Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.1.3.9 (glucose-6-phosphatase)
 3,081 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Mechanisms regulating the energy-dependent calcium sequestering activity of liver microsomes were studied. The possibility for a physiologic mechanism capable of entrapping the transported Ca2+ was investigated. It was found that the addition of glucose 6-phosphate to the incubation system for MgATP-dependent microsomal calcium transport results in a marked stimulation of Ca2+ uptake. The uptake at 30 min is about 50% of that obtained with oxalate when the incubation is carried out at pH 6.8, which is the pH optimum for oxalate-stimulated calcium uptake. However, at physiological pH values (7.2-7.4), the glucose 6-phosphate-stimulated calcium uptake is maximal and equals that obtained with oxalate at pH 6.8. The Vmax of the glucose 6-phosphate-stimulated transport is 22.3 nmol of calcium/mg protein per min. The apparent Km for calcium calculated from total calcium concentrations is 31.9 microM. After the incubation of the system for MgATP-dependent microsomal calcium transport in the presence of glucose 6-phosphate, inorganic phosphorus and calcium are found in equal concentrations, on a molar base, in the recovered microsomal fraction. In the system for the glucose 6-phosphate-stimulated calcium uptake, glucose 6-phosphate is actively hydrolyzed by the glucose-6-phosphatase activity of liver microsomes. The latter activity is not influenced by concomitant calcium uptake. Calcium uptake is maximal when the concentration of glucose 6-phosphate in the system is 1-3 mM, which is much lower than that necessary to saturate glucose-6-phosphatase. These results are interpreted in the light of a possible cooperative activity between the energy-dependent calcium pump of liver microsomes and the glucose-6-phosphatase multicomponent system. The physiological implications of such a cooperation are discussed.
 ...
PMID:Calcium sequestration activity in rat liver microsomes. Evidence for a cooperation of calcium transport with glucose-6-phosphatase. 298 15

The cholesterol content of rat liver microsomal membranes was modified in vitro by incubating microsomes and cytosol with liposomes prepared by sonication of microsomal lipids and cholesterol. In this way, the cholesterol to phospholipid molar ratio was increased from 0.11-0.13 in untreated microsomes to a maximal of 0.8 in treated ones. Cholesterol incorporation in microsomes produced an increase in the diphenyl-hexatriene steady-state fluorescence anisotropy and a decrease in the efficiency of pyrene-excimer formation which indicated a decrease in the rotational and translational mobility, respectively, of these probes in the membranes lipid phase. Cholesterol incorporation in microsomes did not affect significantly the glucose-6-phosphatase activity in 0.1% Triton X-100 totally disrupted microsomes, but diminished the glucose-6-phosphatase activity of 'intact' microsomes. This indicates that possibly the glucose 6-phosphate translocation across the microsomal membrane is impeded by an increase in the membrane apparent 'microviscosity'. Cholesterol incorporation in microsomes decreased NADH-cytochrome c reductase without affecting NADH-ferricyanide reductase activity. The delta 9 desaturation reaction rate was enhanced by cholesterol incorporation at low but not at high palmitic acid substrate concentration. delta 5 and delta 6 desaturase reaction-rates were increased both at low and high fatty acid substrate concentrations. These results suggest that a mechanism involving fatty acid desaturase enzymes, might exist to self-regulate the microsomal membrane lipid phase 'fluidity' in the rat liver.
 ...
PMID:In vitro modification of cholesterol content of rat liver microsomes. Effects upon membrane 'fluidity' and activities of glucose-6-phosphatase and fatty acid desaturation systems. 299 32

The activities of glucose-6-phosphate hydrolase and glucose-6-phosphate translocase were determined in rats fasted for 1-3 days and in animals fasted for one day and then either refed with mixed pellet or given oral or intraperitoneal glucose. The assay was based on the colorimetric measurement of the released inorganic phosphate. Fasting over 24 h significantly increased both the translocase and the hydrolase activity of glucose-6-phosphatase. These parameters showed a further increase when rats were fasted for another 24 h. In animals fasted for 24 h and then refed with standardized pellet diet, a progressive fall of enzyme activity was noticed. However, even 72 h of refeeding did not lead to complete normalization. Glucose given orally or intraperitoneally also suppressed the enzyme activity, although the effect was somewhat delayed. As expected, in fasting rats glucose and insulin levels were significantly decreased. Normoglycaemia was established after just 24 h, regardless of refeeding with pellets or with glucose. The former group exhibited hyper- and the latter hypo-insulinaemic pattern. We speculate that augmented activity of hepatic glucose-6-phosphatase during fasting stimulates the metabolism of glucose through the glucose cycle and is thereby at least partially responsible for insulin resistance accompanying the fasting state.
 ...
PMID:Effects of fasting and refeeding on the activity of hepatic glucose-6-phosphatase in rats. 299 2

Coupled enzyme assays are described for measuring inorganic phosphates, organic phosphates and phosphate-liberating enzymes in biological material. The assays all determine Pi by its reaction with inosine, catalysed by nucleoside phosphorylase; this yields ribose 1-phosphate and hypoxanthine. The hypoxanthine is oxidized to uric acid by xanthine oxidase, and may be measured either by the absorbance of the uric acid, or by the formazan formed when a tetrazolium salt is used as the oxidant. The coupled enzyme assays are characterized by high sensitivity, quantitative utilization of phosphates and stoichiometric formation of the measurable products, measurement at pH 6.0-8.5, determination of phosphates within a single analytical step, and continuous measurement of phosphohydrolase activity in a corresponding rate assay. Examples include determinations of substrates such as Pi, PPi and AMP, and of enzymes such as 5'-nucleotidase, inorganic pyrophosphatase and glucose-6-phosphatase. Directions for further examples are given.
 ...
PMID:Enzymic determination of inorganic phosphates, organic phosphates and phosphate-liberating enzymes by use of nucleoside phosphorylase-xanthine oxidase (dehydrogenase)-coupled reactions. 299 93

A simple, rapid, and reproducible method of determining glucose-6-phosphatase activity is described. The glucose 6-phosphate hydrolysis is accompanied by the disappearance of the protons from the medium owing to a phosphate species pK change from 6.1 (in glucose 6-phosphate) to 6.9 (in inorganic phosphate). Alkalization is registered by a pH meter with a recorder. The method described in this paper may be used in routine determinations of glucose-6-phosphatase activity.
 ...
PMID:A pH-metrical method of determining glucose-6-phosphatase activity. 300 58

Based on cytochemical analysis, the enzyme NADP phosphatase is most concentrated in the so-called intercalary cisternae from the mid-region of the Golgi apparatus stack. Using free-flow electrophoresis to separate different Golgi regions of rat liver Golgi apparatus, the NADP phosphatase activity, based on estimation of the rate of release of inorganic phosphate from NADP under standard conditions, was similarly localized to membrane fractions from the center of electrophoretic separations. Peak specific activities for both a putative cis marker (NADH-cytochrome c reductase) and an established trans marker (galactosyltransferase) coincided with minima in NADP phosphatase activity, in agreement with the cytochemical observations. The pattern of distribution of enzyme activity for NADP phosphatase differed from that of both acid phosphatase and glucose-6-phosphatase. The pH optimum was 5.0, the Km for NADP was 0.6 mM and a corresponding production of NAD and inorganic phosphorus was shown. Taken together with other markers for free-flow electrophoresis separation, the NADP phosphatase will provide considerable utility as a specific marker to help identify intercalary cisternae of the mammalian Golgi apparatus and to monitor electrophoretic separations.
 ...
PMID:NADP phosphatase as a marker in free-flow electrophoretic separations for cisternae of the Golgi apparatus midregion. 300 95

The observations made by Sacks et al. [Neurochem. Res. 8, 661-685 (1983)] on which they based their criticisms of the deoxyglucose method have been examined and found to have no relationship to the conclusions drawn by them. (1) The observations of Sacks et al. (1983) of constant concentrations of [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate, predominantly in the form of product, reflects only the postmortem phosphorylation of the precursor during the dissection of the brain in their experiments. When the brains are removed by freeze-blowing, the time courses of the [14C]deoxyglucose and [14C]deoxyglucose-6-phosphate concentrations in brain during the 45 min after the intravenous pulse are close to those predicted by the model of the deoxyglucose method. (2) Their observation of a reversal of the cerebral arteriovenous difference from positive to negative for [14C]deoxyglucose and not for [14C]glucose after an intravenous infusion of either tracer is, contrary to their conclusions, not a reflection of glucose-6-phosphatase activity in brain but the consequence of the different proportions of the rate constants for efflux and phosphorylation for these two hexoses in brain and is fully predicted by the model of the deoxyglucose method. (3) It is experimentally demonstrated that there is no significant arteriovenous difference for glucose-6-phosphate in brain, that infusion of [32P]glucose-6-phosphate results in no labeling of brain, and that the blood-brain barrier is impermeable to glucose-6-phosphate. Glucose-6-phosphate cannot, therefore, cross the blood-brain barrier, and the observation by Sacks and co-workers [J. Appl. Physiol. 24, 817-827 (1968); Neurochem. Res. 8, 661-685 (1983)] of a positive cerebral arteriovenous difference for [14C]glucose-6-phosphate and a negative arteriovenous difference for [14C]glucose cannot possibly reflect glucose-6-phosphatase activity in brain as concluded by them. Each of the criticisms raised by Sacks et al. has been demonstrated to be devoid of validity.
 ...
PMID:Invalidity of criticisms of the deoxyglucose method based on alleged glucose-6-phosphatase activity in brain. 300 97

The ability of glucose 6-phosphate and carbamyl phosphate to serve as substrates for glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase; EC 3.1.3.9) of intact and disrupted microsomes from rat liver was compared at pH 7.0. Results support carbamyl phosphate and glucose 6-phosphate as effective substrates with both. Km values for carbamyl phosphate and glucose 6-phosphate were greater with intact than with disrupted microsomes, but Vmax values were higher with the latter. The substrate translocase-catalytic unit concept of glucose-6-phosphatase function is thus confirmed. The Km values for 3-O-methyl-D-glucose and D-glucose were larger when determined with intact than with disrupted microsomes. This observation is consistent with the involvement of a translocase specific for hexose substrate as a rate-influencing determinant in phosphotransferase activity of glucose-6-phosphatase.
 ...
PMID:Comparative reactivity of carbamyl phosphate and glucose 6-phosphate with the glucose-6-phosphatase of intact microsomes. 300 88

Alterations of catalytic activities of the microsomal glucose-6-phosphatase system were examined following either ferrous iron- or halothane (CF3CHBrCl) and carbon tetrachloride (CCl4) free-radical-mediated peroxidation of the microsomal membrane. Enzyme assays were performed in native and solubilized microsomes using either glucose 6-phosphate or mannose 6-phosphate as substrate. Lipid peroxidation was assessed by the amounts of malondialdehyde equivalents formed. Regardless of whether the experiments were performed in the presence of NADPH/Fe3+, NADPH/CF3CHBrCl, or NADPH/CCl4, with the onset of lipid peroxidation, mannose-6-phosphatase activity of the native microsomes increased immediately, while further alterations in catalytic activities were only detectable when lipid peroxidation had passed characteristic threshold values: above 2 nmol malondialdehyde/mg microsomal protein, glucose-6-phosphatase activity of the native microsomes was lost, and at 10 nmol malondialdehyde/mg microsomal protein, glucose-6-phosphatase and mannose-6-phosphatase activity of the solubilized microsomes started to decline. It is concluded that the latter alterations are due to an irreversible damage of the phosphohydrolase active site of the glucose-6-phosphatase system, while the changes observed at earlier stages of microsomal lipid peroxidation may also reflect alterations of the transporter components of the glucose-6-phosphatase system. Virtually no changes in the catalytic activities of the glucose-6-phosphatase system occurred under anaerobic conditions, indicating that CF3CHCl and CCl3 radicals are without direct damaging effect on the glucose-6-phosphatase system. Further, maximum effects of carbon tetrachloride and halothane on lipid peroxidation and enzyme activities were observed at an oxygen partial pressure (PO2) of 2 mmHg, providing additional evidence for the crucial role of low PO2 in the hepatotoxicity of both haloalkanes.
 ...
PMID:Alterations of the microsomal glucose-6-phosphatase system evoked by ferrous iron- and haloalkane free-radical-mediated lipid peroxidation. 300 50

Lead is prevalently replaced by cerium as trapping agent in phosphatase cytochemistry to prevent non-specific precipitation. Recently, substrate specific but artefactual lead precipitates have been described in the nuclear envelope (NE) and rough endoplasmic reticulum (RER) due to a local matrix effect. In the present study a verification was carried out of the localization of acid phosphatase and glucose-6-phosphatase in the NE and RER of rat peritoneal macrophages and hepatocytes respectively with cerium. It appeared that precipitates of cerium phosphate in NE and RER of peritoneal macrophages do not represent sites of acid phosphatase activity but are due to the matrix effect. However, in rat hepatocytes these organelles demonstrate true reactive sites for glucose-6-phosphatase.
 ...
PMID:Phosphatase cytochemistry with cerium as trapping agent. Verification of acid phosphatase and glucose-6-phosphatase reactive sites. 301 7


<< Previous 1 2 3 4 5 6 7 8 9 10