Vanadate: A potent inhibitor of multifunctional glucose-6-phosphatase

Vanadate has been found to be a potent inhibitor of both the hydrolytic and synthetic activities of the multi- functional enzyme glucose-6-phosphatase (d-glucose-6-phosphatase phosphohydrolase, EC 3.1.3.9). The enzyme, when studied in both microsomal preparations and in situ using permeable isolated hepatocytes, is inhibited by micromolar concentrations of vanadate. The inhibition by vanadate is greater in detergent-treated than in untreated microsomes. In both the microsomal preparations and permeable hepatocytes, the inhibition by vanadate is competitive with the phosphate substrate and is greater for the phosphotransferase than the hydrolase activity of the enzyme. The K_I values of vanadate for carbamyl-phosphate : glucose phosphotransferase and glucose-6-phosphate phosphohydrolase determined with permeable hepatocytes are in good agreement with the values determined with detergent-dispersed microsomes. The previously described inhibition of glucose-6-phosphate phosphohydrolase by ATP (Nordlie, R.C., Hanson, T.L., Johns, P.T. and Lygre, D.G. (1968) Proc. Natl. Acad. Sci. USA 60, 590–597) can now be explained by the vanadium contamination of the commercially available ATP samples used. In contrast with glucose-6-phosphatase, hepatic glucokinase and hexokinase were not inhibited by vanadate. Physiological implications and utilitarian experimental applicability of vanadate as a selective metabolic probe, based on these observations, are suggested.     

On the nature of the interaction between 4,4'-diisothiocyanostilbene 2,2'-disulfonic acid and microsomal glucose-6-phosphatase. Evidence for the involvement of sulfhydryl groups of the phosphohydrolase

The effect of 4,4'-diisothiocyanostilbene 2,2'-disulfonic acid (DIDS) on microsomal glucose 6-phosphate hydrolysis has been reinvestigated and characterized in order to elucidate the topological and functional properties of the interacting sites of the glucose-6-phosphatase. The studies were performed on microsomal membranes, partially purified and reconstituted glucose-6-phosphatase preparations and show the following. (a) DIDS inhibits activity of the glucose-6-phosphatase of native microsomes as well as the partially purified glucose-6-phosphatase. (b) Inhibition is reversed when the microsomes and the partially purified phosphohydrolase, incorporated into asolectin liposomes, are modified with Triton X-114. (c) Treatment of native microsomes with DIDS and the following purification of glucose-6-phosphatase from these labeled membranes leads to an enzyme preparation which is labeled and inhibited by DIDS. (d) Preincubation of native microsomes or partially purified glucose-6-phosphatase with a 3000-fold excess of glucose 6-phosphate cannot prevent the DIDS-induced inhibition. (e) Inhibition of glucose-6-phosphatase by DIDS is completely prevented when reactive sulfhydryl groups of the phosphohydrolase are blocked by p-mecuribenzoate. (f) Reactivation of enzyme activity is obtained when DIDS-labeled microsomes are incubated with 2-mercaptoethanol or dithiothreitol. Therefore, we conclude that inhibition of microsomal glucose 6-phosphate hydrolysis by DIDS cannot result from binding of this agent to a putative glucose-6-phosphate-carrier protein. Our results rather suggest that inhibition is caused by chemical modification of sulfhydryl groups of the integral phosphohydrolase accessible to DIDS attack itself. An easy interpretation of these results can be obtained on the basis of a modified conformational model representing the glucose-6-phosphatase as an integral channel-protein located within the hydrophobic interior of the microsomal membrane [Schulze et al. (1986) J. Biol. Chem. 261, 16,571-16,578].     

Glucose-6-phosphate oxidation pathway in rat-liver microsomal vesicles. Stimulation under oxidative stress

The glucose-6-phosphate oxidation pathway present in microsomes was studied using intact microsomal membranes. The oxidation activity, which was measured by monitoring the formation of 14CO2 from [1-14C]glucose 6-phosphate, was greatly stimulated when azodicarboxylic acid bis(dimethylamide), methylene blue or cumene hydroperoxide was added to the assay mixture. Glutathione peroxidase and glutathione reductase are suggested to be involved in the oxidation reaction induced by these oxidizing reagents. We detected a significant activity of the glutathione reductase inherent to microsomes. The microsomal glutathione reductase is latent and requires detergent to reveal its activity. 4,4'-Diisothiocyanostilbene 2,2'-disulfonic acid (DIDS) inhibited the 14CO2 formation, but the inhibition was released by the addition of a detergent. Moreover, the inhibitory effect of DIDS was reversed by glucose 6-phosphate but not by mannose 6-phosphate. We conclude that the glucose-6-phosphate oxidation pathway in intact microsomes starts working under oxidative stress and that a transporter specific for glucose 6-phosphate is involved in the reaction.     

Evidence for glucose-6-phosphate transport in rat liver microsomes

The existence of glucose-6-phosphate transport across the liver microsomal membrane is still controversial. In this paper, we show that S3483, a chlorogenic acid derivative known to inhibit glucose-6-phosphatase in intact microsomes, caused the intravesicular accumulation of glucose-6-phosphate when the latter was produced by glucose-6-phosphatase from glucose and carbamoyl-phosphate. S3483 also inhibited the conversion of glucose-6-phosphate to 6-phosphogluconate occurring inside microsomes in the presence of electron acceptors (NADP or metyrapone). These data indicate that liver microsomal membranes contain a reversible glucose-6-phosphate transporter, which furnishes substrate not only to glucose-6-phosphatase, but also to hexose-6-phosphate dehydrogenase.     

Pentamidine activates T1 the hepatic microsomal glucose 6-phosphate transport protein of the glucose-6-phosphatase system

The mechanism of activation of hepatic microsomal glucose-6-phosphatase (EC 3.1.3.9) in vitro by pentamidine has been investigated in both intact and fully disrupted microsomes. The major effect of pentamidine is a 4.7-fold reduction in the K_m of glucose-6-phosphatase activity in intact diabetic rat liver microsomes. The site of action of pentamidine is T₁ the hepatic microsomal glucose 6-phosphate transport protein. The activation of T₁ by pentamidine may contribute to the disturbed blood glucose homeostasis see in many patients after administration of the drug pentamidine.     

The glucose-6-phosphate transport is not mediated by a glucose-6-phosphate/phosphate exchange in liver microsomes

A phosphate-linked antiporter activity of the glucose-6-phosphate transporter (G6PT) has been recently described in liposomes including the reconstituded transporter protein. We directly investigated the mechanism of glucose-6-phosphate (G6P) transport in rat liver microsomal vesicles. Pre-loading with inorganic phosphate (Pi) did not stimulate G6P or Pi microsomal inward transport. Pi efflux from pre-loaded microsomes could not be enhanced by G6P or Pi addition. Rapid G6P or Pi influx was registered by light-scattering in microsomes not containing G6P or Pi. The G6PT inhibitor, S3483, blocked G6P transport irrespectively of experimental conditions. We conclude that hepatic G6PT functions as an uniporter.     

Alterations of the microsomal glucose-6-phosphatase system evoked by ferrous iron- and haloalkane free-radical-mediated lipid peroxidation

Alterations of catalytic activities of the microsomal glucose-6-phosphatase system were examined following either ferrous iron- or halothane (CF3CHBrCl) and carbon tetrachloride (CCl4) free-radical-mediated peroxidation of the microsomal membrane. Enzyme assays were performed in native and solubilized microsomes using either glucose 6-phosphate or mannose 6-phosphate as substrate. Lipid peroxidation was assessed by the amounts of malondialdehyde equivalents formed. Regardless of whether the experiments were performed in the presence of NADPH/Fe3+, NADPH/CF3CHBrCl, or NADPH/CCl4, with the onset of lipid peroxidation, mannose-6-phosphatase activity of the native microsomes increased immediately, while further alterations in catalytic activities were only detectable when lipid peroxidation had passed characteristic threshold values: above 2 nmol malondialdehyde/mg microsomal protein, glucose-6-phosphatase activity of the native microsomes was lost, and at 10 nmol malondialdehyde/mg microsomal protein, glucose-6-phosphatase and mannose-6-phosphatase activity of the solubilized microsomes started to decline. It is concluded that the latter alterations are due to an irreversible damage of the phosphohydrolase active site of the glucose-6-phosphatase system, while the changes observed at earlier stages of microsomal lipid peroxidation may also reflect alterations of the transporter components of the glucose-6-phosphatase system. Virtually no changes in the catalytic activities of the glucose-6-phosphatase system occurred under anaerobic conditions, indicating that CF3CHCl and CCl3 radicals are without direct damaging effect on the glucose-6-phosphatase system. Further, maximum effects of carbon tetrachloride and halothane on lipid peroxidation and enzyme activities were observed at an oxygen partial pressure (PO2) of 2 mmHg, providing additional evidence for the crucial role of low PO2 in the hepatotoxicity of both haloalkanes.     

Changes in the glucose-6-phosphatase complex in hepatomas

Hepatomas tend to have a decreased glucose-6-phosphatase activity. We have observed phenotypic stability for this change in Morris hepatomas transplanted in rats. To determine if this decrease is selective for translocase functions or the hydrolase activity associated with glucose-6-phosphatase, we have compared activities in liver and hepatomas with glucose-6-phosphate or mannose-6-phosphate as substrates and with intact or histone-disrupted microsomes. In five out of seven subcutaneously transplanted rat hepatoma lines, the microsomal mannose-6-phosphatase activity was lower than in preparations from liver of normal or tumor-bearing rats. With liver microsomes and with most hepatoma microsomes, preincubation with calf thymus histones caused a greater increase in mannose-6-phosphatase than in glucose-6-phosphatase activity. In studies with liver and hepatoma microsomes there were similar increases in mannose-6-phosphatase activity with total calf thymus histones and arginine-rich histones. A smaller increase was seen with lysine-rich histones. The effect of polylysine was similar to the action of lysine-rich histones. There was only a small effect with protamine at the same concentration (1 mg/ml). Rat liver or hepatoma H1 histones gave only about half the activation seen with core nucleosomal histones. Our data suggested that microsomes of rat hepatomas tend to have decreased translocase and hydrolase functions of glucose-6-phosphatase relative to activities in untransformed liver. (Mol Cell Biochem122: 17–24, 1993)     

Vanadate: a potent inhibitor of multifunctional glucose-6-phosphatase

Vanadate has been found to be a potent inhibitor of both the hydrolytic and synthetic activities of the multifunctional enzyme glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase, EC 3.1.3.9). The enzyme, when studied in both microsomal preparations and in situ using permeable isolated hepatocytes, is inhibited by micromolar concentrations of vanadate. The inhibition by vanadate is greater in detergent-treated than in untreated microsomes. In both the microsomal preparations and permeable hepatocytes, the inhibition by vanadate is competitive with the phosphate substrate and is greater for the phosphotransferase than the hydrolase activity of the enzyme. The Ki values of vanadate for carbamyl-phosphate : glucose phosphotransferase and glucose-6-phosphate phosphohydrolase determined with permeable hepatocytes are in good agreement with the values determined with detergent-dispersed microsomes. The previously described inhibition of glucose-6-phosphate phosphohydrolase by ATP (Nordlie, R.C., Hanson, T.L., Johns, P.T. and Lygre, D.G. (1968) Proc. Natl. Acad. Sci. USA 60, 590-597) can now be explained by the vanadium contamination of the commercially available ATP samples used. In contrast with glucose-6-phosphatase, hepatic glucokinase and hexokinase were not inhibited by vanadate. Physiological implications and utilitarian experimental applicability of vanadate as a selective metabolic probe, based on these observations, are suggested.     

Modulation of the activity of hepatic glucose-6-phosphatase by methylthioadenosine sulfoxide.

Methylthioadenosine sulfoxide (MTAS), an oxidized derivative of the cell toxic metabolite methylthioadenosine has been used in elucidating the relevance of an interrelationship between the catalytic behavior and the conformational state of hepatic glucose-6-phosphatase and in characterizing the transmembrane orientation of the integral unit in the microsomal membrane. The following results were obtained: (1) Glucose 6-phosphate hydrolysis at 37 degrees C is progressively inhibited when native microsomes are treated with MTAS at 37 degrees C. In contrast, glucose 6-phosphate hydrolysis of the same MTAS-treated microsomes assayed at 0 degrees C is not inhibited. (2) Subsequent modification of the MTAS-treated microsomes with Triton X-114 reveals that glucose-6-phosphatase assayed at 37 degrees C as well as at 0 degrees C is inhibited. (3) Although excess reagent is separated by centrifugation and the MTAS-treated microsomes diluted with buffer before being modified with Triton the temperature-dependent effect of MTAS on microsomal glucose-6-phosphatase is not reversed at all. (4) In native microsomes MTAS is shown to inhibit glucose-6-phosphatase noncompetitively. The subsequent Triton-modification of the MTAS-treated microsomes, however, generates an uncompetitive type of inhibition. (5) Preincubation of native microsomes with MTAS completely prevents the inhibitory effect of 4,4'-diisothiocyanostilbene 2,2'-disulfonate (DIDS) as well as 4,4'-diazidostilbene 2,2'-disulfonate (DASS) on glucose-6-phosphatase. (6) Low molecular weight thiols and tocopherol protect the microsomal glucose-6-phosphatase against MTAS-induced inhibition. (7) Glucose-6-phosphatase solubilized and partially purified from rat liver microsomes is also affected by MTAS in demonstrating the same temperature-dependent behavior as the enzyme of MTAS-treated and Triton-modified microsomes. From these results we conclude that MTAS modulates the enzyme catalytic properties of hepatic glucose-6-phosphatase by covalent modification of reactive groups of the integral protein accessible from the cytoplasmic surface of the microsomal membrane. The temperature-dependent kinetic behavior of MTAS-modulated glucose-6-phosphatase is interpreted by the existence of distinct catalytically active enzyme conformation forms. Detergent-induced modification of the adjacent hydrophobic microenvironment additionally generates alterations of the conformational state leading to changes of the kinetic characteristics of the integral enzyme.     

Maintenance of luminal NADPH in the endoplasmic reticulum promotes the survival of human neutrophil granulocytes

The present study demonstrates the expression of hexose-6-phosphate dehydrogenase and 11 beta-hydroxysteroid dehydrogenase type 1 in human neutrophils, and the presence and activity of these enzymes in the microsomal fraction of the cells. Their concerted action together with the previously described glucose-6-phosphate transporter is responsible for cortisone-cortisol interconversion detected in human neutrophils. Furthermore, the results suggest that luminal NADPH generation by the cortisol dehydrogenase activity of 11 beta-hydroxysteroid dehydrogenase type 1 prevents neutrophil apoptosis provoked by the inhibition of the glucose-6-phosphate transporter. In conclusion, the maintenance of the luminal NADPH pool is an important antiapoptotic factor in neutrophil granulocytes.     

Rat liver glucose-6-phosphatase system: light scattering and chemical characterization

Glucose-6-phosphatase is a multicomponent system located in the endoplasmic reticulum, involving both a catalytic subunit (G6PC) and several substrate and product carriers. The glucose-6-phosphate carrier is called G6PT1. Using light scattering, we determined K(D) values for phosphate and glucose transport in rat liver microsomes (45 and 33mM, respectively), G6PT1 K(D) being too low to be estimated by this technique. We provide evidence that phosphate transport may be carried out by an allosteric multisubunit translocase or by two distinct proteins. Using chemical modifications by sulfhydryl reagents with different solubility properties, we conclude that in G6PT1, one thiol group important for activity is facing the cytosol and could be Cys(121) or Cys(362). Moreover, a different glucose-6-phosphate translocase, representing 20% of total glucose-6-phosphate transport and insensitive to N-ethylmaleimide modification, could coexist with liver G6PT1. In the G6PC protein, an accessible thiol group is facing the cytosol and, according to structural predictions, could be Cys(284).     

Membrane effects on hepatic microsomal glucose-6-phosphatase.

1) Rat liver microsomes exhibit only a weak hydrolyzing activity towards galactose 6-phosphate. Disruption of the microsomal vesicles does not change the apparent Michaelis constant for this substrate but enhances the apparent maximum velocity. 2) The inhibition of microsomal glucose-6-phosphatase (EC 3.1.3.9) by galactose 6-phosphate is of the competitive type in intact and disrupted microsomal vesicles, suggesting that both substrates are hydrolyzed by the same enzyme. 3) The high degree of latency found for the hydrolysis of galactose 6-phosphate compared to glucose 6-phosphate indicates the presence of a carrier for glucose 6-phosphate in the microsomal membrane. 4) Since glucose as a product is not trapped inside the microsomal vesicles, this sugar probably is able to penetrate the microsomal membrane.     

The pentose phosphate pathway in the endoplasmic reticulum

Approximately the same levels of six of the seven enzymes catalyzing reactions of the pentose phosphate pathway are in the cisternae of washed microsomes from rat heart, spleen, lung, and brain. Renal and hepatic microsomes also have detectable levels of these enzymes except ribulose-5-phosphate epimerase and ribose-5-phosphate isomerase. Their location in the cisternae is indicated by their latencies, i.e. requirement for disruption of the membrane for activity. In addition, transketolase, transaldolase, and glucose-6-phosphatase, a known cisternal enzyme, are inactivated by chymotrypsin and subtilisin only in disrupted hepatic microsomes under conditions in which NADPH-cytochrome c reductase, an enzyme on the external surface, is inactivated equally in intact and disrupted microsomes. The failure to detect the epimerase and isomerase in hepatic microsomes is due to inhibition of their assays by ketopentose-5-phosphatase. Xylulose 5-phosphate is hydrolyzed faster than ribulose 5-phosphate. A mild heat treatment destroys hepatic xylulose-5-phosphatase and glucose-6-phosphatase without affecting acid phosphatase. These results plus the established wide distribution of glucose dehydrogenase, the microsomal glucose-6-phosphate dehydrogenase, and its localization to the lumen of the endoplasmic reticulum suggest that most mammalian cells have two sets of enzymes of the pentose phosphate pathway: one is cytoplasmic and the other is in the endoplasmic reticulum. The activity of the microsomal pentose phosphate pathway is estimated to be about 1.5% that of the cytoplasmic pathway.     

Invalidity of Criticisms of the Deoxyglucose Method Based on Alleged Glucose-6-Phosphatase Activity in Brain

The observations made by Sacks et al. [Neurochem. Res. 8, 661-685 (1983)] on which they based their criticisms of the deoxyglucose method have been examined and found to have no relationship to the conclusions drawn by them. (1) The observations of Sacks et al. (1983) of constant concentrations of [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate, predominantly in the form of product, reflects only the postmortem phosphorylation of the precursor during the dissection of the brain in their experiments. When the brains are removed by freeze-blowing, the time courses of the [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate concentrations in brain during the 45 min after the intravenous pulse are close to those predicted by the model of the deoxyglucose method. (2) Their observation of a reversal of the cerebral arteriovenous difference from positive to negative for [14C]deoxyglucose and not for [14C]glucose after an intravenous infusion of either tracer is, contrary to their conclusions, not a reflection of glucose-6-phosphatase activity in brain but the consequence of the different proportions of the rate constants for efflux and phosphorylation for these two hexoses in brain and is fully predicted by the model of the deoxyglucose method. (3) It is experimentally demonstrated that there is no significant arteriovenous difference for glucose-6-phosphate in brain, that infusion of [32P]glucose-6-phosphate results in no labeling of brain, and that the blood-brain barrier is impermeable to glucose-6-phosphate. Glucose-6-phosphate cannot, therefore, cross the blood-brain barrier, and the observation by Sacks and co-workers [J. Appl. Physiol. 24, 817-827 (1968); Neurochem. Res. 8, 661-685 (1983)] of a positive cerebral arteriovenous difference for [14C]glucose-6-phosphate and a negative arteriovenous difference for [14C]glucose cannot possibly reflect glucose-6-phosphatase activity in brain as concluded by them. Each of the criticisms raised by Sacks et al. has been demonstrated to be devoid of validity.     

Microsomal membrane permeability and the hepatic glucose-6-phosphatase system. Interactions of the system with D-mannose 6-phosphate and D-mannose.

We have proposed that glucose-6-phosphatase (EC 3.1.3.9) is a two-component system consisting of (a) a glucose-6-P-specific transporter which mediates the movement of the hexose phosphate from the cytosol to the lumen of the endoplasmic reticulum (or cisternae of the isolated microsomal vesicle), and (b) a nonspecific phosphohydrolase-phosphotransferase localized on the luminal surface of the membrane (Arion, W.J., Wallin, B.K., Lange, A.J., and Ballas, L.M. (1975) Mol. Cell. Biochem. 6, 75-83). Additional support for this model has been obtained by studying the interactions of D-mannose-6-P and D-mannose with the enzyme of untreated (i.e. intact) and taurocholate-disrupted microsomes. An exact correspondence was shown between the mannose-6-P phosphohydrolase activity at low substrate concentrations and the permeability of the microsomal membrane to EDTA. The state of intactness of the membrane influenced the kinetics of mannose inhibition of glucose-6-P hydrolysis; uncompetitive and noncompetitive inhibitions were observed for intact and disrupted microsomes, respectively. The apparent Km for glucose-6-P was smaller with intact preparations at mannose concentrations above 0.3 M. Mannose significantly inhibited total glucose-6-P utilization by intact microsomes, whereas D-glucose had a stimulatory effect. Both hexoses markedly enhanced the rate of glucose-6-P utilization by disrupted microsomes. The actions of mannose on the glucose-6-phosphatase of intact microsomes fully support the postulated transport model. They are predictable consequences of the synthesis and accumulation of mannose-6-P in the cisternae of microsomal vesicles which possess a nonspecific, multifunctional enzyme on the inner surface and a limiting membrane permeable to D-glucose, D-mannose, glucose-6-P, but impermeable to mannose-6-P. The latency of the mannose-6-P phosphohydrolase activity is proposed as a reliable, quantitative index of microsomal membrane integrity. The inherent limitations of the use of EDTA permeability for this purpose are discussed.     

N-Bromoacetylethanolamine phosphate as a probe for the identification of a liver microsomal glucose-6-phosphate transporter peptide in rats and Ehrlich ascites tumor-bearing mice

Hepatic microsomal glucose-6-phosphatase is a multicomponent system composed of substrate/product translocases and a catalytic subunit. Previously we (Foster et al. (1996) Biochim. Biophys. Acta 12, 244-254) demonstrated that N-bromoacetylethanolamine phosphate (BAEP) is a time-dependent, irreversible inhibitor of glucose-6-phosphate hydrolysis in intact but not disrupted microsomes. We proposed that BAEP manifests its inhibitory effect by binding with a glucose-6-phosphate translocase protein of the glucose-6-phosphatase system. Here we provide additional evidence that BAEP inhibits glucose-6-phosphate transport in microsomal vesicles and utilize [(32)P]BAEP as an affinity label in the identification of a glucose-6-phosphate transport protein. In this study, we identify 51-kDa rat and mouse liver microsomal proteins involved in glucose-6-phosphate transport into and out of microsomal vesicles by utilizing (1) an Ehrlich ascites tumor-bearing mouse model, which displays a decreased sensitivity to the time-dependent inhibitory effect of BAEP, and (2) another glucose-6-phosphate translocase inhibitor, tosyl-lysine chloromethyl ketone, in conjunction with [(32)P]BAEP as an affinity label.     

Kinetic and immunologic evidence for the absence of glucose-6-phosphatase in early human chorionic villi and term placenta.

The existence of the enzyme glucose-6-phosphatase (G6Pase) in early and term human placenta was investigated by comparing the characteristics of placental microsomal glucose 6-phosphate (G6P) hydrolytic activity and liver G6Pase. Placental microsomes exhibited similar apparent Km values for G6P and beta-glycerophosphate in intact and deoxycholate-treated microsomes, heat stability at acidic pH, low latency of mannose 6-phosphate hydrolysis, very low activity of pyrophosphate: glucose phosphotransferase, and undetectable [U-14C]G6P transport into the placental microsomes, all of which indicated that specific G6Pase activity does not exist in placenta. Immunological evidence of the absence of both 36.5 kDa and T2 proteins, which represent the G6Pase catalytic protein and the phosphate/pyrophosphate transporter protein, respectively, confirmed that early and term human placenta are devoid of the multicomponent G6Pase enzyme.     

Topological studies on rat liver microsomal cholesterol ester hydrolase

Lateral and transversal distribution of cholesterol ester hydrolase activity in rat liver microsomal membranes has been studied. Total cholesterol ester hydrolase activity was found predominantly (75%) in rough microsomes though specific esterase activities were similar in rough and smooth microsomal fractions. The transversal asymmetry of the enzyme was examined using the criteria of protease sensitivity and latency of mannose-6-phosphate phosphatase. Cholesterol ester hydrolase resulted drastically inhibited by proteolysis with trypsin when microsomal integrity had been previously disrupted with sodium deoxycholate or sodium taurocholate. Under these conditions, most lumenal mannose-6-phosphate phosphatase activity was destroyed. However, cholesterol esterase was unaffected by preincubating microsomes with the detergent alone, which led to the complete expression of latent mannose-6-phosphate phosphatase or by preincubating them with trypsin, where less than a 15% of the lumenal mannose-6-phosphate phosphatase was lost. These findings suggest that cholesterol ester hydrolase activity is located on the lumenal surface of the hepatic microsomal vesicles.     

The kinetics of intact microsome glucose-6-phosphatase are sigmoid at physiologic glucose 6-phosphate concentrations

The kinetics of rat liver glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase, EC 3.1.3.9) were studied with intact and detergent-disrupted microsomes from normal and diabetic rats. Glucose-6-P concentrations employed (12 microM to 1.0 mM) spanned the physiologic range. With the enzyme of intact microsomes from both groups, plots of v versus [glucose-6-P] were sigmoid. Hanes plots (i.e. [glucose-6-P]/v versus [glucose-6-P]) were biphasic (concave upwards). A Hill coefficient of 1.45 was determined with substrate concentrations between 12 and 133 microM. Disruption of microsomal integrity abolished these departures from classic kinetic behavior, indicating that sigmoidicity may result from cooperative interaction of glucose-6-P with the glucose-6-phosphatase system at the substrate translocase specific for glucose-6-P. With the enzyme from normal rats the [glucose-6-P] at which the enzyme was maximally sensitive to variations in [glucose-6-P] (which we term "Smax"), determined from plots of dv/d [glucose-6-P] versus [glucose-6-P], was in the physiologic range. The Smax of 0.13 mM corresponded well with the normal steady-state hepatic [glucose-6-P] of 0.16 mM, consistent with glucose-6-phosphatase's function as a regulatory enzyme. With the diabetic enzyme, in contrast, values were 0.30 and 0.07 mM for the Smax and steady-state level, respectively. We suggest that the decreasing sensitivity of glucose-6-phosphatase activity to progressively diminishing glucose-6-P concentration, inherent in its sigmoid kinetics, constitutes a mechanism for the preservation of a residual pool of glucose-6-P for other hepatic metabolic functions in the presence of elevated concentrations of glucose-6-phosphatase such as in diabetes.