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Query: EC:3.1.3.9 (glucose-6-phosphatase)
 3,081 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. Glucose-6-phosphatase (EC 3.1.3.9 D-glucose-6-phosphate phosphohydrolase) was found to be localized mainly in the endoplasmic reticulum (microsomal fraction) of all species of vertebrate liver tissue examined. 2. Hepatopancreas tissue from gastropod molluscs was found to be unique in showing the localization of glucose-6-phosphatase in the cytosol (soluble fraction).
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PMID:Subcellular localization of glucose-6-phosphatase in animal tissues. 23 8

Human blood platelets contain no detectable activity of the enzymes fructose diphosphatase (EC 3.1.3.11), phospho-enolpyruvate carboxykinase (EC 4.1.1.32) and pyruvate carboxylase (EC 6.4.1.1.). Glucose-6-phosphatase (EC 3.1.3.9) activity is very low. Phosphofructokinase present in human blood platelets, catalyzes a reaction which can be stimulated by AMP in a platelet homogenate, due to the presence of endogenous ADP and myokinase. These enzymes are responsible for the formation of fructose-6-phosphate from fructose-1, 6-diphosphate. Pyruvate kinase (EC 2.7.1.40) in human blood platelets belongs to the M-type, which is not inhibited by ATP, at least not under the conditions applied. The results obtained indicate that gluconeogenesis in human blood platelets is not present in the way which has been established for liver and kidney.
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PMID:Insignificance of gluconeogenesis in human blood platelets. 112 26

Glucose-6-phosphatase activity has been determined in periportal and pericentral zones of the rat liver lobule using a quantitative histochemical method. The study was performed on unfixed cryostat sections of livers from fasted and fed female and male rats. Highest activity was found in periportal zones, and starvation caused a 2-3-fold increase of glucose-6-phosphatase activity in periportal and pericentral zones of both sexes. Unexpectedly, KM values were also significantly different in periportal and pericentral zones and were found to increase linearly with Vmax values, irrespective of sex and feeding condition. Because the cryofixation procedure was shown to permeabilize the biomembranes in the tissue sections, it can be concluded that the rise in KM and Vmax values has to be attributed to the catalytic unit of the glucose-6-phosphatase system. It is suggested that the enzyme exists in a high affinity configuration at low enzyme concentrations but that at high enzyme concentrations a hysteretic mechanism, as proposed by Berteloot et al. (Berteloot, A., Vidal, H., and Van de Werve, G. (1991) J. Biol. Chem. 266, 5497-5507), transforms the enzyme from a high to a low affinity configuration. The present study indicates that the concept of functional heterogeneity of liver parenchyma may be more complex than thus far assumed.
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PMID:In situ kinetic parameters of glucose-6-phosphatase in the rat liver lobulus. 131 22

Controlled proteolytic digestion by trypsin or bacterial proteases limited to the cytosolic side of the native microsomal membrane is not efficient to inhibit glucose-6-phosphate hydrolysis. Modification of the microsomes with deoxycholate prior to protease treatment is prerequisite to allow accessibility of the integral protein and inhibition of enzyme activity. Glucose-6-phosphatase of native microsomes, however, is rapidly inactivated by micromolar concentrations of TPCK as well as TLCK. In deoxycholate-modified microsomes both reagents do not affect glucose-6-phosphate hydrolysis. These results indicate that in the native, intact microsomal membrane glucose-6-phosphatase is not accessible to proteolytic attack from the cytoplasmic surface. The putative inhibitory effect of some trypsin or bacterial protease preparations on glucose-6-phosphatase of native microsomes observed most possibly is a result of contaminating agents as TPCK or TLCK.
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PMID:Protease inhibitors but not proteases inhibit the glucose-6-phosphatase of native rat liver microsomes. 131 35

Liver cell functional heterogeneity has been shown to persist in toxic CCl4 cirrhosis in growing rats, but the zonation observed in cirrhotic nodules may be different in other types of cirrhosis. To investigate this possibility, we looked at the zonal activities of two microsomal enzymes, glucose-6-phosphatase and NADPH dehydrogenase, in cirrhotic nodules from growing rats with chronic cholestasis. Zonal activities were measured by quantitative cytochemistry and microdensitometry. Liver cell heterogeneity was demonstrated, and we confirmed that the metabolic zonation is the mirror image of that observed in toxic cirrhosis, with periportal activity at the nodule periphery and perivenular activity at the nodule centers. Glucose-6-phosphatase activity was 2.06 times higher at the peripheries of the nodules than at the centers, whereas NADPH dehydrogenase activity at the nodule periphery was 72% of the nodule center activity. We conclude that a liver cell functional heterogeneity persists in biliary rat cirrhosis, with zonation the reverse of that previously found in toxic CCl4 cirrhosis.
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PMID:Liver metabolic zonation in rat biliary cirrhosis: distribution is reverse of that in toxic cirrhosis. 131 72

Methylthioadenosine sulfoxide (MTAS), an oxidized derivative of the cell toxic metabolite methylthioadenosine has been used in elucidating the relevance of an interrelationship between the catalytic behavior and the conformational state of hepatic glucose-6-phosphatase and in characterizing the transmembrane orientation of the integral unit in the microsomal membrane. The following results were obtained: (1) Glucose 6-phosphate hydrolysis at 37 degrees C is progressively inhibited when native microsomes are treated with MTAS at 37 degrees C. In contrast, glucose 6-phosphate hydrolysis of the same MTAS-treated microsomes assayed at 0 degrees C is not inhibited. (2) Subsequent modification of the MTAS-treated microsomes with Triton X-114 reveals that glucose-6-phosphatase assayed at 37 degrees C as well as at 0 degrees C is inhibited. (3) Although excess reagent is separated by centrifugation and the MTAS-treated microsomes diluted with buffer before being modified with Triton the temperature-dependent effect of MTAS on microsomal glucose-6-phosphatase is not reversed at all. (4) In native microsomes MTAS is shown to inhibit glucose-6-phosphatase noncompetitively. The subsequent Triton-modification of the MTAS-treated microsomes, however, generates an uncompetitive type of inhibition. (5) Preincubation of native microsomes with MTAS completely prevents the inhibitory effect of 4,4'-diisothiocyanostilbene 2,2'-disulfonate (DIDS) as well as 4,4'-diazidostilbene 2,2'-disulfonate (DASS) on glucose-6-phosphatase. (6) Low molecular weight thiols and tocopherol protect the microsomal glucose-6-phosphatase against MTAS-induced inhibition. (7) Glucose-6-phosphatase solubilized and partially purified from rat liver microsomes is also affected by MTAS in demonstrating the same temperature-dependent behavior as the enzyme of MTAS-treated and Triton-modified microsomes. From these results we conclude that MTAS modulates the enzyme catalytic properties of hepatic glucose-6-phosphatase by covalent modification of reactive groups of the integral protein accessible from the cytoplasmic surface of the microsomal membrane. The temperature-dependent kinetic behavior of MTAS-modulated glucose-6-phosphatase is interpreted by the existence of distinct catalytically active enzyme conformation forms. Detergent-induced modification of the adjacent hydrophobic microenvironment additionally generates alterations of the conformational state leading to changes of the kinetic characteristics of the integral enzyme.
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PMID:Modulation of the activity of hepatic glucose-6-phosphatase by methylthioadenosine sulfoxide. 165 32

Glycogen and protein concentrations and the activities of liver glycogen metabolic enzymes were measured in 22 children aged 4 to 15, suffering from extrahepatic portal hypertension. Glucose-6-phosphatase, amylo-1,6-glucosidase, fructose-1,6-diphosphatase, phosphorylases alpha and beta, phosphoglucomutase, and phosphohexose isomerase levels were analyzed. Liver biopsy specimens obtained by surgical marginal biopsy were used in the study. No or drastic reduction of phosphorylase alpha activity and reduction of glycogen concentration and glucose-phosphatase activity were found characteristic of extrahepatic hypertension. Analysis of correlations of the findings has demonstrated a medium correlation in 4 cases and a strong correlation between the findings in 1 case, the possibility being estimated as 0.95-0.99. The highest number of correlations was observed with phosphorylase alpha and glucose-6-phosphatase (3 correlations). Liver blood stream impairments result in injury to one of its main biochemical functions, i.e., the maintenance of blood glucose homeostasis, this leading to reduction of the adaptation potential of the body; this should be borne in mind when planning therapeutic measures for patients with extrahepatic hypertension.
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PMID:[Carbohydrate metabolism enzymes in children with extrahepatic portal hypertension]. 172 40

Glucose-6-phosphatase activity was measured in rat liver or pancreatic islet crude homogenates and microsomes. The data recorded in the liver were comparable to those reported in prior studies. However, in the islets, the hydrolysis of D-glucose 6-phosphate by disrupted microsomes represented, when expressed relative to the protein content, less than 2% of the value recorded in liver microsomes. Moreover, no phosphotransferase activity was detected in the islets. These findings impose reservation on both the presence of glucose-6-phosphatase in rat islets and its participation to stimulus-secretion coupling.
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PMID:Hexose metabolism in pancreatic islets: the glucose-6-phosphatase riddle. 184 30

It was known in the 1950s that hepatic microsomal glucose-6-phosphatase plays an important role in the regulation of blood glucose levels. All attempts since then to purify a single polypeptide with glucose-6-phosphatase activity have failed. Until recently, virtually nothing was known about the molecular basis of glucose-6-phosphatase or its regulation. Recent studies of the type 1 glycogen storage diseases, which are human genetic deficiencies that result in impaired glucose-6-phosphatase activity, have greatly increased our understanding of glucose-6-phosphatase. Glucose-6-phosphatase has been shown to comprise at least five different polypeptides, the catalytic subunit of glucose-6-phosphatase with its active site situated in the lumen of the endoplasmic reticulum; a regulatory Ca2+ binding protein; and three transport proteins, T1, T2, and T3, which respectively allow glucose-6-phosphate, phosphate, and glucose to cross the endoplasmic reticulum membrane. Purified glucose-6-phosphatase proteins, immunospecific antibodies, and improved assay techniques have led to the diagnosis of a variety of new type 1 glycogen storage diseases. Recent studies of the type 1 glycogen storage diseases have led to a much greater understanding of the role and regulation of each of the glucose-6-phosphatase proteins.
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PMID:Molecular pathology of glucose-6-phosphatase. 216 25

We describe a novel technique for the histochemical and cytochemical demonstration of glucose-6-phosphatase activity. In this method, lead is replaced by cobalt. After activity of glucose-6-phosphatase, cobalt phosphate Co3(PO4)2 is formed, and in the presence of ammonium sulfide (NH4)2S, the precipitate is transformed into a sulfide that fixes osmium and provides good electron density. Glucose-6-phosphatase activity was determined mostly in rat kidney cells, but controls were also performed in liver cells. A strong reaction was seen in proximal tubule cells, but the reaction was weak in distal convoluted tubule cells. This technique showed the same endoplasmic reticulum (ER) organization in proximal and distal nephron as that seen with the osmium impregnation technique. In collecting tubules, intercalated cells had irregular reactivity, while principal cells had none. Our results indicate that the cobalt technique is valid, reliable, and sensitive enough to detect low glucose-6-phosphatase activity. Moreover, the technique can be used with 1-mm-thick specimens and obviates the need for use of frozen tissue sections.
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PMID:A new method based on cobalt for histochemical and cytochemical demonstration of glucose-6-phosphatase activity. 216 94


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