Investigations on the possible involvement of phospholipids in the glucose-6-phosphate transport system of rat-liver microsomal glucose-6-phosphatase

Kinetic properties of the glucose 6-phosphatase of the liver from arthritic rats

According to previous reports, adjuvant-induced arthritic rats present reduced activities of the hepatic glucose 6-phosphatase. A kinetic study was done in order to characterize this phenomenon. Microsomes were isolated from livers of arthritic and control rats (Holtzman strain) and the glucose 6-phosphatase was measured at various temperatures (13-37 degrees C) and glucose 6-phosphate concentrations. Irrespective of the temperature, the enzyme from arthritic rats presented a reduction of both V(max) and K(M). Detergent treatment of liver microsomes from control rats increased the activity, but no increase was found when microsomes from arthritic rats were treated in the same way. The mannose 6-phosphatase activity of detergent-treated microsomes from arthritic rats was only 25% of the activity found with detergent-treated microsomes from control rats. Without detergent treatment, the mannose 6-phosphatase activities of both control and arthritic rats were minimal. The activation energy, derived from V(max), was not changed by arthritis. In vivo arthritic rats presented higher hepatic glucose 6-phosphate concentrations, a phenomenon that is consistent with a reduced activity of glucose 6-phosphatase. It was concluded that in arthritic rats, the hydrolase is probably reduced, without a similar change in the translocase activity.

New lessons in the regulation of glucose metabolism taught by the glucose 6-phosphatase system

The operation of glucose 6-phosphatase (EC 3.1.3.9) (Glc6Pase) stems from the interaction of at least two highly hydrophobic proteins embedded in the ER membrane, a heavily glycosylated catalytic subunit of m 36 kDa (P36) and a 46-kDa putative glucose 6-phosphate (Glc6P) translocase (P46). Topology studies of P36 and P46 predict, respectively, nine and ten transmembrane domains with the N-terminal end of P36 oriented towards the lumen of the ER and both termini of P46 oriented towards the cytoplasm. P36 gene expression is increased by glucose, fructose 2,6-bisphosphate (Fru-2,6-P2) and free fatty acids, as well as by glucocorticoids and cyclic AMP; the latter are counteracted by insulin. P46 gene expression is affected by glucose, insulin and cyclic AMP in a manner similar to P36. Accordingly, several response elements for glucocorticoids, cyclic AMP and insulin regulated by hepatocyte nuclear factors were found in the Glc6Pase promoter. Mutations in P36 and P46 lead to glycogen storage disease (GSD) type-1a and type-1 non a (formerly 1b and 1c), respectively. Adenovirus-mediated overexpression of P36 in hepatocytes and in vivo impairs glycogen metabolism and glycolysis and increases glucose production; P36 overexpression in INS-1 cells results in decreased glycolysis and glucose-induced insulin secretion. The nature of the interaction between P36 and P46 in controling Glc6Pase activity remains to be defined. The latter might also have functions other than Glc6P transport that are related to Glc6P metabolism.

Evidence that the transit of glucose into liver microsomes is not required for functional glucose-6-phosphatase

We show that the production of glucose from glucose-6-phosphate hydrolysis outside microsomes is a function of glucose-6-phosphatase independent of its property to form glucose inside microsomes. Indeed, during development (before 1 day of age), mouse liver microsomes had glucose-6-phosphatase producing glucose solely outside microsomes. Furthermore, in vivo treatment of rats with the glucocorticoid analogue triamcinolone resulted in increased glucose-6-phosphatase activity outside but not inside microsomes and without change in the catalytic subunit 40 kDa glucose-6-phosphatase mRNA abundance or protein level, indicating that other factors induced by triamcinolone (e.g., altered membrane lipid environment and/or a regulatory protein) were responsible for the activity change. Triamcinolone treatment also lessened the inhibition of glucose-6-phosphatase by pyridoxal 5'-phosphate (PLP), but this effect was not due to an interaction of PLP with the active site. Accordingly, reversal of the inhibition was observed after permeabilization of the microsomes. The two distinct orientations of liver microsomal glucose-6-phosphate phosphohydrolase suggest different physiological roles played by this enzyme in the endoplasmic reticulum membrane.

Photodynamic effects induced by meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin using rat hepatic microsomes as model membranes

Porphyrins, in combination with light, offer an alternate approach to the treatment of cancer, in the form of photodynamic therapy (PDT). With a view to locate new porphyrins for use in PDT, we evaluated the ability of a novel water-soluble porphyrin, meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin (T4CPP) to induce photodamage in membranes, using rat hepatic microsomes as a model system. Hepatic microsomes treated with T4CPP and exposed to visible light showed significant lipid peroxidation, as assessed by the formation of conjugated dienes, lipid hydroperoxides, and thiobarbituric acid-reactive substances. The peroxidation induced was both time- and concentration-dependent. T4CPP plus light also resulted in the destruction of the microsomal enzymes adenosine triphosphatase and glucose-6-phosphatase. Analysis of the products of peroxidation and selective inhibition by specific inhibitors showed that the oxidative damage induced was mainly due to singlet oxygen and partly due to hydroxyl radical. The porphyrin T4CPP was efficiently labeled with 99mTc. When this 99mTc-labeled porphyrin was injected into a mammary-tumor-bearing rat, it accumulated in the tumor. Our studies suggest that T4CPP, due to its potential to localize in tumors and to induce membrane damage as exemplified by alteration in rat liver microsomes, may have possible applications in this new modality of cancer treatment.

Chlorogenic acid and synthetic chlorogenic acid derivatives: novel inhibitors of hepatic glucose-6-phosphate translocase

The enzyme system glucose-6-phosphatase (EC 3.1.3.9) plays a major role in the homeostatic regulation of blood glucose. It is responsible for the formation of endogenous glucose originating from gluconeogenesis and glycogenolysis. Recently, chlorogenic acid was identified as a specific inhibitor of the glucose-6-phosphate translocase component (Gl-6-P translocase) of this enzyme system in microsomes of rat liver. Glucose 6-phosphate hydrolysis was determined in the presence of chlorogenic acid or of new synthesized derivatives in intact rat liver microsomes in order to assess the inhibitory potency of the compounds on the translocase component. Variation in the 3-position of chlorogenic acid had only poor effects on inhibitory potency. Introduction of lipophilic side chain in the 1-position led to 100-fold more potent inhibitors. Functional assays on isolated perfused rat liver with compound 29i, a representative of the more potent derivatives, showed a dose-dependent inhibition of gluconeogenesis and glycogenolyosis, suggesting glucose-6-phosphatase as the locus of interference of the compound for inhibition of hepatic glucose production also in the isolated organ model. Gl-6-P translocase inhibitors may be useful for the reduction of inappropriately high rates of hepatic glucose output often found in non-insulin-dependent diabetes.

Glucose transport and glucose 6-phosphate hydrolysis in intact rat liver microsomes

Glucose transport was investigated in rat liver microsomes in relation to glucose 6-phosphatase (Glu-6-Pase) activity using a fast sampling, rapid filtration apparatus. 1) The rapid phase in tracer uptake and the burst phase in glucose 6-phosphate (Glu-6-P) hydrolysis appear synchronous, while the slow phase of glucose accumulation occurs during the steady-state phase of glucose production. 2) [14C]Glucose efflux from preloaded microsomes can be observed upon addition of either cold Glu-6-P or Glu-6-Pase inhibitors, but not cold glucose. 3) Similar steady-state levels of intramicrosomal glucose are observed under symmetrical conditions of Glu-6-P or vanadate concentrations during influx and efflux experiments, and those levels are directly proportional to Glu-6-Pase activity. 4) The rates of both glucose influx and efflux are characterized by t1/2 values that are independent of Glu-6-P concentrations. 5) Glucose efflux in the presence of saturating concentrations of vanadate was not blocked by 1 mM phloretin, and the initial rates of efflux appear directly proportional to intravesicular glucose concentrations. 6) It is concluded that glucose influx into microsomes is tightly linked to Glu-6-Pase activity, while glucose efflux may occur independent of hydrolysis, so that microsomal glucose transport appears unidirectional even though it can be accounted for by diffusion only over the accessible range of sugar concentrations.

The purification of a detergent-soluble glucose-6-phosphatase from rat liver

A highly active and soluble glucose-6-phosphatase has been purified to near homogeneity from rat liver. Successful purification has been initiated by covalent labeling of the enzyme in native rat liver microsomes with pyridoxal 5'-phosphate and NaBH4, followed by solubilization of the microsomes with Triton X-100, chromatography on phenyl-Sepharose, hydroxyapatite, DEAE-Sephacel and a second chromatography step on hydroxyapatite. The final enzyme preparation obtained was approximately 700-fold purified over the activity of starting microsomes. As judged by SDS/PAGE the purified glucose-6-phosphatase is composed of a single protein with a molecular mass of 35 kDa. The present work demonstrates that the purified glucose-6-phosphatase must be arranged in the native microsomal membrane so that it is accessible to pyridoxal 5'-phosphate from the cytoplasmic side.

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.

Topographical localization and characterization of microsomal glucose-6-phosphatase binding sites accessible to 4,4'-diazidostilbene 2,2'-disulfonic acid

The effect of the photoactivated reagent 4,4'-diazidostilbene 2,2'-disulfonic acid (DASS) on rat liver microsomal glucose-6-phosphatase has been investigated in order to analyze the accessibility and the chemical nature of functional sites of the integral enzyme protein. The following results were obtained. (i) When native rat liver microsomes are irradiated with the photoactive reagent, the activity of glucose-6-phosphatase is progressively inhibited. However, complete reactivation is obtained by modification of the DASS-labeled microsomes with Triton X-114. (ii) Inhibition of glucose-6-phosphatase is also reversed when the DASS-labeled microsomes are treated with p-mercuribenzoate or dithiothreitol. (iii) When native microsomes are labeled with DASS an intensely fluorescent adduct is formed whose emission and excitation maximum corresponds with those obtained when cysteine or 3-mercaptopropionic acid are irradiated in the presence of the photolabile reagent. (iv) The data from fluorescence measurements show that p-mercuribenzoate and dithiothreitol reduce fluorescence labeling of the microsomes whereas Triton modification of the DASS-labeled membranes does not affect the DASS-induced fluorescence. (v) Glucose 6-phosphate hydrolysis of the partially purified glucose-6-phosphatase is also inhibited as observed with native microsomes. The DASS-induced inhibition is reversed and prevented by p-mercuribenzoate; however, the partially purified enzyme cannot be reactivated by Triton X-114. (vi) When glucose-6-phosphatase is partially purified from the DASS-labeled microsomes this enzyme preparation is fluorescence labeled and inhibited. From these results we conclude that DASS directly reacts with the integral phosphohydrolase mainly by chemical modification of essential sulfhydryl groups of the enzyme protein accessible from the cytoplasmic surface of the native microsomal membrane. The Triton-induced reactivation of the glucose-6-phosphatase of DASS-labeled microsomes is explained in terms of conformational changes of the integral protein elicited during modification of the surrounding membrane by detergent.

Glucose-6-phosphate phosphohydrolase activity in guinea pig liver microsomes is influenced by phosphatidylcholine. Interaction with cholesterol-enriched membranes

Guinea pig liver microsomal membranes were cholesterol-enriched by feeding guinea pigs a high-cholesterol diet. Cholesterol enrichment as well as partial lipid removal of normal native microsomes by acetone-butanol extraction resulted in 40-50% loss in activity of the glucose-6-phosphate phosphohydrolase (G-6-Pase) (EC 3.1.3.9) enzyme system. The activity was restored by supplementation of microsomal total phospholipid (PL) and its phosphatidylcholine (PC) species but not with microsomal neutral lipids, cholesterol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, sphingomyelin or diphosphatidylglycerol (cardiolipin). The activity was decreased by sodium deoxycholate but enhanced by dimethylsulfoxide. Egg-yolk PC and asolectin influenced the activity of the enzyme to the same extent as microsomal PC did. Lipid depletion and cholesterol produced an increase in Km while the Vmax was lowered. The non-linearity in the Arrhenius plot of the native microsomes was lost on lipid removal and cholesterol enrichment. The energy of activation (Ea) calculated from the continuous line was found to be lowered to the level that was observed above the break points in intact microsomes. Addition of microsomal PC to the assay system decreased the Km of the enzymatic reaction in native membranes, in partially lipid-depleted and cholesterol-enriched membranes, but did not alter the Vmax values and only marginally influenced the non-linear relationship of the Arrhenius expression of temperature dependence. The ability of immature rat liver phospholipid exchange protein to introduce alien PL into microsomal membrane was used to study the lipid dependence of G-6-Pase. Protein-catalyzed and detergent (cholate)-mediated membrane PL exchange for egg-yolk PC from the PC/cholesterol unilamellar liposomes resulted in substantial loss of enzyme activity. The discrepancies in the influence of PC on G-6-Pase were interpreted by assuming that the enzyme was a two-component system, a surface-located substrate transporter unit and a membrane integral catalytic phosphohydrolase unit. The lipid microenvironment and PL requirement in particular, could be different for the two components, although they represented a single functional unit at the time of enzymatic reaction.

Determination of glucose-6-phosphatase activity using the glucose dehydrogenase-coupled reaction

We present a method to determine glucose 6-phosphate activity. This assay measures the rate of glucose released in the glucose-6-phosphatase reaction. The glucose is oxidized to beta-D-gluconolactone by glucose dehydrogenase in a coupled reaction that uses NAD(P)+. The determination is rapid, reproducible, and does not require withdrawal, precipitation, centrifugation, or neutralization steps. This method provides a simple resolution to the problem of the nonspecific appearance of Pi, which is especially important in studies of regulation of glucose-6-phosphatase performed in the presence of ATP.

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].

Is thermostability of glucose-6-phosphatase indeed dependent on a stabilizing protein?

Partial purification of glucose-6-phosphatase from rat liver microsomes by solubilization of the membranes with the non-ionic detergent Triton X-114 at pH 6.5 and the removal of inactivating detergent by hydrophobic chromatography results in a thermostable enzyme protein which is not dependent on stabilizing phospholipids or proteins. The readdition of low amounts of detergent immediately causes a conversion into a thermo-unstable phosphohydrolase protein. Thus these findings present evidence that heat instability of partially purified glucose-6-phosphatase derives from traces of inactivating detergent changing the structural properties of the phosphohydrolase rather than from the absence of the postulated specific stabilizing protein.

Latency studies on rat liver microsomal glucose-6-phosphatase. Correlation of membrane modification and solubilization by Triton X-114 with the enzymatic activity

Interrelationships between the catalytic properties of glucose-6-phosphatase and the membrane structure of rat liver microsomes were investigated. Membrane modification and solubilization employing the nonionic surfactant Triton X-114 were standardized and analysed by ultracentrifugation, surface tension- and turbidity measurements. The effect of Triton X-114 on the glucose-6-phosphatase activity was studied systematically and the whole magnitude of time- and temperature-dependent inactivation of this enzyme has been demonstrated. The results show that the activity measured is always a resultant of two processes, the beginning of inactivation and the release of latency. Maximal activation of about 600% (83% of apparent latency) was obtained at 0 degree C. A correlation between membrane modification and solubilization and the conditions under preincubation and test incubation reveals that studies on detergent-disrupted microsomes are performed on structures reassembled from solubilizates and this implies a modified microenvironment in the reconstitutes. Kinetic analyses suggest interrelationships between Triton X-114 and the permeability barrier of the glucose-6-phosphatase system. At 0 degree C 2-propanol and ethanol are more potent tools for membrane modification than Triton X-114 and release 88% and 86% latent activity corresponding to an activation of the glucose-6-phosphatase of about 850% and 700%, respectively. These observations suggest that detergent treatment of microsomes could not preserve the functional integrity of the glucose-6-phosphate phosphohydrolase, which is one dogma of the substrate-transport hypothesis developed by Arion and his co-workers (Arion, W.J., et al. (1975) Mol. Cell. Biochem. 6, 75-83).

Studies on the properties of glucose-6-phosphatase from carp liver microsomes (Cyprinus carpio L.)

The effects of temperature and pH on the phosphohydrolase activity of carp hepatic glucose-6-phosphatase (EC 3.1.3.9) have been investigated. The enzyme activity was maximum at about 308 K and in the pH range 5-6.5. The apparent Michaelis constant (KM) and Vmax of the reaction with glucose-6-phosphate were found to be 14.8 mM and 2.27 nmol/min/mg protein. The enzyme activity was partly inhibited by EDTA, while in the presence of sufficient PCMB virtually total inhibition was observed.

Identification and purification of a liver microsomal glucose 6-phosphatase

1. Hepatic glucose 6-phosphatase activity was purified 65-fold in good yield over that in cholate-solubilized microsomal fractions. 2. This preparation still contained five major polypeptides and numerous minor contaminants. 3. The smallest of the five major polypeptides (Mr approx. 18 500) could be purified from heat-treated microsomal fractions. 4. Antisera raised against the heat-stable protein doublet was used to immunoprecipitate specifically glucose 6-phosphatase activity from cholate-solubilized microsomal fractions. 5. This work indicates that hepatic microsomal glucose 6-phosphatase appears to be one or both of the low-molecular-weight heat-stable polypeptides.

The membrane relationship of microsomal nucleoside diphosphatase

1. The effects of some agents which alter the activity or sedimentability of microsomal nucleoside diphosphatase (NDPase) were studied. 2. Trypsin activated NDPase by stimulating endogenous lipid peroxidation. 3. Eighty percent of the NDPase in vesicles was protected from anaerobic proteolytic attack as shown by activity measurements and electrophoresis. 4. Toluene activated NDPase which remained particulate. 5. NDPase and other proteins could then be easily released from microsomes. 6. The nature of the NDPase-membrane interaction is discussed.