Metabolic regulation by multifunctional glucose-6-phosphatase

From Glucose to Lactate and Transiting Intermediates Through Mitochondria, Bypassing Pyruvate Kinase: Considerations for Cells Exhibiting Dimeric PKM2 or Otherwise Inhibited Kinase Activity

A metabolic hallmark of many cancers is the increase in glucose consumption coupled to excessive lactate production. Mindful that L-lactate originates only from pyruvate, the question arises as to how can this be sustained in those tissues where pyruvate kinase activity is reduced due to dimerization of PKM2 isoform or inhibited by oxidative/nitrosative stress, posttranslational modifications or mutations, all widely reported findings in the very same cells. Hereby 17 pathways connecting glucose to lactate bypassing pyruvate kinase are reviewed, some of which transit through the mitochondrial matrix. An additional 69 converging pathways leading to pyruvate and lactate, but not commencing from glucose, are also examined. The minor production of pyruvate and lactate by glutaminolysis is scrutinized separately. The present review aims to highlight the ways through which L-lactate can still be produced from pyruvate using carbon atoms originating from glucose or other substrates in cells with kinetically impaired pyruvate kinase and underscore the importance of mitochondria in cancer metabolism irrespective of oxidative phosphorylation.

The cAMP-dependent protein kinase downregulates glucose-6-phosphatase expression through RORα and SRC-2 coactivator transcriptional activity

Fasting hormones activate the cAMP/PKA signaling pathway and stimulate expression of hepatic gluconeogenic enzymes including glucose-6-phosphatase (G6Pase). Previously it was shown that steroid receptor coactivator 2 (SRC-2) knock-out mice exhibit fasting hypoglycemia and that SRC-2 coactivates RAR-related orphan receptor alpha (RORα) at the proximal G6Pase promoter. We have investigated the upstream regulation and functional implications of this RORα/SRC-2 complex on G6Pase expression. In HepG2 cells, overexpression of the catalytic PKA subunit (PKA-Cα) reduced the SRC-2 protein level, recruitment to the G6Pase promoter, and its ability to coactivate RORα. Knock-down and transactivation experiments employing G6Pase promoter constructs demonstrated that RORα and SRC-2 are required for PGC-1α to stimulate G6Pase expression. These results suggest that PKA inhibits SRC-2 coactivation of RORα and in turn reduces PGC-1α dependent regulation of G6Pase. This indirect feedback mechanism may underlie the suppression of gluconeogenesis throughout long-term starvation.

A retrospective review of the roles of multifunctional glucose-6-phosphatase in blood glucose homeostasis: Genesis of the tuning/retuning hypothesis

In a scientific career spanning from 1955 to 2000, my research focused on phosphoenolpyruvate carboxykinase and glucose-6-phosphatase. Grounded in basic enzymology, and initially pursuing the steady-state rate behavior of isolated preparations of these critically important gluconeogenic enzymes, our key findings were confirmed and extended by in situ enzyme rate experiments exploiting isolated liver perfusions. These efforts culminated in the discovery of the liver cytosolic isozyme of carboxykinase, known today as (GTP)PEPCK-C (EC4.1.1.32) and also revealed a biosynthetic function and multicomponent nature of glucose-6-phosphatase (EC3.1.3.9). Discovery that glucose-6-phosphatase possessed an intrinsically biosynthetic activity, now known as carbamyl-P:glucose phosphotransferase - along with a deeper consideration of the enzyme's hydrolytic activity as well as the action of liver glucokinase resulted in the evolution of Tuning/Retuning Hypothesis for blood glucose homeostasis in health and disease. This THEN & NOW review shares with the reader the joy and exhilaration of major scientific discovery and also contrasts the methodologies and approaches on which I relied with those currently in use.

Absence of the SRC-2 coactivator results in a glycogenopathy resembling Von Gierke's disease

Hepatic glucose production is critical for basal brain function and survival when dietary glucose is unavailable. Glucose-6-phosphatase (G6Pase) is an essential, rate-limiting enzyme that serves as a terminal gatekeeper for hepatic glucose release into the plasma. Mutations in G6Pase result in Von Gierke's disease (glycogen storage disease-1a), a potentially fatal genetic disorder. We have identified the transcriptional coactivator SRC-2 as a regulator of fasting hepatic glucose release, a function that SRC-2 performs by controlling the expression of hepatic G6Pase. SRC-2 modulates G6Pase expression directly by acting as a coactivator with the orphan nuclear receptor RORalpha. In addition, SRC-2 ablation, in both a whole-body and liver-specific manner, resulted in a Von Gierke's disease phenotype in mice. Our results position SRC-2 as a critical regulator of mammalian glucose production.

Histone 2A stimulates glucose-6-phosphatase activity by permeabilization of liver microsomes

Histone 2A increases glucose-6-phosphatase activity in liver microsomes. The effect has been attributed either to the conformational change of the enzyme, or to the permeabilization of microsomal membrane that allows the free access of substrate to the intraluminal glucose-6-phosphatase catalytic site. The aim of the present study was the critical reinvestigation of the mechanism of action of histone 2A. It has been found that the dose-effect curve of histone 2A is different from that of detergents and resembles that of the pore-forming alamethicin. Inhibitory effects of EGTA on glucose-6-phosphatase activity previously reported in histone 2A-treated microsomes have been also found in alamethicin-permeabilized vesicles. The effect of EGTA cannot therefore simply be an antagonization of the effect of histone 2A. Histone 2A stimulates the activity of another latent microsomal enzyme, UDP-glucuronosyltransferase, which has an intraluminal catalytic site. Finally, histone 2A renders microsomal vesicles permeable to non-permeant compounds. Taken together, the results demonstrate that histone 2A stimulates glucose-6-phosphatase activity by permeabilizing the microsomal membrane.

The glucose-6-phosphatase system

Glucose-6-phosphatase (G6Pase), an enzyme found mainly in the liver and the kidneys, plays the important role of providing glucose during starvation. Unlike most phosphatases acting on water-soluble compounds, it is a membrane-bound enzyme, being associated with the endoplasmic reticulum. In 1975, W. Arion and co-workers proposed a model according to which G6Pase was thought to be a rather unspecific phosphatase, with its catalytic site oriented towards the lumen of the endoplasmic reticulum [Arion, Wallin, Lange and Ballas (1975) Mol. Cell. Biochem. 6, 75--83]. Substrate would be provided to this enzyme by a translocase that is specific for glucose 6-phosphate, thereby accounting for the specificity of the phosphatase for glucose 6-phosphate in intact microsomes. Distinct transporters would allow inorganic phosphate and glucose to leave the vesicles. At variance with this substrate-transport model, other models propose that conformational changes play an important role in the properties of G6Pase. The last 10 years have witnessed important progress in our knowledge of the glucose 6-phosphate hydrolysis system. The genes encoding G6Pase and the glucose 6-phosphate translocase have been cloned and shown to be mutated in glycogen storage disease type Ia and type Ib respectively. The gene encoding a G6Pase-related protein, expressed specifically in pancreatic islets, has also been cloned. Specific potent inhibitors of G6Pase and of the glucose 6-phosphate translocase have been synthesized or isolated from micro-organisms. These as well as other findings support the model initially proposed by Arion. Much progress has also been made with regard to the regulation of the expression of G6Pase by insulin, glucocorticoids, cAMP and glucose.

Overexpression of the P46 (T1) translocase component of the glucose-6-phosphatase complex in hepatocytes impairs glycogen accumulation via hydrolysis of glucose 1-phosphate

The final step of gluconeogenesis and glycogenolysis is catalyzed by the glucose-6-phosphatase (Glc-6-Pase) enzyme complex, located in the endoplasmic reticulum. The complex consists of a 36-kDa catalytic subunit (P36), a 46-kDa glucose 6-phosphate translocase (P46), and putative glucose and inorganic phosphate transporters. Mutations in the genes encoding P36 or P46 have been linked to glycogen storage diseases type Ia and type Ib, respectively. However, the relative roles of these two proteins in control of the rate of glucose 6-phosphate hydrolysis have not been defined. To gain insight into this area, we have constructed a recombinant adenovirus containing the cDNA encoding human P46 (AdCMV-P46) and treated rat hepatocytes with this virus, or a virus encoding P36 (AdCMV-P36), or the combination of both viruses, resulting in large and equivalent increases in expression of the transgenes within 8-24 h of viral treatment. The overexpressed P46 protein was appropriately targeted to hepatocyte microsomes and caused a 58% increase in glucose 6-phosphate hydrolysis in nondetergent-treated (intact) microsomal preparations relative to controls, whereas overexpression of P36 caused a 3.6-fold increase. Overexpression of P46 caused a 50% inhibition of glycogen accumulation in hepatocytes from fasted rats incubated at 25 mm glucose relative to cells treated with a control virus (AdCMV-betaGAL). Furthermore, in hepatocytes from fed rats cultured at 25 mm glucose and then exposed to 15 mm glucose, AdCMV-P46 treatment activated glycogenolysis, as indicated by a 50% reduction in glycogen content relative to AdCMV-betaGAL-treated controls. In contrast, overexpression of P46 had only small effects on glycolysis, whereas overexpression of P36 had large effects on both glycogen metabolism and glycolysis, even in the presence of co-overexpressed glucokinase. Finally, P46 overexpression enhanced glucose 1-phosphate but not fructose 6-phosphate hydrolysis in intact microsomes, providing a mechanism by which P46 overexpression may exert its preferential effects on glycogen metabolism.

Negligible glucose-6-phosphatase activity in cultured astroglia

2-Deoxy[14C]glucose-6-phosphate (2-[14C]DG-6-P) dephosphorylation and glucose-6-phosphatase (G-6-Pase) activity were examined in cultured rat astrocytes under conditions similar to those generally used in assays of glucose utilization. Astrocytes were loaded with 2-[14C]DG-6-P by preincubation for 15 min in medium containing 2 mM glucose and 50 microM 2-deoxy[14C]glucose (2-[14C]DG). The medium was then replaced with identical medium including 2 mM glucose but lacking 2-[14C]DG, and incubation was resumed for 5 min to diminish residual free 2-[14C]DG levels in the cells by either efflux or phosphorylation. The medium was again replaced with fresh 2-[14C]DG-free medium, and the incubation was continued for 5, 15, or 30 min. Intracellular and extracellular 14C contents were measured at each time point, and the distribution of 14C between 2-[14C]DG and 2-[14C]DG-6-P was characterized by paper chromatography. The results showed little if any hydrolysis of 2-[14C]DG-6-P or export of free 2-[14C]DG from cells to medium; there were slightly increasing losses of 2-[14C]DG and 2-[14C]DG-6-P into the medium with increasing incubation time, but they were in the same proportions found in the cells, suggesting they were derived from nonadherent or broken cells. Experiments carried out with medium lacking glucose during the assay for 2-deoxyglucose-6-phosphatase activity yielded similar results. Evidence for G-6-Pase activity was also sought by following the selective detritiation of glucose from the 2-C position when astrocytes were incubated with [2-3H]glucose and [U-14C]glucose in the medium. No change in the 3H/14C ratio was found in incubations for as long as 15 min. These results indicate negligible G-6-Pase activity in cultured astrocytes.

Alterations of carbohydrate and lipid intermediary metabolism during inhibition of glucose-6-phosphatase in rats

S 4048 (1-[2-(4-Chloro-phenyl)-cyclopropylmethoxy]-3, 4-dihydroxy-5-(3-imidazo[4, 5-b]pyridin-1-yl-3-phenyl-acryloyloxy)-cyclohexanecarboxylic acid), a derivative of chlorogenic acid, specifically inhibits the glucose-6-phosphate translocating component T1 of the glucose-6-phosphatase system. Its pharmacological effect was studied on carbohydrate and lipid parameters in rats. In starved and fed rats, S 4048 caused a dose-dependent reduction of blood glucose levels with a corresponding increase in hepatic and renal glycogen and glucose-6-phosphate. The major quantitative route of carbon flux in the liver during S 4048-induced inhibition of the glucose-6-phosphatase activity seemed to be glycogenesis. Plasma free fatty acids were increased secondarily due to the S 4048-induced hypoglycemia. Hepatic triglycerides were increased possibly due to increased re-esterification of the readily available free fatty acids. Glucose-6-phosphate translocase inhibitors may be useful for experimentally studying aspects of type 1 glycogen storage disease in laboratory animals as well as for the therapeutic modulation of inappropriately high rates of hepatic glucose production in type 2 diabetes.

Diabetes affects similarly the catalytic subunit and putative glucose-6-phosphate translocase of glucose-6-phosphatase

The effect of streptozocin diabetes on the expression of the catalytic subunit (p36) and the putative glucose-6-phosphate translocase (p46) of the glucose-6-phosphatase system (G6Pase) was investigated in rats. In addition to the documented effect of diabetes to increase p36 mRNA and protein in the liver and kidney, a approximately 2-fold increase in the mRNA abundance of p46 was found in liver, kidney, and intestine, and a similar increase was found in the p46 protein level in liver. In HepG2 cells, glucose caused a dose-dependent (1-25 mM) increase (up to 5-fold) in p36 and p46 mRNA and a lesser increase in p46 protein, whereas insulin (1 microM) suppressed p36 mRNA, reduced p46 mRNA level by half, and decreased p46 protein by about 33%. Cyclic AMP (100 microM) increased p36 and p46 mRNA by >2- and 1.5-fold, respectively, but not p46 protein. These data suggest that insulin deficiency and hyperglycemia might each be responsible for up-regulation of G6Pase in diabetes. It is concluded that enhanced hepatic glucose output in insulin-dependent diabetes probably involves dysregulation of both the catalytic subunit and the putative glucose-6-phosphate translocase of the liver G6Pase system.

Metabolic engineering with recombinant adenoviruses

Fuel homeostasis in mammals is accomplished by the interplay between tissues and organs with distinct metabolic roles. These regulatory mechanisms are disrupted in obesity and diabetes, leading to a renewed emphasis on discovery of molecular and pharmacologic methods for reversing metabolic disorders. In this chapter, we review the use of recombinant adenoviral vectors as tools for delivering metabolic regulatory genes to cells in culture and to tissues of intact animals. Included are studies on the use of these vectors for gaining insights into the biochemical mechanisms that regulate glucose-stimulated insulin secretion from pancreatic islet beta-cells. We also highlight their use for understanding the function of newly discovered genes that regulate glycogen metabolism in liver and other tissues, and for evaluating "candidate" genes such as glucose-6-phosphatase, which may contribute to development of metabolic dysfunction in pancreatic islets and liver. Finally, we discuss the use of adenoviral and related vectors for causing chronic increases in the levels of circulating hormones. These examples serve to highlight the power of viral gene transfer vectors as tools for understanding metabolic regulatory mechanisms.

Regulation of glucose production by the liver

Glucose is an essential nutrient for the human body. It is the major energy source for many cells, which depend on the bloodstream for a steady supply. Blood glucose levels, therefore, are carefully maintained. The liver plays a central role in this process by balancing the uptake and storage of glucose via glycogenesis and the release of glucose via glycogenolysis and gluconeogenesis. The several substrate cycles in the major metabolic pathways of the liver play key roles in the regulation of glucose production. In this review, we focus on the short- and long-term regulation glucose-6-phosphatase and its substrate cycle counter-part, glucokinase. The substrate cycle enzyme glucose-6-phosphatase catalyzes the terminal step in both the gluconeogenic and glycogenolytic pathways and is opposed by the glycolytic enzyme glucokinase. In addition, we include the regulation of GLUT 2, which facilitates the final step in the transport of glucose out of the liver and into the bloodstream.

Perturbation of fuel homeostasis caused by overexpression of the glucose-6-phosphatase catalytic subunit in liver of normal rats

The terminal step in hepatic gluconeogenesis is catalyzed by glucose-6-phosphatase, an enzyme activity residing in the endoplasmic reticulum and consisting of a catalytic subunit (glucose-6-phosphatase (G6Pase)) and putative accessory transport proteins. We show that Zucker diabetic fatty rats (fa/fa), which are known to exhibit impaired suppression of hepatic glucose output, have 2.4-fold more glucose-6-phosphatase activity in liver than lean controls. To define the potential contribution of increased hepatic G6Pase to development of diabetes, we infused recombinant adenoviruses containing the G6Pase cDNA (AdCMV-G6Pase) or the beta-galactosidase gene into normal rats. Animals were studied by one of three protocols as follows: protocol 1, fed ad libitum for 7 days; protocol 2, fed ad libitum for 5 days, fasted overnight, and subjected to an oral glucose tolerance test; protocol 3, fed ad libitum for 4 days, fasted for 48 h, subjected to oral glucose tolerance test, and then allowed to refeed overnight. Hepatic glucose-6-phosphatase enzymatic activity was increased by 1.6-3-fold in microsomes isolated from AdCMV-G6Pase-treated animals in all three protocols, and the resultant metabolic profile was similar in each case. AdCMV-G6Pase-treated animals exhibited several of the abnormalities associated with early stage non-insulin-dependent diabetes mellitus, including glucose intolerance, hyperinsulinemia, decreased hepatic glycogen content, and increased peripheral (muscle) triglyceride stores. These animals also exhibited significant decreases in circulating free fatty acids and triglycerides, changes not normally associated with the disease. Our studies show that overexpression of G6Pase in liver is sufficient to perturb whole animal glucose and lipid homeostasis, possibly contributing to the development of metabolic abnormalities associated with diabetes.

Three thiol groups are important for the activity of the liver microsomal glucose-6-phosphatase system. Unusual behavior of one thiol located in the glucose-6-phosphate translocase

Liver microsomal glucose-6-phosphatase (Glc-6-Pase) is a multicomponent system involving both substrate and product carriers and a catalytic subunit. We have investigated the inhibitory effect of N-ethylmaleimide (NEM), a rather specific sulfhydryl reagent, on rat liver Glc-6-Pase activity. Three thiol groups are important for Glc-6-Pase system activity. Two of them are located in the glucose-6-phosphate (Glc-6-P) translocase, and one is located in the catalytic subunit. The other transporters (phosphate and glucose) are not affected by NEM treatment. The NEM alkylation of the catalytic subunit sulfhydryl residue is prevented by preincubating the disrupted microsomes with saturating concentrations of substrate or product. This suggests either that the modified cysteine is located in the protein active site or that substrate binding hides the thiol group via a conformational change in the enzyme structure. Two other thiols important for the Glc-6-Pase system activity are located in the Glc-6-P translocase and are more reactive than the one located in the catalytic subunit. The study of the NEM inhibition of the translocase has provided evidence of the existence of two distinct areas in the protein that can behave independently, with conformational changes occurring during Glc-6-P binding to the transporter. The recent cloning of a human putative Glc-6-P carrier exhibiting homologies with bacterial phosphoester transporters, such as Escherichia coli UhpT (a Glc-6-P translocase), is compatible with the fact that two cysteine residues are important for the bacterial Glc-6-P transport.

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

The insulin-like action of tungstate in diabetic rats (A. Barberà et al., 1994, J. Biol. Chem. 269, 20047-20053) prompted us to examine the effects of tungstate on the glucose-6-phosphatase system. Our results indicate that tungstate is a potent inhibitor of glucose-6-phosphatase, with a Ki in the 10-25 microM range determined with native microsomes and in the 1-7 microM range determined with detergent-treated microsomes. With both preparations, simple linear competitive inhibition was observed versus glucose 6-phosphate (glucose-6-P) as substrate with the glucose-6-P phosphohydrolase activity of the enzyme. Tungstate was a simple linear competitive inhibitor versus carbamyl phosphate (carbamyl-P) and a linear noncompetitive inhibitor versus glucose with the carbamyl-P:glucose phosphotransferase activity of the glucose-6-phosphatase system. These findings, in addition to the observation that tungstate protected the enzyme against thermal inactivation, indicate that tungstate binds with high affinity and competes at the active site of the enzyme where the substrates glucose-6-P and carbamyl-P bind prior to catalysis. Our results suggest that potent inhibition of glucose-6-P hydrolysis by tungstate is likely responsible, at least in part, for the normalization of glycemia and the rebound in hepatic glucose-6-P levels observed in earlier studies in which tungstate exhibited insulin-like action in diabetic rats.

Pharmacodynamic profile of a novel inhibitor of the hepatic glucose-6-phosphatase system

The glucose-6-phosphatase (G-6-Pase) system catalyzes the terminal enzymatic step of gluconeogenesis and glycogenolysis. Inhibition of the G-6-Pase system in the liver is expected to result in a reduction of hepatic glucose production irrespective of the relative contribution of gluconeogenesis or glycogenolysis to hepatic glucose output. In isolated perfused rat liver, S-3483, a derivative of chlorogenic acid, produced concentration-dependent inhibition of gluconeogenesis and glycogenolysis in a similar concentration range. In fed rats, glucagon-induced glycogenolysis resulted in hyperglycemia for nearly 2 h. Intravenous infusion of 50 mg . kg-1. h-1 S-3483 prevented the hyperglycemic peak and subsequently caused a further lowering of blood glucose. In 24-h starved rats, in which normoglycemia is maintained predominantly by gluconeogenesis, intravenous infusion of S-3483 resulted in a constant reduction of blood glucose levels. Intrahepatic concentrations of glucose-6-phosphate (G-6-P) and glycogen were significantly increased at the end of both in vivo studies. In contrast, lowering of blood glucose in starved rats by 3-mercaptopicolinic acid was accompanied by a reduction of G-6-P and glycogen. Our results demonstrate for the first time in vivo a pharmacologically induced suppression of hepatic G-6-P activity with subsequent changes in blood glucose levels.

Effects of ionic strength and chloride ion on activities of the glucose-6-phosphatase system: regulation of the biosynthetic activity of glucose-6-phosphatase by chloride ion inhibition/deinhibition

Certain amino acids stimulate glycogenesis from glucose. The regulatory volume decrease mechanism explaining these effects was defined by Meijer et al. (1992, J. Biol. Chem. 267, 5823-5828). It involves amino acid-induced swelling of hepatocytes resulting in loss of chloride ions which leads to deinhibition of glycogen synthase phosphatase. This results in enhanced conversion of the inactive to active form of glycogen synthase and thus enhanced glycogen synthesis. We have studied the effects of amino acids and chloride ion on the glucose-6-phosphatase system (Glc-6-Pase) with rat liver microsomal preparations, and correlated our results with those reported by others with glycogen synthase. Glc-6-Pase activities are increased by elevated ionic strength varied by increasing the concentration of various buffers or charged amino acids but are not affected by changes in osmolarity, varied with disaccharides or uncharged amino acids. With undisrupted microsomes, chloride ion competitively inhibits carbamyl phosphate: glucose phosphotransferase (KCP,t,UMi,Cl- = 19 mM) more extensively than Glc-6-P phosphohydrolase (KG6P,h,UMi,Cl- = 117 mM). Inhibition by chloride ion and activation due to ionic strength may be important considerations when assessing in vitro Glc-6-Pase activities where an attempt is made to replicate physiologic conditions. Further we propose that amino acids may play a role in increasing biosynthetic activity of Glc-6-Pase, as well as previously characterized glycogen synthase (Meijer et al., op. cit.), via the regulatory volume decrease mechanism through diminished chloride ion inhibition. Reduced concentration of chloride ion will (1) deinhibit the biosynthetic activity of Glc-6-Pase, while still inhibiting Glc-6-P hydrolysis, leading to an increased cellular concentration of Glc-6-P (an important glycogenic intermediate as well as allosteric activator of glycogen synthase) and (2) increase the active form of glycogen synthase by deinhibiting glycogen synthase phosphatase both through the previously defined mechanism (see above) and via Glc-6-P-enhanced conversion of glycogen synthase from its inactive to active form. We propose that the biosynthetic activity of Glc-6-Pase may act in concert with glycogen synthase during amino acid-induced glycogenesis from glucose.

Direct evidence for the involvement of two glucose 6-phosphate-binding sites in the glucose-6-phosphatase activity of intact liver microsomes. Characterization of T1, the microsomal glucose 6-phosphate transport protein by a direct binding assay

S 5627 is a synthetic analogue of chlorogenic acid. S 5627 is a potent linear competitive inhibitor of glucose 6-phosphate (Glc-6-P) hydrolysis by intact microsomes (Ki = 41 nM) but is without effect on the enzyme in detergent- or NH4OH-disrupted microsomes. 3H-S 5627 was synthesized and used as a ligand in binding studies directed at characterizing T1, the Glc-6-P transporter. Binding was evaluated using Ca2+-aggregated microsomes, which can be sedimented at low g forces. Aside from a modest reduction in K values for both substrate and S 5627, Ca2+ aggregation had no effect on glucose-6-phosphatase (Glc-6-Pase). Scatchard plots of binding data are readily fit to a simple "two-site" model, with Kd = 21 nM for the high affinity site and Kd = 2 microM for the low affinity site. Binding to the high affinity site was competitively blocked by Glc-6-P (Ki = 9 microM), whereas binding was unaffected by mannose-6-phosphate, Pi, and PPi and only modestly depressed by 2-deoxy-D-glucose 6-phosphate, a poor substrate for Glc-6-Pase in intact microsomes. Thus the high affinity 3H-S 5627 binding site fits the criteria for T1. Permeabilization of the membrane with 0.3% (3-[(chloramidopropyl)-dimethylammonio]-1-propanesulfonate) activated Glc-6-Pase and broadened its substrate specificity, but it did not significantly alter the binding of 3H-S 5627 to the high affinity sites or the ability of Glc-6-P to block binding. These data demonstrate unequivocally that two independent Glc-6-P binding sites are involved in the hydrolysis of Glc-6-P by intact microsomes. The present findings are the strongest and most direct evidence to date against the notion that the substrate specificity and the intrinsic activity of Glc-6-Pase in native membranes are determined by specific conformational constraints imposed on the enzyme protein. These data constitute compelling evidence for the role of T1 in Glc-6-Pase activity.

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.

Chlorogenic acid and hydroxynitrobenzaldehyde: new inhibitors of hepatic glucose 6-phosphatase

We have studied the interactions of chlorogenic acid (CHL) and 2-hydroxy-5-nitrobenzaldehyde (HNB) with the components of the rat hepatic glucose 6-phosphatase (Glc-6-Pase) system. CHL and HNB are competitive inhibitors of glucose 6-phosphate (Glc-6-P) hydrolysis in intact microsomes with Ki values of 0.26 and 0.22 mm, respectively. CHL is without effect on the enzyme of fully disrupted microsomes or the system inorganic pyrophosphatase (PPiase) activity. HNB is a potent competitive inhibitor of the system PPiase activity (Ki = 0.56 mm) and a somewhat weaker noncompetitive inhibitor of enzyme activity (Ki = 2.1 mm). These findings indicate CHL binds to T1, the Glc-6-P transporter, and HNB inhibits through interaction with both T1 and T2 the phosphate (Pi)-PPi transporter. Binding of CHL and HNB is freely reversible. However, the inhibition of both PPiase and Glc-6-Pase by HNB becomes irreversible following incubation of HNB-exposed microsomes with 2.5 mm sodium borohydride, indicating that inhibition involves the formation of a Schiff base. The presence of CHL effectively protects T1, but not T2, against the irreversible inhibition by HNB. In contrast, PPi and Pi are effective in protecting T2, but not T1. This is the first report describing an effective inhibitor of the system PPiase activity (T2). CHL is the most specific T1 inhibitor described to date.

An altered T2 beta translocase of the glucose-6-phosphatase system in the membrane of the endoplasmic reticulum from livers of Ehrlich-ascites-tumour-bearing mice

The inhibitory interactions of orthophosphate (P1) with the glucose-6-phosphatase system of intact microsomes derived from the livers of normal and Ehrlich-ascites-tumour-bearing mice reveal the appearance of a novel form of the T2 beta translocase component of the glucose-6-phosphatase system in tumour-stressed mice. Kinetic studies, with and without 20 mM P1, show a strictly classical competitive inhibition, with a K1,P1 of 4.2 mM, with disrupted microsomes from both control and tumour-bearing mouse liver. Inhibition was also observed with intact microsomes from livers of control mice, and contributions by both competitive and non-competitive components of inhibition were quantified by calculation of Kis,P1 and Kii,P1 values respectively. However, little inhibition was noted with intact microsomes from the livers of tumour-bearing mice. It is concluded that this novel form of T2 beta is less able to transport Pi, from the cytosol to the endoplasmic reticulum lumen, perhaps because of the tumour-related increased Km for Pi transport in this direction.

Glucosamine-sensitive and -insensitive detritiation of [2-3H]glucose in isolated rat hepatocytes: a study of the contributions of glucokinase and glucose-6-phosphatase

Glucosamine, a potent inhibitor of glucokinase (hexokinase IV or D), was used to estimate the contribution of this enzyme to glucose phosphorylation in freshly isolated rat hepatocytes and its sensitivity to fructose 6-phosphate in situ. Experiments with radiolabelled glucosamine indicated that this amino sugar, at concentrations of 5 or 40 mM, readily penetrated hepatocytes to reach in 1 min a total (i.e., glucosamine+metabolites) intracellular concentration equal to 0.8-1.2-fold its extracellular concentration. In marked contrast, N-acetylglucosamine barely penetrated the cells. The detritiation of [2-3H]glucose, used to estimate glucose phosphorylation in intact cells, was inhibited by glucosamine much more potently than by N-acetylglucosamine, half-maximal effects being reached at about 2.5 and 30 mM respectively. Extrapolation of the data indicated that about 12% of the detritiation was resistant to glucosamine. Dihydroxyacetone (10 mM), lactate (10 mM) + pyruvate (1 mM), and glucagon (1 microM) increased up to 8-fold the concentration of hexose 6-phosphates (glucose 6-phosphate+fructose 6-phosphate) and, against expectations, modestly decreased the detritiation rate measured in the absence of glucosamine. In the presence of 40 mM glucosamine, these agents increased the detritiation rate, which then positively correlated with the concentration of hexose 6-phosphates. This hexose 6-phosphates-dependent detritiation was sensitive to inhibition by vanadate, and was also catalysed by gel-filtered cell-free extracts, as well as by liver microsomes in the presence of phosphoglucoisomerase; it can be explained by an exchange reaction catalysed by glucose-6-phosphatase. When this exchange reaction is taken into account, it appears that the rate of glucose detritiation attributable to glucokinase decreases when the concentration of hexose 6-phosphates increases. This is in agreement with the known effect of fructose 6-phosphate to potentiate the inhibition of glucokinase by its regulatory protein.

Time-dependent inhibition of glucose 6-phosphatase by 3-mercaptopicolinic acid

3-Mercaptopicolinate (3-MP) inhibits D-glucose-6-phosphate (G6P) phosphohydrolase activity of the glucose-6-phosphatase system (Bode et al. (1993) Biochem. Cell Biol. 71, 113-121). We therefore attempted to maximize the inhibition by varying the physical state of microsomes, the concentration of 3-MP, and the time of preliminary incubation of 3-MP with the enzyme. The inhibition was irreversible and time- and inhibitor-concentration-dependent, with G6P phosphohydrolase activity of intact rat liver microsomes, but there was no inhibition with detergent-treated microsomes. The effectiveness of 3-MP as a time-dependent inhibitor of glucose 6-phosphatase was demonstrated in situ by measuring glycogenolysis in isolated, perfused livers from fed rats. We first exposed the livers to 2 mM 3-MP for 40 min, and then assessed the inhibitory effects on glycogenolysis. It was lowered by 50%. These observations establish that 3-MP at the mM level may be useful as an experimental probe in the study of the role(s) of G6P in the regulation of glycogenolysis as well as glycogenesis. Further, they validate the use of much lower (microM) concentrations of 3-MP to block gluconeogenesis (at the phosphoenolpyruvate carboxykinase step) without interfering with glucose 6-phosphatase. We also explored the mechanism of 3-MP inhibition. The time-dependent inhibition of carbamoyl-phosphate:glucose phosphotransferase activity with microsomes incubated with 1 mM 3-MP for 60 or 90 min and then assayed with 1 mM carbamoyl phosphate and 180 mM glucose was modest compared with inhibition of G6P phosphohydrolase. When G6P production by carbamoyl-phosphate:glucose phosphotransferase was reduced by decreasing glucose concentration to 60 mM, no inhibition by 3-MP was discernible. There was no inhibition of inorganic pyrophosphatase activity. These studies support the model of time-dependent, irreversible reaction of 3-MP with the G6P translocase component of the glucose-6-phosphatase system.

Hysteresis at near-physiologic substrate concentrations underlies apparent sigmoid kinetics of the glucose-6-phosphatase system

Although Canfield and Arion (J. Biol. Chem. 263, 7458-7460 (1990)) have described the kinetics as hyperbolic, Traxinger and Nordlie (J. Biol. Chem. 262, 10015-10019 (1987)) reported sigmoid kinetics in the glucose-6-phosphatase system of intact microsomes at near-physiologic glucose-6-P concentrations. We show here that apparent sigmoidal kinetics, most clearly seen as sharp upward inflections in Hanes plots as substrate concentration approaches zero, are a consequence of the hysteretic lag in product formation during the first minutes of incubation of the enzyme with low concentrations of substrate. The appearance of sigmoidicity, observed when reaction velocities are calculated from changes in Pi concentration between 0 and 6 min of incubation, is not present when velocity is determined from slopes of [product]-time plots after linearity is achieved. The Km,glucose-6-P value, 0.86 mM, based on these hysteresis-corrected velocity values determined with intact microsomes from normal, control rats at low substrate concentrations, approached the upper limit of physiologic hepatic glucose-6-P concentrations. This suggests that glucose-6-phosphatase activity may be regulated by factors other than substrate concentrations alone. We propose that the hysteretic behavior, not sigmoid kinetics of the glucose-6-phosphatase enzyme system, may be a prime regulatory feature.

The hepatic glucose-6-phosphatase system in Ehrlich-ascites-tumour-bearing mice

To examine the effects of the presence of Ehrlich ascites tumours on both the catalytic unit and the substrate/product translocase components of the glucose-6-phosphatase system in vivo, we isolated microsomes from the livers of control and tumour-bearing mice. Samples were analysed immunochemically for the quantity of catalytic unit, stabilizing protein and translocases T2 and T3 proteins. In comparison experiments, a variety of kinetic studies were performed. The most striking findings in tumour-bearing mice were: a 2.5-fold increase in the quantity of translocase T2 protein; increases in the Km and Vmax. for glucose 6-phosphate phosphohydrolase; and a decrease in the Km value for carbamoyl phosphate (carbamoyl-P) of carbamoyl-P:glucose phosphotransferase, all with intact microsomes. The percentage latency at Vmax. decreased for PPi phosphohydrolase and for glucose 6-phosphate phosphohydrolase, but was unaffected for carbamoyl-P:glucose phosphotransferase. These observations support a tumour-related increase in translocase T2 capacity in vivo, as it transports Pi from the microsomal lumen to the medium and carbamoyl-P or PPi from the medium to the microsomal lumen.

Contribution of glucose/glucose 6-phosphate cycle activity to insulin resistance in type 2 (non-insulin-dependent) diabetes mellitus

It has been suggested that increased glucose/glucose 6-phosphate substrate cycling impairs net hepatic glucose uptake in Type 2 (non-insulin-dependent) diabetes mellitus and contributes to hyperglycaemia. To investigate glucose/glucose 6-phosphate cycle activity and insulin action in Type 2 diabetes we studied eight patients and eight healthy control subjects, using the euglycaemic glucose clamp and isotope dilution techniques with purified [2-3H]- and [6-3H] glucose tracers, in the post-absorptive state and eight patients and five healthy control subjects during consecutive insulin infusions at rates of 0.4 and 2.0 mU.kg-1 x min-1. [2-3H]glucose and [6-3H]glucose radioactivity in plasma samples were determined using selective enzymatic detritiation, allowing calculation of glucose turnover rates for each isotope, the difference being glucose/glucose 6-phosphate cycling. Endogenous glucose production ([6-3H]glucose) was greater in diabetic than control subjects in the post-absorptive state (15.6 +/- 1.5 vs 11.3 +/- 0.4 mumol.kg-1 x min-1, p < 0.05) and during the 0.4 mU insulin infusion (10.1 +/- 1.3 vs 5.2 +/- 0.3 mumol.kg-1 x min-1, p < 0.01) indicating hepatic insulin resistance. Glucose/glucose 6-phosphate cycling was significantly greater in diabetic than in control subjects in the post-absorptive state (2.6 +/- 0.4 vs 1.6 +/- 0.2 mumol.kg-1 x min-1, p < 0.05) but not during the 0.4 mU insulin infusion (2.0 +/- 0.4 vs 2.0 +/- 0.3 mumol.kg-1 x min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Hysteretic behavior of the hepatic microsomal glucose-6-phosphatase system

Carbamyl-P:glucose and PPi:glucose phosphotransferase, but not inorganic pyrophosphatase, activities of the hepatic microsomal glucose-6-phosphatase system demonstrate a time-dependent lag in product production with 1 mM phosphate substrate. Glucose-6-P phosphohydrolase shows a similar behavior with [glucose-6-P] less than or equal to 0.10 mM, but inorganic pyrophosphatase activity does not even at the 0.05 or 0.02 mM level. The hysteretic behavior is abolished when the structural integrity of the microsomes is destroyed by detergent treatment. Calculations indicate that an intramicrosomal glucose-6-P concentration of between 20 and 40 microM must be achieved, whether in response to exogenously added glucose-6-P or via intramicrosomal synthesis by carbamyl-P:glucose or PPi:glucose phosphotransferase activity, before the maximally active form of the enzyme system is achieved. It is suggested that translocase T1, the transport component of the glucose-6-phosphatase system specific for glucose-6-P, is the target for activation by these critical intramicrosomal concentrations of glucose-6-P.

Regional metabolic rate of exogenous glucose in the isoprenaline and dobutamine stimulated canine myocardium as estimated by the 2-deoxy-D[1-14C]glucose method

The effect of beta-adrenoceptor stimulation by isoprenaline and dobutamine on the transmural distribution pattern of regional myocardial metabolic rate of exogenous glucose (RMMRGlc) was studied in the anesthetized closed chest dog using the 2-deoxy-D[1-14C]glucose method. In a previous series a lumped constant (LC) value of 0.93 +/- 0.47 (1 SD) was measured for [14C]2-deoxyglucose in the canine myocardium. In the control group (N = 12) RMMRGlc was significantly higher in the subendocardial layer of the left ventricular free wall than in both the middle and subepicardial layer, where it was quite evenly distributed (P less than or equal to 0.05). With i.v. dobutamine (N = 8) RMMRGlc was significantly lower in the midportion of left ventricular free wall than in the subepicardial layer (P less than or equal to 0.05), but it was not different from the inner wall section. Significant differences between the subepicardial and subendocardial portions of the left ventricular free wall could not be found, either. In the isoprenaline group (N = 9) no transmural gradients of RMMRGlc were observed in the left ventricular myocardium. In all groups, both the interventricular septum and the right ventricular free wall exhibited homogeneous distribution patterns of RMMRGlc. It is concluded that transmural distribution patterns of exogenous glucose utilization probably reflect corresponding gradients in energy demands of the left ventricular wall. Redistribution of RMMRGlc in the isoprenaline and dobutamine groups may result from altered working conditions, a change in local inotropic state of the left ventricular myocardium, or from regional differences in the proportions of substrate utilization, and from regional differences in adrenoceptor density.

Mechanisms underlying enhanced glycogenolysis in livers of 3,5,3'-triiodothyronine-treated rats

Rats trained on a diurnal controlled meal-feeding schedule and injected with a single dose of 3,5,3'-triiodothyronine (T3) failed to accumulate liver glycogen and incorporated less D-[6-3H]glucose into glycogen than normally observed during the feeding period. In the experimental group, the concentration of liver adenosine 3',5'-cyclic monophosphate (cAMP) did not fall during feeding and the pattern of activities of glycogen phosphorylase, glycogen synthase, and phosphorylase kinase remained conductive to glycogenolysis. Liver lysosomal alpha-glucosidase activity normally fell during feeding periods. After T3 treatment the activities of alpha-glucosidase and two lysosomal cathepsins (B1 and D) were elevated. The evidence suggests that T3 may induce both liver phosphorylase kinase and lysosomal alpha-glucosidase. This outcome of T3 excess, in concert with previously described T3-inducible systems, provides a plausible explanation for the failure of glycogen accumulation in this experimental model.

A minimal model of liver glycogen metabolism; feasibility for predicting flux rates

A minimal model of glycogen metabolism can allow the estimation of the flux rates in the glycogen pathway from the time course of the intermediates in the pathway, measured during substrate administration and hormonal stimulation. The comprehensive model of El-Refai & Bergman (Am. J. Physiol. 231, 1608, 1976) consisting of six compartments and 26 non-estimable parameters has successfully accounted for the responses of hepatic glycogenic intermediates in response to a glucose load in hepatocytes (Katz et al., J. biol. Chem. 253, 4530, 1978), in perfused liver (Nordlie et al., J. biol. Chem. 255, 1834, 1980) and during refeeding in vivo (Van DeWerve & Jeanrenaud, Am. J. Physiol. 247, E271, 1984). The comprehensive model is here reduced to a minimal model, consisting of five compartments representing extracellular and intracellular glucose, glucose-phosphate, uridine diphosphate glucose (UDPG), glycogen, and five parameters estimated from the hepatic response to a given stimulus. Estimation of these parameters requires the measurement of the net hepatic glucose balance, the net gluconeogenic flux, and the time course of glycogenic intermediates responding to a hormone or substrate stimulus. The hepatic glycogenolytic response predicted by the comprehensive model in response to an increase in glucagon is closely fitted by the minimal model. When Gaussian distributed random error was added, 0-5% SD in the glucose and glycogen compartments and 0-10% SD in the glucose-phosphate and UDPG compartments, the hepatic response predicted by the minimal model was virtually free of the added error, and the model parameters were found to be within 30% of their true values. When the minimal model was used to interpret the experimental response to an increase in glucose concentration it predicted that: (1) glucokinase can phosphorylate glucose at rates similar to maximal rates of net glycogen synthesis; (2) futile cycling at the glycogen/glucose-1-phosphate level can limit glycogen synthesis; and (3) glucose-6-phosphatase inhibition by glucose has a significant role in net glycogen synthesis.

The microsomal glucose-6-phosphatase enzyme of pancreatic islets

Microsomal fractions isolated from pancreatic islet cells were shown to contain high specific glucose-6-phosphatase activity. The islet-cell glucose-6-phosphatase enzyme has the same Mr (36,500), similar immunological properties and kinetic characteristics to the hepatic microsomal glucose-6-phosphatase enzyme.

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].

Impaired carbohydrate metabolism of polymorphonuclear leukocytes in glycogen storage disease Ib

This study measures hexose monophosphate (HMP) shunt activity, glycolytic rate, and glucose transport in PMN and lymphocytes of patients with glycogen storage disease (GSD) type Ib as compared with controls and with GSD Ia patients. HMP shunt activity and glycolysis were significantly lower in intact PMN cells of GSD Ib patients as compared with GSD Ia patients and with controls. These activities were above normal levels in disrupted GSD Ib PMN. HMP shunt activity and glycolytic rates in lymphocytes were similar in all three groups studied. The rate of 2-deoxyglucose transport into GSD Ib PMN was 30% of that into cells of normal controls. In GSD Ib lymphocytes or in GSD Ia PMN and lymphocytes transport was normal. The striking limitation of glucose transport across the cell membrane of the PMN of GSD Ib patients may account for the impairment of leukocyte function that is characteristic of GSD Ib, but not found in GSD Ia patients.

Regionality of glucose-6-phosphate hydrolysis in the liver lobule of the rat: metabolic heterogeneity of "portal" and "septal" sinusoids

To investigate intercellular compartmentation of liver metabolism, we have recently introduced new procedures for quantitative assessment of metabolic liver cell heterogeneity both along sinusoids of portal and septal origins as well as at the level of the parenchymal unit, and also for three-dimensional imaging of enzyme and metabolite distribution. As part of the evaluation of the role of metabolic liver cell heterogeneity for the regulation of net substrate flux in the glucose-6-phosphatase/glucokinase system, and for the reduction of of these antagonistic enzymes, these techniques were used on livers from male rats. They served to obtain distribution data on glucose-6-phosphatase (the hydrolytic component of the glucose-6-phosphatase/glucokinase system) and its substrate, glucose-6-P, during the postresorptive phase (i.e., a metabolic state of net glucose release). Glucose-6-phosphatase (Vmax) and glucose-6-P were shown to decrease along the sinusoidal axis, and values of both parameters were significantly higher along sinusoids of portal than septal origin. Distribution of in vivo rates of glucose-6-P hydrolysis indicates the importance of metabolite distribution for in vivo regulation of liver cell function, insofar as it considerably increases the degree of heterogeneity among hepatocytes over that maximal rates of glucose formation. Histo- and microchemical data support the concept of a "lobular parenchymal unit" composed of "primary lobules," and justify the conclusion that hepatocyte function, in addition to the hormonal and nutritional states of the animal, not only depends upon cell location along the sinusoidal axis, but also on the origin of sinusoids.

The levels of nicotinamide nucleotides in liver microsomes and their possible significance to the function of hexose phosphate dehydrogenase

The concentrations of NAD and NADP have been determined in detergent extracts of washed rat liver microsomes. Precautions were taken during the preparation of the microsomes to remove nicotinamide nucleotides from their external surface both by hydrolysis by nucleotide pyrophosphatase (EC 3.6.1.9) and by washing them three times in 0.15 M-Tris/HCl, pH 8.0, to remove soluble proteins which bind these nucleotides. The mannose phosphatase was essentially completely latent, indicating that the microsomes were intact. Assuming these nucleotides are in the cisternae of the microsomes, the concentrations in the cisternae are 240 +/- 25 microM-NAD and 55 +/- 12 microM-NADP. These levels of nucleotides are compatible with both the glucose:NAD+ and the glucose 6-phosphate:NADP+ oxidoreductase activities of hexose phosphate dehydrogenase (EC 1.1.1.47). Since the organ and subcellular distributions of this dehydrogenase and glucose-6-phosphatase are similar, and Pi stimulates the glucose:NAD+ oxidoreductase activity, it is proposed that the combined action of these two enzymes leads to the reduction of both coenzymes in the lumen of the endoplasmic reticulum. A modification of the colorimetric method of Nisselbaum & Green [(1969) Anal. Biochem. 27, 212-217] for the determination of NADP+ is described. Colour formation is linear with the concentration of NADP+ and is sensitive to less than 0.3 nmol of NADP+.

Effects of phenobarbital and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene on differentiated functions in mouse liver

The promoters of murine hepatocarcinogenesis phenobarbital (PB) and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) given to adult C3Hf female mice increased the content of total liver DNA by 1.6-1.8-fold each week after the beginning of treatment. Both compounds increased the aminopyrine-N-demethylase activity, decreased the glucose 6-phosphatase (G6Pase), alkaline phosphodiesterase I and alkaline phosphatase specific activities, but did not modify the gamma-glutamyltransferase levels. Both compounds decreased the abundance of tyrosine aminotransferase- and metallothionein I-related RNA transcripts. These findings confirmed the PB-like activity of TCPOBOP and showed that both chemicals had a pleiotropic effect on mouse liver, that was not limited to stimulation of drug metabolism, but also affected other hepatocyte functions.

Anomer specificity of glucose-6-phosphatase and glucokinase

The anomeric form of glucose produced by glucose-6-phosphatase was studied using an apparatus that specifically measures beta-D-glucose. The time course of beta-D-glucose formation from glucose-6-P by glucose-6-phosphatase is essentially linear. In the presence of mutarotase, this rate is reduced to 70% of that obtained in the absence of mutarotase. When detergent treated microsomes were used, the rate of beta-D-glucose formation is unaffected by mutarotase. These results suggest that only beta-anomer of glucose is produced by microsomal glucose-6-phosphatase and this specificity is determined by translocase for glucose-6-P or glucose. It was also demonstrated that alpha-D-glucose is the substrate for glucokinase.

Inhibition of gluconeogenesis by hypoglycin in the rat. Evidence for inhibition of glucose-6-phosphatase in vivo

Treatment of rats with hypoglycaemic doses of hypoglycin has been shown to abolish the relative detritiation of [2-3H,U-14C]glucose [Osmundsen, Billington, Taylor & Sherratt (1978) Biochem. J. 170, 337-342], indicating that both the Cori and the glucose/glucose 6-phosphate cycles were inhibited in vivo. This inhibition was confirmed and, in addition, it was shown that the conversion in vivo of both [14C]lactate and [14C]fructose into glucose was decreased after hypoglycin treatment. These results suggest that hypoglycin poisoning results in the inhibition in vivo of glucose-6-phosphatase activity, which participates in the overall inhibition of gluconeogenesis and hypoglycaemia. Clofibrate feeding apparently protected the rats against the inhibition of the fructose-to-glucose conversion by hypoglycin. However, in isolated hepatocytes prepared from hypoglycin-treated rats, the conversion of [14C]fructose into glucose and the recycling of [2-3H,U-14C]glucose were not different from that in control hepatocytes. This suggests that the inhibition was lost during preparation of the hepatocytes. The direct measurement of glucose-6-phosphatase activity showed that it was inhibited when measured in concentrated, but not dilute, homogenates prepared from hypoglycin-treated rats.

N-acetylglucosamine-insensitive glucose phosphorylation in isolated hepatocytes from rats fasted for 48 h

Glucose phosphorylation in isolated hepatocytes was studied by the release of 3H from D-[2-3H]glucose. Glucokinase activity was decreased by fasting rats for 48 h and was further reduced in cells by adding 30 mM GlcNAc, a potent competitive inhibitor. Although this treatment resulted in the loss of more than 97% of glucokinase activity in hepatocytes, glucose phosphorylation proceeded at an appreciable rate. These observations demonstrate the involvement of a high -K0.5 enzyme system in addition to glucokinase in hepatocyte glucose phosphorylation.

The effect of 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide on the rat liver microsomal glucose-6-phosphatase system

The effect of 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide (CMC) on the reactions catalyzed by the glucose-6-phosphatase system of rat liver microsomes was studied. Modification of the intact microsomes by CMC leads to the inhibition of the glucose-6-phosphatase, pyrophosphate:glucose and carbamoyl-phosphate : glucose phosphotransferase activities of the system. The activities are restored by the disruption of the microsomal permeability barrier. The mannose-6-phosphate, pyrophosphate, and carbamoyl-phosphate phosphohydrolase activities of the intact as well as the disrupted microsomes were not affected by CMC. It follows from the results obtained that CMC inactivates the microsomal glucose-6-phosphate translocase, the inactivation is a result of the modification of a single sulfhydryl or amino group of the translocase; pyrophosphate, carbamoyl phosphate and inorganic phosphate are transported across the microsomal membrane without participation of the glucose-6-phosphate translocase; pyrophosphate and carbamoyl phosphate may act as the phosphate donors in the glucose phosphorylation reactions in vivo.

Use of 3-D-computer graphics for imaging of distribution of hepatic metabolites

In conjunction with the investigation of intercellular compartmentation of liver metabolism and as a logical further step, following the introduction of a new sample isolation procedure for microchemical analysis of functional liver cell heterogeneity, the possible benefit of computer-assisted three-dimensional imaging procedures for the reconstruction of hepatic metabolite distribution was investigated. In this intent, we elected to access a central computer facility by means of a small microcomputer system which, nevertheless, permitted to take full advantage of a large capacity main-frame computer and a high quality graphics plotter, at comparatively low overall costs. Commercially available software (SAS/GRAPH) was tailored to the specific requirements of this application. The three-dimensional imaging process recombines microchemical data (metabolite or enzyme values) with those of the size and location of samples within a particular cross-sectional area of a liver unit and provides an integrated view of metabolite distributions. The three-dimensional images were then used to define general distribution characteristics, as well as, differences in metabolite distribution along sinusoids of portal and septal origin. Glucose increased, whereas glucose-6-P decreased along sinusoids from the beginning to the end and values of both metabolites were found to be higher along 'portal/central' than along 'septal/central' sinusoids. Co-distribution of glucose-6-phosphatase with its substrate (glucose-6-P) was indicated by histochemical and microchemical results and is anticipated to be of considerable regulatory importance, since it further enhances the differences among hepatocytes at different locations along sinusoids with respect to their ability to produce glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat hepatic microsomal glucose-6-phosphatase protein levels are increased in streptozotocin-induced diabetes

Hepatic microsomal glucose-6-phosphatase activity levels and the hepatic output of glucose are increased in diabetes. We have used protein chemistry and immunological techniques to determine the mechanism by which the activity levels of the glucose-6-phosphatase system are increased in streptozotocin-induced diabetic rats. In the streptozotocin-induced diabetic rats, the activity of the glucose-6-phosphatase enzyme increased four-fold without appreciably altering the transport capacity of the glucose-6-phosphatase system. The solubilized diabetic rat liver glucose-6-phosphatase enzyme appeared to be very similar to the solubilized enzyme from control rat liver microsomes. They exhibit the same Km, are labile at 30 degrees C, are stabilized by sodium fluoride and they migrate to the same position during density gradient centrifugation. Immunological studies demonstrated that a greater amount of hepatic microsomal glucose-6-phosphatase enzyme protein is present in diabetic rats than in control rats. Thus, we have determined for the first time that increased levels of the glucose-6-phosphatase protein are present in streptozotocin-induced diabetes. The significance of this finding in relation to the regulation of the hepatic microsomal glucose-6-phosphatase system is discussed.