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1.
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.  相似文献   

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The temperature dependence of glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase EC 3.1.3.9) was studied in rat liver and kidney microsomal fractions. Arrhenius plots were non-linear and showed four distinct discontinuities in enzyme activity over the temperature range 2-41 degrees C. The discontinuities occurred at approx. 39, 30, 20 and 12 degrees C in the liver and were similar to this in the kidney. Changes in the energy of activation for the enzyme were noted at approx. 20 degrees C in both tissues. The multiple discontinuities in glucose-6-phosphatase activity are viewed as a reflection of complex reorganization and/or change in physical state of the membrane components, primarily lipid.  相似文献   

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Progress has continued to be made over the past 4 years in our understanding of the glucose-6-phosphatase (G6Pase) system. The gene for a second component of the system, the putative glucose-6-P transporter (G6PT), was cloned, and mutations in this gene were found in patients diagnosed with glycogen storage disease type 1b. The functional characterization of this putative G6PT has been initiated, and the relationship between substrate transport via the G6PT and catalysis by the system's catalytic subunit continues to be explored. A lively debate over the feasibility of various aspects of the two proposed models of the G6Pase system persists, and the functional/structural relationships of the individual components of the system remain a hot topic of interest in G6Pase research. New evidence supportive of physiologic roles for the biosynthetic functions of the G6Pase system in vivo also has emerged over the past 4 years.  相似文献   

6.
Purification of particulate glucose-6-phosphatase   总被引:2,自引:0,他引:2  
C F Cori  R C Garland  H W Chang 《Biochemistry》1973,12(16):3126-3130
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7.
R C Nordlie 《Life sciences》1979,24(26):2397-2404
Glucose-6-phosphatase is a multifunctional enzyme, displaying potent ability to synthesize as well as hydrolyze Glc-6-P. These multifunctional characteristics have been exploited in studies of the extended distribution of the enzyme, and their physiological significance has been examined. The enzyme is considerably more widely distributed than previously suspected. It has been found in pancreas, adrenals, lung, testes, spleen, and brain as well as in liver, kidney, and mucosa of small intestine. Approximately 15–20% of total hepatic glucose-6-phosphatase-phosphotransferase is present in nuclear membrane, 75–80% is found in endoplasmic reticulum, and small amounts have been detected also in plasma membrane and repeatedly-washed mitochondria. Both hydrolytic and synthetic functions, in constant proportions, have been found in livers of 21 species of birds, amphibia, reptiles, crustacea, fishes, and mammals (including man) studied. With 5 mM phosphoryl donor and 100 mM D-glucose as substrates, carbamyl-P:glucose phosphotransferase activity of glucose-6-phosphatase exceeded that of glucokinase by 5–50 fold. While latencies of activities of isolated microsomal preparations are extensive, those of nuclear membranes are not. Latencies of activities of intact endoplasmic reticulum of permeable hepatocytes are 28% for Glc-6-P phosphohydrolase and 56% for carbamyl-P:glucose phosphotransferase. Studies with isolated perfused livers from fasted rats suggest rather convincingly that such phosphotransferase activities may function as an hepatic glucose-phosphorylating system supplemental to glucokinase and hexokinase. This conclusion is based both on comparisons of rates of glucose uptake with hepatic enzyme levels (glucokinase, hexokinase, phosphotransferase), and on observed inhibitibility of glucose uptake by ornithine and 3-0-methyl-D-glucose. The question of availability of adequate concentrations of suitable phosphoryl donor(s) in cytosol of the liver cell constitutes a principal focus for continuing studies regarding physiological functions of this enzyme.  相似文献   

8.
Molecular pathology of glucose-6-phosphatase   总被引:3,自引:0,他引:3  
A Burchell 《FASEB journal》1990,4(12):2978-2988
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.  相似文献   

9.
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).  相似文献   

10.
L-Proline's glycogenic action is unlike that of other amino acids in that it produces effects beyond those explainable by a simple increase in osmolarity (Baquet, A., Hue, L., Meijer, A. J., van Woerkom, G. M., and Plomp, P. J. A. M. (1990) J. Biol. Chem. 265, 955-959). We postulate that this effect may relate to inhibition of hepatic glucose-6-P hydrolysis by a proline-derived metabolite. We tested this hypothesis with isolated livers from rats fasted 48 h which were perfused with L-proline or L-glutamine. Net glucose and net glycogen production and levels of glucose-6-P and certain other hepatic metabolites were measured. The data obtained support our hypothesis by demonstrating fundamental differences in the metabolic fates of proline and glutamine in the liver. Both pass through alpha-ketoglutarate in the initial stage of gluconeogenesis, but proline supports hepatic glycogen formation while glutamine does not. The concomitant increase in hepatic glucose-6-P and proline-associated glyconeogenesis suggests that inhibition of glucose-6-P hydrolysis by a proline-derived metabolite may divert glucose-6-P produced from proline from glucose production and to glycogen synthesis. This conclusion is supported by the effects of perfusions with and without proline (3-mercaptopicolinate present) on (a) glyconeogenesis and glucose formation from dihydroxyacetone, (b) net glucose uptake and glycogen formation with 30 mM glucose as substrate, and (c) glucose production from endogenous glycogen in perfused livers from fed rats.  相似文献   

11.
The mechanism of activation of hepatic microsomal glucose-6-phosphatase (EC 3.1.3.9) by histone 2A has been investigated in both intact and disrupted microsomes. Histone 2A increased the Vmax and decreased the Km of glucose-6-phosphatase in intact microsomes but had no effect on glucose-6-phosphatase activity in disrupted microsomes. Histone 2A was shown to activate glucose-6-phosphatase in intact microsomes by disrupting the membrane vesicles and thereby allowing the direct measurement of the activity of the latent glucose-6-phosphatase enzyme. The study demonstrated that disrupting microsomes with histone 2A is an excellent method for directly assaying glucose-6-phosphatase activity as it poses none of the problems encountered with all of the previously used methods.  相似文献   

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Membrane effects on hepatic microsomal glucose-6-phosphatase.   总被引:1,自引:0,他引:1  
1) Rat liver microsomes exhibit only a weak hydrolyzing activity towards galactose 6-phosphate. Disruption of the microsomal vesicles does not change the apparent Michaelis constant for this substrate but enhances the apparent maximum velocity. 2) The inhibition of microsomal glucose-6-phosphatase (EC 3.1.3.9) by galactose 6-phosphate is of the competitive type in intact and disrupted microsomal vesicles, suggesting that both substrates are hydrolyzed by the same enzyme. 3) The high degree of latency found for the hydrolysis of galactose 6-phosphate compared to glucose 6-phosphate indicates the presence of a carrier for glucose 6-phosphate in the microsomal membrane. 4) Since glucose as a product is not trapped inside the microsomal vesicles, this sugar probably is able to penetrate the microsomal membrane.  相似文献   

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The thermal stability of glucose-6-phosphatase in rat liver microsomes was examined in untreated and cholate-treated microsomes. Activity of the enzyme was measured with both glucose-6-P and mannose-6-P as substrates. Heat treatment did not cause glucose-6-phosphatase activity to decline to zero with a single rate constant in untreated microsomes. Instead, heat treatment produced an enzyme with a small residual activity that was stable. The residual level of activity was not stimulated by addition of detergent. In untreated microsomes the energies of activation for the processes of decay were different for glucose-6-phosphatase and mannose-6-phosphatase activities, suggesting that the rate-limiting steps for the hydrolysis of these compounds were different. Treatment of microsomes with detergent increased the rate constants for the thermal decay of glucose-6-phosphatase by about 150 times, and, in contrast to untreated microsomes, glucose-6-phosphatase and mannose-6-phosphatase decayed to zero with a single rate constant in cholate-treated microsomes. Also, rate constants for thermal inactivation of glucose-6-phosphatase and mannose-6-phosphatase were the same in cholate-treated microsomes. Removal of cholate increased the stability of glucose-6-phosphatase but did not regenerate the form of the enzyme present in untreated microsomes. The data for the stability of glucose-6-phosphatase under different conditions provide evidence that the enzyme can exist in at least five different stable states that are enzymatically active.  相似文献   

16.
A comparative study was made of the effect of X-radiation on the membrane-bound glucoso-6-phosphatase of the nuclear membrane and microsomal fraction of calf thymus cells. Dose- and concentration-dependencies of inactivation of glucoso-6-phosphatase are indicative of a higher radiosensitivity of glucoso-6-phosphatase of nuclear membranes than that of microsomes. This difference in radiosensitivity is associated with the peculiarities of the composition and structural organization of these two membrane systems of a cell.  相似文献   

17.
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.  相似文献   

18.
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).  相似文献   

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