This approach is based on previous work of Arion et al. [J. Biol. Chem. (1976) 251, 4901-4907] and consists of measurement of mannose-6-phosphatase activity in the untreated microsomal fraction and in the corresponding microsomes that are fully disrupted in order to eliminate the membrane permeability barrier. Complete disruption of rat liver microsomes was achieved by incubation for 60 min at 0 degree C in the presence of 4 mM zwitterionic detergent 3-[(3-cholamido-propyl)dimethyl-ammonio]-2-hydroxy-1-propane sulphonate (Chapso). That the microsomal membrane permeability barrier was eliminated under those conditions was suggested by the fact that the enzyme activation (up to 50-fold) produced by this pretreatment was at least as large as the effect of any other previously reported disruptive procedure. Disruption of the microsomes by Chapso or by ultrasonication markedly enhanced the thermolability of the mannose-6-phosphatase activity. In addition, exposure of the microsomes to high concentrations of Chapso produced enzyme inactivation that could be partially reversed by dilution of the detergent prior to assaying the enzymic activity. Investigation of these enzyme inactivation phenomena under various incubation conditions for disruption of the microsomes by Chapso and for subsequent assay of mannose-6-phosphatase activity in the presence of Chapso enabled us to define conditions under which instability of the enzyme was undetectable. Using these optimized procedures for disruption of microsomes and assay of hexose-6-P phosphohydrolase, we found that the low-Km mannose-6-phosphatase activity of untreated rat liver microsomes consistently was less than 5% of the total enzyme activity in the fully disrupted microsomes. Accurate and precise assay of the structural latency of mannose-6-phosphatase in membrane preparations must be performed under well-controlled conditions, with special attention to the marked thermolability of the enzyme in the presence of detergent, and is a prerequisite for using this approach for the purpose of assessing intactness of microsomal preparations.

Radioassay of UDP-glucuronyltransferase-catalysed formation of bilirubin monoglucuronides and bilirubin diglucuronide in liver microsomes

A radioassay for specific determination of the rates of UDP-glucuronic acid-dependent conversion of bilirubin into the two isomeric (C-8, C-12) bilirubin monoglucuronides and bilirubin diglucuronide is described and illustrated by its application to rat liver microsomes. The method is based on measurement of the relative amounts of radiolabel in unesterified bilirubin and its mono- and di-esterified reaction products after incubation with [14C]bilirubin as substrate. This analysis is performed by the alkaline-methanolysis procedure, combined with one of two t.l.c. systems developed in order to enhance the sensitivity, accuracy and precision of the radioassay. Results for rates of total bilirubin glucuronide formation obtained with the new assay and the standard enzyme assay based on the ethyl anthranilate diazo-method were identical. However, the sensitivity of the latter technique is approx. 10-fold lower than that of the radioassay.

Induction of hepatic glycogen synthesis by glucocorticoids is not mediated by insulin

Administration of 0.1 or 1 mg of prednisolone to fed mice caused a 5-fold activation of glycogen synthase in the liver after 3h, without significant changes in the circulating levels of glucose or insulin, or the hepatic concentration of cyclic AMP. Adrenalectomized fasted rats responded to cortisol (10 mg) with an increased glycaemia and a progressive activation of hepatic glycogen synthase after 2-4 h. but without an increase in the very low insulinaemia. These results are incompatible with the prevailing hypothesis that glucocorticoids provoke hepatic glycogen synthesis through an extra secretion of insulin. It is discussed that the acute effect of glucocorticoids is to inhibit rather than stimulate the release of insulin.

The role of glycogen synthase phosphatase in the glucocorticoid-induced deposition of glycogen in foetal rat liver

1. The mechanism that underlies the induction of glycogen synthesis in the foetal rat liver by glucocorticoids was reinvestigated in conditions where the accumulation of glycogen is either precociously induced with dexamethasone or inhibited by steroid deprivation. It appears that glucocorticoids act as the physiological trigger for glycogen synthesis by inducing both glycogen synthase (a known effect) and its activating enzyme, glycogen synthase phosphatase. 2. The activity of glycogen synthase phosphatase in adult liver stems from the interaction of two protein components [Doperé, Vanstapel & Stalmans (1980) Eur. J. Biochem. 104, 137--146]. Two independent experimental approaches indicate that the cytosolic 'S-component' is already well developed in the foetal liver before the onset of glycogen synthesis. The manifold glucocorticoid-dependent increase in synthase phosphatase activity during late gestation must be attributed to the specific development of the glycogen-bound 'G-component'.

Glycogen-synthase phosphatase activity in rat liver. Two protein components and their requirement for the activation of different types of substrate

Three subfractions of glycogen synthase b (termed b1, b2, b3) have been isolated from the glycogen fraction of dog liver on the basis of a different affinity for DEAE-cellulose. Their kinetic properties and chromatographic behaviour are compatible with the presence of an increasing number of phosphorylated sites from synthase b1 towards b3. Synthase phosphatase activity in rat liver stems from two heat-labile and trypsin-labile proteins. These components are conveniently prepared from the cytosolic fraction of glycogen-depleted liver; the 'G-component' of the phosphatase co-sediments with added particulate glycogen, whereas the 'S-component' remains in the supernatant. The G-component alone did not convert any available synthase b to the a form. The synthase phosphatase activity of the S-component was variable according to the actual type of substrate. When acting on synthase b2 and b3, the S-component had a low phosphatase activity that was increased 7-fold and 11-fold, respectively, upon addition of the G-component. Synthase b1, however, was efficiently activated by the S-component, and only 35% faster in the presence of both components. When the cytosolic fraction of glycogen-depleted livers was analysed by sucrose-gradient centrifugation a single peak of phosphatase activity (S20, W = 10.2 S; provisional Mr = 254000) was detected with synthase b2 as substrate. In addition to this peak, presumably an S-G complex, synthase b1 also identified free S-component of lower and heterogeneous molecular weight. Our results illustrate in general the influence of the type of synthase b on the detection of synthase phosphatase activity, and specifically may provide an explanation for some discrepant reports on the subcellular distribution of the enzyme.