Permeability of rat liver microsomal membrane to glucose 6-phosphate

Proinsulin C-peptide potentiates the inhibitory action of insulin on glucose synthesis in primary cultured rabbit kidney-cortex tubules: Metabolic studies

Effects of equimolar concentrations of proinsulin C-peptide and insulin on glucose synthesis were studied in primary cultures of rabbit kidney-cortex tubules grown in the presence of alanine, glycerol, and octanoate. The rhodamine-labeled C-peptide entered renal tubular cells and localized in nuclei, both in the presence and absence of insulin; preincubations with the unlabeled compound inhibited internalization. C-peptide did not affect glucose formation when added alone but potentiated the inhibitory action of insulin by about 20% due to a decrease in flux through glucose-6-phosphate isomerase (GPI) and (or) glucose-6-phosphatase (G6Pase). GPI inhibition was caused by: (i) increased intracellular contents of fructose-1,6-bisphosphate and fructose-1-phosphate, inhibitors of the enzyme and (ii) reduced level of the phosphorylated GPI, which exhibits higher enzymatic activity in the presence of casein kinase 2. A decrease in flux through G6Pase, due to diminished import of G6P by G6P-transporter from the cytoplasm into endoplasmic reticulum lumen, is also suggested. The data show for the first time that in the presence of insulin and C-peptide, both GPI and G6P-ase may act as regulatory enzymes of renal gluconeogenic pathway.

Evidence that adrenal hexose-6-phosphate dehydrogenase can effect microsomal P450 cytochrome steroidogenic enzymes

The role of adrenal hexose-6-phosphate dehydrogenase in providing reducing equivalents to P450 cytochrome steroidogenic enzymes in the endoplasmic reticulum is uncertain. Hexose-6-phosphate dehydrogenase resides in the endoplasmic reticulum lumen and co-localizes with the bidirectional enzyme 11β-hydroxysteroid dehydrogenase 1. Hexose-6-phosphate dehydrogenase likely provides 11β-hydroxysteroid dehydrogenase 1 with NADPH electrons via channeling. Intracellularly, two compartmentalized reactions generate NADPH upon oxidation of glucose-6-phosphate: cytosolic glucose-6-phosphate dehydrogenase and microsomal hexose-6-phosphate dehydrogenase. Because some endoplasmic reticulum enzymes require an electron donor (NADPH), it is conceivable that hexose-6-phosphate dehydrogenase serves in this capacity for these pathways. Besides 11β-hydroxysteroid dehydrogenase 1, we examined whether hexose-6-phosphate dehydrogenase generates reduced pyridine nucleotide for pivotal adrenal microsomal P450 enzymes. 21-hydroxylase activity was increased with glucose-6-phosphate and, also, glucose and glucosamine-6-phosphate. The latter two substrates are only metabolized by hexose-6-phosphate dehydrogenase, indicating that requisite NADPH for 21-hydroxylase activity was not via glucose-6-phosphate dehydrogenase. Moreover, dihydroepiandrostenedione, a non-competitive inhibitor of glucose-6-phosphate dehydrogenase, but not hexose-6-phosphate dehydrogenase, did not curtail activation by glucose-6-phosphate. Finally, the most compelling observation was that the microsomal glucose-6-phosphate transport inhibitor, chlorogenic acid, blunted the activation by glucose-6-phosphate of both 21-hydroxylase and 17-hydroxylase indicating that luminal hexose-6-phosphate dehydrogenase can supply NADPH for these enzymes. Analogous kinetic observations were found with microsomal 17-hydroxylase. These findings indicate that hexose-6-phosphate dehydrogenase can be a source, but not exclusively so, of NADPH for several adrenal P450 enzymes in the steroid pathway. Although the reduced pyridine nucleotides are produced intra-luminally, these compounds may also slowly transverse the endoplasmic reticulum membrane by unknown mechanisms.

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.

Hexose-6-phosphate dehydrogenase in the endoplasmic reticulum

Hexose-6-phosphate dehydrogenase (H6PD) is a luminal enzyme of the endoplasmic reticulum that is distinguished from cytosolic glucose-6-phosphate dehydrogenase by several features. H6PD converts glucose-6-phosphate and NADP(+) to 6-phosphogluconate and NADPH, thereby catalyzing the first two reactions of the pentose-phosphate pathway. Because the endoplasmic reticulum has a separate pyridine nucleotide pool, H6PD provides NADPH for luminal reductases. One of these enzymes, 11beta-hydroxysteroid dehydrogenase type 1 responsible for prereceptorial activation of glucocorticoids, has been the focus of much attention as a probable factor in the pathomechanism of several human diseases including insulin resistance and the metabolic syndrome. This review summarizes recent advances related to the functions of H6PD.

The glucose-6-phosphate transporter-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system of the adipose tissue

11beta-hydroxysteroid dehydrogenase type 1, expressed mainly in the endoplasmic reticulum of adipocytes and hepatocytes, plays an important role in the prereceptorial activation of glucocorticoids. In liver endoplasmic reticulum-derived microsomal vesicles, nicotinamide adenine dinucleotide phosphate reduced supply to the enzyme is guaranteed by a tight functional connection with hexose-6-phosphate dehydrogenase and the glucose-6-phosphate transporter (G6PT). In adipose tissue, the proteins and their activities supporting the action of 11beta-hydroxysteroid dehydrogenase type 1 have not been explored yet. Here we report the occurrence of the hexose-6-phosphate dehydrogenase in rat epididymal fat, as detected at the level of mRNA, protein, and activity. In the isolated microsomes, the activity was evident only on the permeabilization of the membrane because of the poor permeability to the cofactor nicotinamide adenine dineucleotide phosphate (NADP(+)), which is consistent with the intralumenal compartmentation of both the enzyme and a pool of pyridine nucleotides. In fat cells, the access of the substrate, glucose-6-phosphate to the intralumenal hexose-6-phosphate dehydrogenase appeared to be mediated by the liver-type G6PT. In fact, the G6PT expression was revealed at the level of mRNA and protein. Accordingly, the transport of glucose-6-phosphate was demonstrated in microsomal vesicles, and it was inhibited by S3483, a prototypic inhibitor of G6PT. Furthermore, isolated adipocytes produced cortisol on addition of cortisone, and the production was markedly inhibited by S3483. The results show that adipocytes are equipped with a functional G6PT-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system and indicate that all three components are potential pharmacological targets for modulating local glucocorticoid activation.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Immunodetection of the expression of microsomal proteins encoded by the glucose 6-phosphate transporter gene

Glucose 6-phosphate transport has been well characterized in liver microsomes. The transport is required for the functioning of the glucose-6-phosphatase enzyme that is situated in the lumen of the hepatic endoplasmic reticulum. The genetic deficiency of the glucose 6-phosphate transport activity causes a severe metabolic disease termed type 1b glycogen storage disease. The cDNA encoding a liver transporter for glucose 6-phosphate was cloned and was found to be mutated in patients suffering from glycogen storage disease 1b. While related mRNAs have been described in liver and other tissues, the encoded protein(s) has not been immunologically characterized yet. In the present study, we report (using antibodies against three different peptides of the predicted amino acid sequence) that a major protein encoded by the glucose 6-phosphate transporter gene is expressed in the endoplasmic reticulum membranes of rat and human liver. The protein has an apparent molecular mass of approx. 33 kDa using SDS/PAGE, but several lines of evidence indicate that its real molecular mass is 46 kDa, as expected. The glucose 6-phosphate transporter protein was also immunodetected in kidney microsomes, but not in microsomes derived from human fibrocytes, rat spleen and lung, and a variety of cell lines. Moreover, little or no expression of the glucose 6-phosphate transporter protein was found in liver microsomes obtained from three glycogen storage disease 1b patients, even bearing mutations that do not directly interfere with protein translation, which can be explained by a (proteasome-mediated) degradation of the mutated transporter.

Cholesterol depletion perturbs calcium handling by rat submandibular glands

The sensitivity to cholesterol depletion of calcium handling by rat submandibular glands was investigated. The glands were digested with collagenase. After homogenization, the lysate was extracted at 4 degrees C with 0.5% Triton X-100 and the extract was submitted to an ultracentrifugation in a sucrose discontinuous gradient. A population of detergent-resistant membranes (DRM) was collected at the 5%-35% interface. The DRM had a higher content of cholesterol, saturated and long-chain fatty acids. Caveolin-1 and alpha(q/11) were located in these membranes. They were more ordered than vesicles from total cellular lysate as determined by anisotropy measurement. They disappeared after cholesterol extraction with methyl-beta-cyclodextrin (MCD). Exposure of the cellular suspension with MCD nearly abolished the response to carbachol, epinephrine, and substance P and inhibited the activation of phospholipase C (PLC) by these agonists and by sodium fluoride. MCD did not affect the mobilization of intracellular pools of calcium by thapsigargin. It increased the uptake of extracellular calcium or barium and did not inhibit the uptake of calcium after depletion of the intracellular stores of this ion. From these results, it is concluded that Triton X-100 can extract a fraction of membrane resistant to detergents. Treatment of the cells with MCD disrupts these membranes. The coupling between the heterotrimeric GTP-binding protein G(q/11) and poly-phosphoinositide-specific PLC is affected by disruption of these membrane fractions. At the opposite, the store-operated calcium channel (SOCC) is not affected by DRM-disruption.

Electron microscopical enzyme histochemistry on unfixed tissues and cells. Bridging the gap between LM and EM enzyme histochemistry

In principle, enzyme histochemistry should be performed on unfixed tissues and cells to avoid inhibition of enzyme activity by chemical fixation. For EM enzyme histochemistry, unfixed tissue specimens include fresh tissue blocks, non-frozen tissue chopper sections, cryostat sections and cell preparations. Studies on localization of enzyme activity at the ultrastructural level in unfixed specimens, be it fresh or frozen, are reviewed here. Preservation of ultrastructural morphology is discussed with special attention to the effects of freezing. It is concluded that unfixed cryostat sections are the best alternative for EM histochemistry of tissues, when interposing a semipermeable membrane in between cryostat section and gelled incubation medium. It is an adequate method to preserve structural integrity of unfixed tissue on the one hand and to avoid inactivation of the enzyme by chemical fixation on the other. For EM cytochemistry on individual cells, a better preservation of ultrastructure may be obtained because freezing can be avoided, but mild pretreatment with a fixative or detergent may be necessary to permeabilize cellular membranes for demonstration of intracellular enzyme activity.

Inhibition of microsomal glucose-6-phosphate transport in human neutrophils results in apoptosis: a potential explanation for neutrophil dysfunction in glycogen storage disease type 1b

Mutations in the gene of the hepatic glucose-6-phosphate transporter cause glycogen storage disease type 1b. In this disease, the altered glucose homeostasis and liver functions are accompanied by an impairment of neutrophils/monocytes. However, neither the existence of a microsomal glucose-6-phosphate transport, nor the connection between its defect and cell dysfunction has been demonstrated in neutrophils/monocytes. In this study we have characterized the microsomal glucose-6-phosphate transport of human neutrophils and differentiated HL-60 cells. The transport of glucose-6-phosphate was sensitive to the chlorogenic acid derivative S3483, N-ethylmaleimide, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, known inhibitors of the hepatic microsomal glucose-6-phosphate transporter. A glucose-6-phosphate uptake was also present in microsomes from undifferentiated HL-60 and Jurkat cells, but it was insensitive to S3483. The treatment with S3484 of intact human neutrophils and differentiated HL-60 cells mimicked some leukocyte defects of glycogen storage disease type 1b patients (ie, the drug inhibited phorbol myristate acetate-induced superoxide anion production and reduced the size of endoplasmic reticulum Ca(2+) stores). Importantly, the treatment with S3484 also resulted in apoptosis of human neutrophils and differentiated HL-60 cells, while undifferentiated HL-60 and Jurkat cells were unaffected by the drug. The proapoptotic effect of S3483 was prevented by the inhibition of nicotinamide adenine dinucleotide phosphate oxidase or by antioxidant treatment. These results suggest that microsomal glucose-6-phosphate transport has a role in the antioxidant protection of neutrophils, and that the genetic defect of the transporter leads to the impairment of cellular functions and apoptosis.

T cell receptor can be recruited to a subset of plasma membrane rafts, independently of cell signaling and attendantly to raft clustering

The constitutive/inducible association of the T cell receptor (TCR) with isolated detergent-resistant, lipid raft-derived membranes has been studied in Jurkat T lymphocytes. Membranes resistant to 1% Triton X-100 contained virtually no CD3epsilon, part of the TCR complex, irrespective of cell stimulation. On the other hand, membranes resistant either to a lower Triton X-100 concentration (i.e. 0.2%) or to the less hydrophobic detergent Brij 58 (1%) contained (i) a low CD3epsilon amount (approximate 2.7% of total) in resting cells and (ii) a several times higher amount of the TCR component, after T cell stimulation with either antigen-presenting cells or with phytohemagglutinin. It appeared that CD3/TCR was constitutively associated with and recruited to a raft-derived membrane subset because (i) all three membrane preparations contained a similar amount of the raft marker tyrosine kinase Lck but no detectable amounts of the conventional membrane markers, CD45 phosphatase and transferrin receptor; (ii) a larger amount of particulate membranes were resistant to solubilization with 0.2% Triton X-100 and Brij 58 than to solubilization with 1% Triton X-100; and (iii) higher cholesterol levels were present in membranes resistant to either the lower Triton X-100 concentration or to Brij 58, as compared with those resistant to 1% Triton X-100. The recruitment of CD3 to the raft-derived membrane subset appeared (i) to occur independently of cell signaling events, such as protein-tyrosine phosphorylation and Ca(2+) mobilization/influx, and (ii) to be associated with clustering of plasma membrane rafts induced by multiple cross-linking of either TCR or the raft component, ganglioside GM(1). We suggest that during T cell stimulation a lateral reorganization of rafts into polarized larger domains can determine the recruitment of TCR into these domains, which favors a polarization of the signaling cascade.

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).

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.

Identification of two novel and potent competitive inhibitors of the glucose-6-phosphatase catalytic protein

AIM

In this study, we show that inhibitors of the glucose-6-phosphatase (G-6-Pase) catalytic protein could be an alternative approach to the recent G-6-Pase T1-translocase inhibitors to target this enzyme for the treatment of type 2 diabetes.

METHOD

The active enantiomers of 4-methoxyphenyl-[4-(4-methoxyphenyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridin-5-yl]methanone (Compound A-1) and 4-methoxyphenyl-[4-(4-trifluoromethoxyphenyl)-4,5,6,7-tetrahydrothieno[3,2-c]pyridin-5-yl]methanone (Compound B-1) were characterized as inhibitors of the G-6-Pase catalytic protein using pig and rat liver microsomes and cultured rat hepatocytes.

RESULTS

Both compounds were found to be potent competitive inhibitors of the G-6-Pase catalytic protein obtained from pig and rat liver microsomes. The K(i) values (microM) were calculated to be 0.61 +/- 0.02 and 0.63 +/- 0.08 for compound A-1 and B-1 on intact pig microsomes, and 0.27 +/- 0.02 and 0.29 +/- 0.06 on disrupted pig microsomes. The corresponding values for rat liver microsomes were found to be 3.3 +/- 0.6 and 4.0 +/- 1.2 for compound A-1 and B-1 on intact microsomes, and 1.54 +/- 0.1 and 1.21 +/- 0.1 on disrupted microsomes. Compound A-1 was also able to inhibit pyrophosphatase activities from both intact and disrupted microsomes with equal potency (IC50; 0.43-0.55 microm). Using cultured rat hepatocytes and glycerol as the substrate, these compounds were able to prevent glucose production up to 60% with a concomitant increase in the G-6-P content (2.3-fold) using compound A-1. No increase in glycogen levels was seen.

CONCLUSION

These data demonstrated that these compounds were more potent inhibitors on G-6-Pase obtained from pig microsomes and were able to penetrate the microsomal membrane. The hepatocyte data further support the kinetic data, and are also consistent with the evoked mechanism of action.

Probing into the function of the gene product responsible for glycogen storage disease type Ib

This study aimed at directly assessing glucose 6-phosphate (G6P) transport by intact rat liver microsomes. Tracer uptake from labeled G6P occurred with T(1/2) values that proved insensitive to unlabeled G6P or 100 microM vanadate, and could not be activated over background levels by intravesicular phosphate in the complete absence of G6P hydrolysis. [(32)P]Phosphate efflux was similarly unaffected by G6P or phosphate in the incubation medium. We conclude that the gene product responsible for glycogen storage disease type Ib is functionally distinct from the bacterial hexose phosphate transporter, which operates as an obligatory phosphate:phosphate or G6P:phosphate exchanger.

Glucose-6-phosphatase in the insulin secreting cell line INS-1

The glucose-6-phosphatase system of the glucose sensitive insulin secreting rat insulinoma cells (INS-1) was investigated. INS-1 cells contain easily detectable levels of glucose-6-phosphatase enzyme protein (assessed by Western blotting) and have a very significant enzymatic activity. The features of the enzyme (Km and Vmax values, sensitivity to acidic pH, partial latency, and double immunoreactive band) are similar to those of the hepatic form. On the other hand, hardly detectable levels of glucose-6-phosphatase activity and protein were present in the parent glucose insensitive RINm5F cell line. The mRNA of the glucose-6-phosphate transporter was also more abundant in the INS-1 cells. The results support the view that the glucose-6-phosphatase system of the beta-cell is associated with the regulation of insulin secretion.

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.

Beta-glucuronidase latency in isolated murine hepatocytes

The physiological function of microsomal beta-glucuronidase is unclear. Substrates may be either glucuronides produced in the lumen of endoplasmic reticulum (ER) or those taken up by hepatocytes. In the latter case, efficient inward transport of glucuronides at the plasma membrane and the ER membrane would be required. Therefore, the potential role of beta-glucuronidase in ER was investigated. Isolated mouse hepatocytes and mouse and rat liver microsomal vesicles were used in the experiments. Selective permeabilization of the plasma membrane of isolated hepatocytes with saponin or digitonin resulted in an almost 4-fold elevation in the rate of beta-nitrophenol glucuronide hydrolysis, while the permeabilization of plasma membrane plus ER membrane by Triton X-100 caused a further 2-fold elevation. In microsomal vesicles, the p-nitrophenol glucuronide or phenolphthalein glucuronide beta-glucuronidase activity showed about 50% latency as revealed by alamethicin or Triton X-100 treatment. A light-scattering study indicated that the microsomes are relatively impermeable to both glucuronides and to glucuronate. On the basis of our results, the role of liver microsomal beta-glucuronidase in the deconjugation of glucuronides taken up by the liver seems unlikely. Hydrolysis of the glucuronides produced in the ER lumen may play a role in substrate supply for ascorbate synthesis or in "proofreading" of glucuronidation.

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.

Identification of protein components of the microsomal glucose 6-phosphate transporter by photoaffinity labelling

The glucose-6-phosphatase system catalyses the terminal step of hepatic glucose production from both gluconeogenesis and glycogenolysis and is thus a key regulatory factor of blood glucose homoeostasis. To identify the glucose 6-phosphate transporter T1, we have performed photoaffinity labelling of human and rat liver microsomes by using the specific photoreactive glucose-6-phosphate translocase inhibitors S 0957 and S 1743. Membrane proteins of molecular mass 70, 55, 33 and 31 kDa were labelled in human microsomes by [3H]S 0957, whereas in rat liver microsomes bands at 95, 70, 57, 54, 50, 41, 33 and 31 kDa were detectable. The photoprobe [3H]S 1743 led to the predominant labelling of a 57 kDa and a 50 kDa protein in the rat. Stripping of microsomes with 0.3% CHAPS retains the specific binding of T1 inhibitors; photoaffinity labelling of such CHAPS-treated microsomes resulted in the labelling of membrane proteins of molecular mass 55, 33 and 31 kDa in human liver and 50, 33 and 31 kDa in rat liver. Photoaffinity labelling of human liver tissue samples from a healthy individual and from liver samples of patients with a diagnosed glycogen-storage disease type 1b (GSD type 1b; von Gierke's disease) revealed the absence of the 55 kDa protein from one of the patients with GSD type 1. These findings support the identity of the glucose 6-phosphate transporter T1, with endoplasmic reticulum protein of molecular mass 50 kDa in rat liver and 55 kDa in human liver.

Preferential transport of glutathione versus glutathione disulfide in rat liver microsomal vesicles

A bi-directional, saturable transport of glutathione (GSH) was found in rat liver microsomal vesicles. GSH transport could be inhibited by the anion transport blockers flufenamic acid and 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid. A part of GSH taken up by the vesicles was metabolized to glutathione disulfide (GSSG) in the lumen. Microsomal membrane was virtually nonpermeable toward GSSG; accordingly, GSSG generated in the microsomal lumen could hardly exit. Therefore, GSH transport, contrary to previous assumptions, is preferred in the endoplasmic reticulum, and GSSG entrapped and accumulated in the lumen creates the oxidized state of its redox buffer.

Structure and mutation analysis of the glycogen storage disease type 1b gene

Glycogen storage disease (GSD) 1b is the deficiency of endoplasmic reticulum glucose-6-phosphate (G6P) transport. We here report the structure of the gene encoding a protein likely to be responsible for G6P transport, and its mapping to human chromosome 11q23.3. The gene is composed of nine exons spanning a genomic region of approximately 4 kb. Primers based on the genomic sequence were used in single strand conformation polymorphism (SSCP) analysis and mutations were found in six out of seven GSD 1b patients analysed.

Ontogeny of the murine glucose-6-phosphatase system

A deficiency in microsomal glucose-6-phosphatase (G6Pase) activity causes glycogen storage disease type 1 (GSD-1), a clinically and biochemically heterogeneous group of diseases. It has been suggested that catalysis by G6Pase involves multiple components, with defects in the G6Pase catalytic unit causing GSD-1a and defects in the putative substrate and product translocases causing GSD-1b, 1c, and 1d. However, this model is open to debate. To elucidate the G6Pase system, we have examined G6Pase mRNA expression, G6Pase activity, and glucose 6-phosphate (G6P) transport activity in the murine liver and kidney during normal development. In the liver, G6Pase mRNA and enzymatic activity were detected at 18 days gestation and increased markedly at parturition, before leveling off to adult levels. In the kidney, G6Pase mRNA and enzymatic activity appeared at 19 days gestation and peaked at weaning, suggesting that kidney G6Pase may have a different metabolic role. In situ hybridization analysis demonstrated that, in addition to the liver and kidney, the intestine expressed G6Pase. Despite the expression of G6Pase in the embryonic liver, microsomal G6P transport activity was not detectable until birth, peaking at about age 4 weeks. Our study strongly supports the multicomponent model for the G6Pase system.

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.

Gulonolactone oxidase activity-dependent intravesicular glutathione oxidation in rat liver microsomes

The orientation of gulonolactone oxidase activity was investigated in rat liver microsomes. Ascorbate formation upon gulonolactone addition resulted in higher intravesicular than extravesicular ascorbate concentrations in native microsomal vesicles. The intraluminal ascorbate accumulation could be prevented or the accumulated ascorbate could be released by permeabilising the vesicles with the pore-forming alamethicin. The formation of the other product of the enzyme, hydrogen peroxide caused the preferential oxidation of intraluminal glutathione in glutathione-loaded microsomes. In conclusion, these results suggest that the orientation of the active site of gulonolactone oxidase is intraluminal and/or the enzyme releases its products towards the lumen of the endoplasmic reticulum.

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.

Effects of 3-mercaptopicolinic acid and a derivative of chlorogenic acid (S-3483) on hepatic and islet glucose-6-phosphatase activity

Glucose-6-phosphatase activity was measured in hepatic microsomes and in pancreatic islets from ob/ob mice. In hepatic microsomes vanadate, phlorizin, 3-mercaptopicolinic acid and a derivative of chlorogenic acid (S-3483) inhibited the translocase activity of the enzyme, vanadate in addition inhibited hydrolase activity. In islets, vanadate inhibited both components of the enzyme, phlorizin inhibited only hydrolase activity while 3-mercaptopicolinic acid and compound S-3483 were without effect. Similarly, when islets were incubated with 3H2O and unlabeled glucose, the incorporation of 3H into medium glucose was inhibited by vanadate and phlorizin, but not by 3-mercaptopicolinic acid and S-3483. These findings suggest that, as with glucokinase, different isoenzymes of glucose-6-phosphatase are present in islets and liver.

Microsomal steroid sulfatase: interactions with cytosolic steroid sulfotransferases

Net sulfation of 4-methylumbelliferone in intact hepatocytes is regulated, in part, by substrate cycling between sulfotransferases (SULT) and arylsulfatases (ARS). Thus, ARS have the potential to influence rates of net sulfate conjugation of a variety of compounds in intact cells via interaction with SULT. Unlike ARSA and ARSB, which are lysosomal, steroid sulfate sulfatase (ARSC, also known as STS) is localized exclusively in the endoplasmic reticulum (ER). The present study was designed to assess the existence and extent of substrate cycling between steroids and their sulfate conjugates through ARSC and SULT, and also to initiate studies of the topology of the catalytic site of ARSC in the rat liver ER. Addition of rat liver microsomes to cytosol and 3'-phosphoadenosine 5'-phosphosulfate (PAPS) reduced rates of sulfation of dehydroepiandrosterone (DHEA) by SULT, and similarly hydrolysis of DHEA sulfate (DHEAS) was reduced when recombinant human hydroxysteroid SULT was added to rat liver microsomes in the presence of PAPS. There was no evidence for ARSC latency in the presence of detergent at either 4 or 37 degrees C, indicating that facilitated transport of steroid sulfates across the ER membrane may not be required for ARSC activity. The effect of proteases on ARSC activity in intact and disrupted microsomes was determined and compared with effects on components of the glucose-6-phosphatase system known to be localized on the lumenal and cytoplasmic surfaces of the ER. In contrast to the components of the glucose-6-phosphatase system, activity of ARSC in both intact and disrupted microsomes was substantially more resistant to protease inactivation. Our results indicate that substrate cycling of steroids and their sulfates does occur, and suggest that the active site of ARSC may be located within the ER membrane.

Dehydroascorbate and ascorbate transport in rat liver microsomal vesicles

Ascorbate and dehydroascorbate transport was investigated in rat liver microsomal vesicles using radiolabeled compounds and a rapid filtration method. The uptake of both compounds was time- and temperature-dependent, and saturable. Ascorbate uptake did not reach complete equilibrium, it had low affinity and high capacity. Ascorbate influx could not be inhibited by glucose, dehydroascorbate, or glucose transport inhibitors (phloretin, cytochalasin B) but it was reduced by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and by the alkylating agent N-ethylmaleimide. Ascorbate uptake could be stimulated by ferric iron and could be diminished by reducing agents (dithiothreitol, reduced glutathione). In contrast, dehydroascorbate uptake exceeded the level of passive equilibrium, it had high affinity and low capacity. Glucose cis inhibited and trans stimulated the uptake. Glucose transport inhibitors were also effective. The presence of intravesicular reducing compounds increased, while extravesicular reducing environment decreased dehydroascorbate influx. Our results suggest that dehydroascorbate transport is preferred in hepatic endoplasmic reticulum and it is mediated by a GLUT-type transporter. The intravesicular reduction of dehydroascorbate leads to the accumulation of ascorbate and contributes to the low intraluminal reduced/oxidized glutathione ratio.

Liver microsomal transport of glucose-6-phosphate, glucose, and phosphate in type 1 glycogen storage disease

The transport of glucose-6-phosphate (G6P), glucose, and orthophosphate into liver microsomes, isolated from six patients with various subtypes of type 1 glycogen storage disease (GSD), was measured using a light-scattering method. We found that G6P, glucose, and phosphate could all cross the microsomal membrane, in four cases of type 1a GSD. In contrast, liver microsomal transport of G6P and phosphate was deficient in the GSD 1b and 1c patients, respectively. These results support the involvement of multiple proteins (and genes) in GSD type 1. The results obtained with the light-scattering method are in accordance with conventional kinetic analysis of the microsomal glucose-6-phosphatase system. Therefore, this technique could be used to directly diagnose type 1b and 1c GSD.

Sequence of a putative glucose 6-phosphate translocase, mutated in glycogen storage disease type Ib

We report the sequence of a human cDNA that encodes a 46 kDa transmembrane protein homologous to bacterial transporters for phosphate esters. This protein presents at its carboxy terminus the consensus motif for retention in the endoplasmic reticulum. Northern blots of rat tissues indicate that the corresponding mRNA is mostly expressed in liver and kidney. In two patients with glycogen storage disease type Ib, mutations were observed that either replaced a conserved Gly to Cys or introduced a premature stop codon. The encoded protein is therefore most likely the glucose 6-phosphate translocase that is functionally associated with glucose-6-phosphatase.

Demonstration of a metabolically active glucose-6-phosphate pool in the lumen of liver microsomal vesicles

Glucose-6-phosphate transport was investigated in rat or human liver microsomal vesicles using rapid filtration and light-scattering methods. Upon addition of glucose-6-phosphate, rat liver microsomes accumulated the radioactive tracer, reaching a steady-state level of uptake. In this phase, the majority of the accumulated tracer was glucose, but a significant intraluminal glucose-6-phosphate pool could also be observed. The extent of the intravesicular glucose pool was proportional with glucose-6-phosphatase activity. The relative size of the intravesicular glucose-6-phosphate pool (irrespective of the concentration of the extravesicular concentration of added glucose-6-phosphate) expressed as the apparent intravesicular space of the hexose phosphate was inversely dependent on glucose-6-phosphatase activity. The increase of hydrolysis by elevating the extravesicular glucose-6-phosphate concentration or temperature resulted in lower apparent intravesicular glucose-6-phosphate spaces and, thus, in a higher transmembrane gradient of glucose-6-phosphate concentrations. In contrast, inhibition of glucose-6-phosphate hydrolysis by vanadate, inactivation of glucose-6-phosphatase by acidic pH, or genetically determined low or absent glucose-6-phosphatase activity in human hepatic microsomes of patients suffering from glycogen storage disease type 1a led to relatively high intravesicular glucose-6-phosphate levels. Glucose-6-phosphate transport investigated by light-scattering technique resulted in similar traces in control and vanadate-treated rat microsomes as well as in microsomes from human patients with glycogen storage disease type 1a. It is concluded that liver microsomes take up glucose-6-phosphate, constituting a pool directly accessible to intraluminal glucose-6-phosphatase activity. In addition, normal glucose-6-phosphate uptake can take place in the absence of the glucose-6-phosphatase enzyme protein, confirming the existence of separate transport proteins.

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.

Inhibition of the glucose-6-phosphatase system by N-bromoacetylethanolamine phosphate, a potential affinity label for auxiliary proteins

N-Bromoacetylethanolamine phosphate (BAEP) has been used previously as an affinity label to study the hexose phosphate binding sites of fructose-6-P, 2-kinase:fructose-2, 6-bisphosphatase (Sakakibara et al. (1984) J. Biol. Chem. 259, 14023-14028). We have employed this compound to probe components of the glucose-6-phosphatase system using a combination of time-dependent and immediate inhibition kinetic techniques. Inhibition of D-glucose-6-phosphate (G6P) phosphohydrolase activity of native microsomes was irreversible and time- and inhibitor-concentration-dependent. Only a partial time-dependent, irreversible inhibition of the PPi phosphohydrolase activity of native microsomes was observed. BAEP inhibited PPi:glucose phosphotransferase activity of native microsomes in a concentration-dependent, irreversible manner which was more extensive than that seen with PPi phosphohydrolase, but less extensive than was observed with G6P phosphohydrolase. Disruption of microsomal integrity by detergent-treatment either prior to incubation with BAEP or subsequent to preliminary incubation with BAEP but prior to assay for activity abolished the time-dependent inhibition. These irreversible, time- and concentration-dependent inhibitory actions of BAEP thus are manifest at a site or sites where the intact membrane-bound enzyme first makes contact with substrates G6P and PPi. An additional site of inhibition by BAEP, through relatively weak, reversible competitive inhibition at the active catalytic site, is indicated by classical steady-state kinetic analysis. The irreversible, time- and concentration-dependent inhibitions by BAEP seen with G6P and PPi as substrates strongly suggest the potential utility of radio-labeled BAEP as an affinity label for the identification and ultimate isolation and study of uncharacterized auxiliary components of the glucose-6-phosphatase system.

Ultrastructural localization of activity of phosphatases by low temperature incubation of unfixed cryostat sections

In the present study, we demonstrate the activity of several phosphatases ultrastructurally in long-term (up to 24 months) cold-stored (-80 degrees C) rat tissues. Phosphatase activity was histochemically studied with the use of unfixed cryostat sections in combination with low temperature (4 degrees C) incubation conditions in order to prevent inactivation of enzyme activity and to limit the loss of ultrastructure. 5'-Nucleotidase activity was observed at plasma membranes, mainly at bile canalicular membranes of hepatocytes in liver. Thiamine pyrophosphatase activity was detected not only in trans side cisternae but also in medial and cis side cisternae of Golgi complexes in the parotid gland. Glucose-6-phosphatase activity was localized in endoplasmic reticulum as well as at the outer membrane of the nuclear envelope. Acid phosphatase reaction product was found in lysosomes. Furthermore, the localization patterns of 5'-nucleotidase and thiamine pyrophosphatase activity were compared with those obtained after different fixation procedures such as immediate chemical fixation of tissues or fixation of tissues after freezing and thawing. The results showed similar localization patterns of these enzymes after the different pretreatments. However, with respect to the ultrastructural morphology, some damage was observed in unfixed material after incubation. It can be concluded that the procedure described here enables ultrastructural localization of activity of phosphatases in long-term cold-stored tissues. This procedure will be useful for a retrospective study on archival material when histochemical parameters are needed.

Evidence for an UDP-glucuronic acid/phenol glucuronide antiport in rat liver microsomal vesicles

The transport of glucuronides synthesized in the luminal compartment of the endoplasmic reticulum by UDP-glucuronosyltransferase isoenzymes was studied in rat liver microsomal vesicles. Microsomal vesicles were loaded with p-nitrophenol glucuronide (5 mM), phenolphthalein glucuronide or UDP-glucuronic acid, by a freeze-thawing method. In was shown that: (i) the loading procedure resulted in millimolar intravesicular concentrations of the different loading compounds; (ii) addition of UDP-glucuronic acid (5 mM) to the vesicles released both intravesicular glucuronides within 1 min; (iii) glucuronides stimulated the release of UDP-glucuronic acid from UDP acid-loaded microsomal vesicles; (iv) trans-stimulation of UDP-glucuronic acid entry by loading of microsomal vesicles with p-nitrophenol glucuronide, phenolphthalein glucuronide, UDP-glucuronic acid and UDP-N-acetyl-glucosamine almost completely abolished the latency of UDP-glucuronosyltransferase, although mannose 6-phosphatase latency remained unaltered; (v) the loading compounds by themselves did not stimulate UDP-glucuronosyltransferase activity. This study indicates that glucuronides synthesized in the lumen of endoplasmic reticulum can leave by an antiport, which concurrently transports USP-glucuronic acid into the lumen of the endoplasmic reticulum.

Low levels of glucose-6-phosphate hydrolysis in the sarcoplasmic reticulum of skeletal muscle: involvement of glucose-6-phosphatase

Glucose-6-phosphate hydrolysis was measured in a fraction obtained from rabbit fast-twitch skeletal muscle and corresponding to total sarcoplasmic reticulum, as well as in three subfractions containing longitudinal tubules, terminal cisternae or both structures. In all cases the levels of hydrolysis measured both in native and disrupted membranes were approximately 60-100 times lower than the microsomal glucose-6-phosphatase activity of the corresponding livers. In contrast to liver microsomes, most (up to 80%) of the glucose-6-phosphate hydrolysing activity in muscle sarcoplasmic reticulum membranes was not inactivated by pH 5.0 pre-incubation indicating that it was not catalysed by the specific glucose-6-phosphatase enzyme. Osmotically induced changes in light-scattering intensity of sarcoplasmic reticulum vesicles revealed that, in contrast to liver microsomes, sarcoplasmic reticulum vesicles were not selectively permeable to glucose-6-phosphate as mannose-6-phosphate was also permeable and in addition they were poorly permeable to glucose. Immunoblot experiments using antibodies raised against the glucose-6-phosphatase enzyme, and liver endoplasmic reticulum glucose and Pi translocases, failed to detect the presence of these protein components in sarcoplasmic reticulum membranes. Southern blot analysis of reverse transcriptase-polymerase chain reaction products from rat muscle revealed that glucose-6-phosphatase mRNA is present in muscle. Quantification of Northern blot analysis of liver and muscle mRNA indicated that muscle contains less than 2% of the amount of glucose-6-phosphate mRNA found in corresponding livers. We conclude that very low levels of specific glucose-6-phosphatase (e.g. as in liver; E.C. 3.1.3.9) are present in muscle sarcoplasmic reticulum and that the muscle and liver glucose-6-phosphatase systems have several different properties.

Endoplasmic reticulum phosphate transport

The major role of the liver endoplasmic reticulum phosphate/pyrophosphate transport proteins is the regulation of blood glucose levels. The glucose-6-phosphatase enzyme is an endoplasmic reticulum enzyme system which hydrolyzes glucose-6-phosphate to glucose and phosphate. Glucose-6-phosphatase is the terminal step of both gluconeogenesis and glycogenolysis. The glucose-6-phosphatase enzyme is a very hydrophobic membrane protein and its active site is inside the lumen of the endoplasmic reticulum. The substrates and products of the enzyme therefore have to cross the endoplasmic reticulum membrane. The glucose-6-phosphatase enzyme is associated with a calcium binding protein (SP). There are also transport proteins for the substrate glucose-6-phosphate (T1) and the products phosphate (T2) and glucose (T3). There appear to be at least two different liver endoplasmic reticulum proteins that can transport phosphate. One of the proteins T2b can also transport pyrophosphate and carbamyl phosphate which are also substrates for the glucose-6-phosphatase enzyme. The metabolic regulation, genetic deficiencies, ontogeny and tissue distribution of the endoplasmic reticulum T2 proteins will be described.

Evidence for glucuronide (small molecule) sorting by human hepatic endoplasmic reticulum

The entry of substrates into, and the export of glururonides from, the lumen of hepatic endoplasmic reticulum (ER) in vitro (sealed microsomes) has been measured using radioactivity-labelled materials and a rapid filtration assay. Analysis of liver microsomes from a jaundiced patient showed the accumulation of bilirubin glucuronides within the lumen of the ER. Further analysis of these hepatic microsomes revealed that newly synthesized 1-naphthol glucuronide could exit from the microsomes whereas bilirubin glucuronide was accumulated within the microsomes. These results suggest the existence of mechanisms for the sorting of small molecules, destined for export through bile canalicular or basolateral plasma membranes, by ER. Furthermore, these sorting processes may be regulated by specific transporters within the ER.

Fatty acyl-CoA esters inhibit glucose-6-phosphatase in rat liver microsomes

In native rat liver microsomes glucose 6-phosphatase activity is dependent not only on the activity of the glucose-6-phosphatase enzyme (which is lumenal) but also on the transport of glucose-6-phosphate, phosphate and glucose through the respective translocases T1, T2 and T3. By using enzymic assay techniques, palmitoyl-CoA or CoA was found to inhibit glucose-6-phosphatase activity in intact microsomes. The effect of CoA required ATP and fatty acids to form fatty acyl esters. Increasing concentrations (2-50 microM) of CoA (plus ATP and 20 microM added palmitic acid) or of palmitoyl-CoA progressively decreased glucose-6-phosphatase activity to 50% of the control value. The inhibition lowered the Vmax without significantly changing the Km. A non-hydrolysable analogue of palmitoyl-CoA also inhibited, demonstrating that binding of palmitoyl-CoA rather than hydrolysis produces the inhibition. Light-scattering measurements of osmotically induced changes in the size of rat liver microsomal vesicles pre-equilibrated in a low-osmolality buffer demonstrated that palmitoyl-CoA alone or CoA plus ATP and palmitic acid altered the microsomal permeability to glucose 6-phosphate, but not to glucose or phosphate, indicating that T1 is the site of palmitoyl-CoA binding and inhibition of glucose-6-phosphatase activity in native microsomes. The type of inhibition found suggests that liver microsomes may comprise vesicles heterogeneous with respect to glucose-6-phosphate translocase(s), i.e. sensitive or insensitive to fatty acid ester inhibition.

Glucose-6-phosphatase proteins of the endoplasmic reticulum

Hepatic glucose-6-phosphatase (G-6-Pase) catalyses the terminal step of hepatic glucose production and it plays a key role in the maintenance of blood glucose homeostasis. Hepatic G-6-Pase is an integral resident endoplasmic reticulum (ER) protein and it is part of a multicomponent system. Its active site is situated inside the lumen of the ER and transport proteins are needed to allow its substrates, glucose-6-phosphate (G-6-P) (and pyrophosphate), and its products, phosphate and glucose to cross the ER membrane. In addition, a calcium-binding protein is also associated with the G-6-Pase enzyme. Recent immunological studies have shown that G-6-Pase (which has conventionally been thought to be present only in the gluconeogenic organs) is present in minor cell types in a variety of human tissues and that its distribution changes dramatically during human development. In all the tissues, enzymatic analysis, direct transport assays and/or immunological detection of the ER glucose and phosphate transport proteins have been used to demonstrate the presence and activity of the whole G-6-Pase system. The G-6-Pase protein is very hydrophobic and has proved difficult to purify to homogeneity. Four proteins of the system have now been isolated and polyclonal antibodies have been raised against them; two have also been cloned. The available sequences, together with topological studies, have given some information about both the topology of the proteins in the ER and the probable mechanisms by which the proteins are retained in the ER.

Astrocytic glucose-6-phosphatase and the permeability of brain microsomes to glucose 6-phosphate

Cells from primary rat astrocyte cultures express a 36.5 kDa protein that cross-reacts with polyclonal antibodies to the catalytic subunit of rat hepatic glucose-6-phosphatase on Western blotting. Glucose-6-phosphate-hydrolysing activity of the order of 10 nmol/min per mg of total cellular protein can be demonstrated in cell homogenates. This activity shows latency, and is localized to the microsomal fraction. Kinetic analysis shows a Km of 15 mM and a Vmax. of 30 nmol/min per mg of microsomal protein in disrupted microsomes. Approx. 40% of the total phosphohydrolase activity is specific glucose-6-phosphatase, as judged by sensitivity to exposure to pH 5 at 37 degrees C. Previous reports that the brain microsomal glucose-6-phosphatase system does not distinguish glucose 6-phosphate and mannose 6-phosphate are confirmed in astrocyte microsomes. However, we demonstrate significant phosphomannose isomerase activity in brain microsomes, allowing for ready interconversion between mannose 6-phosphate and glucose 6-phosphate (Vmax. 15 nmol/min per mg of microsomal protein; apparent Km < 1 mM; pH optimum 5-6 for the two-step conversion). This finding invalidates the past inference from the failure of brain microsomes to distinguish mannose 6-phosphate and glucose 6-phosphate that the cerebral glucose-6-phosphatase system lacks a 'glucose 6-phosphate translocase' [Fishman and Karnovsky (1986) J. Neurochem. 46, 371-378]. Furthermore, light-scattering experiments confirm that a proportion of whole brain microsomes is readily permeable to glucose 6-phosphate.

Isolation and the complete amino acid sequence of lumenal endoplasmic reticulum glucose-6-phosphate dehydrogenase

I have isolated glucose-6-phosphate dehydrogenase from rabbit liver microsomes and determined its complete amino acid sequence. Sequence determination was achieved by automated Edman degradation of peptides generated by chemical and enzymatic cleavages. The microsomal enzyme consists of 763 residues and is quite dissimilar from the previously characterized cytosolic enzymes. The N terminus of the microsomal enzyme is blocked by a pyroglutamyl residue. Carbohydrate is attached at Asn-138 and Asn-263, implying that the bulk of the protein is oriented on the lumenal side of the endoplasmic membrane. The amino acid sequence of the microsomal protein shows limited homology to the extensively sequenced cytosolic glucose-6-phosphate dehydrogenases. Clusters of up to six identical residues can be identified in four regions: peptide segments at residues 10-21, 154-163, and 173-261. In addition, another array of identical residues, requiring a 100-residue deletion in the sequence of the microsomal enzyme, spans residues 436-462 and corresponds to residues 348-373 of the cytosolic protein. Two segments with a Gly-Xaa-Gly-Xaa-Xaa-Gly motif, related to a coenzyme binding fold, were identified at Gly-399 and Gly-491. In the cytosolic enzymes, a variation of this sequence motif occurs at Gly-37 and Gly-241. The 300-residue C-terminal segment of the microsomal enzyme is unique and has no counterpart in the cytosolic or the bacterial enzymes. An unexpected finding with regard to the microsomal enzyme is that it lacks an identifiable membrane-spanning region or the lumenal-protein C-terminal consensus sequences Lys-Asp-Glu or His-Ile/Thr-Glu-Leu. Thus, the mode of transport and retention of this protein in the lumen of endoplasmic reticulum remains to be determined.

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.