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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)

The transport model of glucose-6-phosphatase (EC 3.1.3.9) was recently challenged by a report that detergent treatment had no effect on the presteady state kinetics of glucose-6-P hydrolysis catalyzed at 0 degree C by the enzyme in liver microsomes previously frozen in 0.25 M mannitol (Zakim, D., and Edmondson, D. E. (1982) J. Biol. Chem. 257, 1145-1148). The lack of response to detergent is shown to be the expected consequence of the conditions used in the presteady state measurements. First, when the assay temperature was reduced from 30 to 0 degree C the depression in the glucose-6-P phosphohydrolase activity of intact microsomes (i.e. the system) was much greater than that of fully disrupted microsomes (i.e. enzyme). This indicates that temperature influences transport much more than hydrolysis of glucose-6-P. As a result, the contribution of a small fraction of enzyme associated with disrupted structures is markedly exaggerated, so it becomes the predominant hydrolytic activity before detergent treatment. Second, freezing microsomes in 0.25 M mannitol caused such extensive disruption that all of the activity manifest at 0 degree C could be attributed to enzyme in disrupted structures. The present findings underscore the importance of assessing the state of intactness of "untreated" microsomes and quantifying the contribution of the disrupted component in kinetic analyses of the glucose-6-phosphatase system. The proposition that the detergent-induced changes in the kinetic properties of glucose 6-phosphatase represent removal of constraints imposed on the enzyme by the membrane environment rather than increased access of enzyme to substrate is critically analyzed.
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PMID:The importance of membrane integrity in kinetic characterizations of the microsomal glucose-6-phosphatase system. 628 74

In 3 fish species (Labeo rohita, Clarias batrachus and Channa punctatus) subjected to a sublethal concentration (0.035 mg/l) of DDT continuously for 20 days both hepatic and renal acid and alkaline phosphatases were elevated, whereas the rise in glucose-6-phosphatase and fructose-1,6-diphosphatase was restricted to hepatic tissue. The variations in phosphatase levels were more pronounced in L. rohita than in the other two species.
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PMID:DDT toxicity: gluconeogenic enzymes and non-specific phosphomonoesterases in three teleosts. 629 22

Rats exposed to cosmetic kerosene mists (odourless kerosene), concentration of 75 and 300 mg/m3 for 14 days, underwent morphological and cytoenzymatic liver tests and biochemical tests of lipids composition in this organ. In addition, lipids concentration and activity of test--enzymes in blood serum were determined. The findings were: passive congestion, fine--droplet fatty degeneration in I zones of clusters and increased number of Browicz--Kupffer's phagocytes near liver triads. Those changes were accompanied by: decreased activity of succinic dehydrogenese (SDH), tetrazolic NADPH--reductase (NADPH-r.t.) and glucose-6-phosphatase (G-6-P-ase) and increased activity of adenosine triphosphatase (Mg++-ATP-ase) and acid phosphatase (AcP). In blood serum medium increase of base phosphatase (AP), 5-nucleotidase (5-Nt) and leucyloaminepeptidase (LAP) and decreased activity of prothrombin (Pt) were found. In addition, it was demonstrated that liver steatosis was characterized by cumulation of free fatty acids, phospholipids and cholesterol esters with simultaneous decrease in triglycerides content in this organ. The obtained results indicate that changes induced by kerosene hydrocarbons in liver are focal and cumulate in I zones of liver clusters. The degree of lesion varies with the extent of exposure, and results from toxic effects of this preparation on hepatic cells lypoproteid membranes.
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PMID:[Comparative studies on the toxicity of various dieelectrics, kerosene derivatives, used in the electroerosion technic. I. Morphological, cytoenzymatic and biochemical changes in the liver of rats chronically exposed to kerosene hydrocarbons]. 630 48

The availability of a fresh, unfrozen liver biopsy specimen permitted the characterization of a unique type of glycogen storage disease. The subject, an 11-year-old female, showed the classic clinical symptoms of type I glycogenosis. However, her hepatic D-glucose-6-phosphate phosphohydrolase (EC 3.1.3.9) level as determined with detergent-activated homogenate was normal. The underlying mechanism was studied with intact microsomes from this fresh liver homogenate. Glucose-6-P phosphohydrolase was 75% latent, compared with 25% in normal controls matched for age and sex. Inorganic pyrophosphatase, PPi:glucose phosphotransferase, and carbamyl-P:glucose phosphotransferase activities of glucose 6-phosphatase were totally latent. While not observed with intact microsomes, these activities were fully manifested with detergent-disrupted microsomes. D-Glucose inhibited glucose-6-P phosphohydrolase activity of both intact and disrupted microsomes, but exogenous Pi inhibited only with the detergent-disrupted preparation. These observations are interpreted on the basis of the multicomponent glucose 6-phosphatase system of Arion et al. (Arion, W. J., Lange, A. J., Walls, H. E., and Ballas, L. M. (1980) J. Biol. Chem. 255, 10396-10406). All are consistent with a defect in T2, the putative translocase specific for Pi, PPi, and carbamyl-P. However, Pi produced endogenously from glucose-6-P hydrolysis within the microsomal lumen did not inhibit. This suggests that (i) a pathway for egress of Pi from the microsomal lumen exists independently of T2, (ii) T2 in this case works only unidirectionally, or (iii) the catalytic unit of glucose 6-phosphatase in situ has become desensitized to interactions with Pi, PPi, and carbamyl-P in this mutant model. Defects in both T1, the translocase specific for glucose-6-P, and T2 thus appear involved in this unique glycogenosis.
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PMID:Type Ic, a novel glycogenosis. Underlying mechanism. 630 84

Although the activity of glucose-6-phosphatase in rat liver is altered markedly following the administration of a variety of hormones in vivo, it is not certain whether the hormones act directly on the hepatocyte. To study this problem hepatocytes were isolated by a collagenase-perfusion technique and cultured on collagen gel/nylon mesh membranes. The activity of glucose 6-phosphatase in cells cultured with fetal calf serum and with Dulbecco's modified Eagle's medium or Leibovitz L-15 medium decreased to less than 10-30% of the activity in freshly isolated cells by 96 h. However, when L-15 plus newborn or fetal calf serum was supplemented with glucagon (10(-6)M), epinephrine (10(-6)M), triiodothyronine (10(-6)M), and dexamethasone (10(-5)M) (L-15-GETD), the activity of glucose-6-phosphatase was maintained so that, after 144 h, the activity was at least 80% of that detected in freshly isolated cells. In cells cultured in L-15 plus serum for 72 or 96 h and then in L-15-GETD, glucose-6-phosphatase increased 30-50% over that in control cultures after 24 h. Insulin, which decreases glucose-6-phosphatase activity when administered to intact animals, also decreased the glucose-6-phosphatase activity in cultured hepatocytes to 20-50% of that in controls.
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PMID:Effects of hormones on the activity of glucose-6-phosphatase in primary cultures of rat hepatocytes. 631 98

To establish on a quantitative basis the subcellular distribution of the enzymes that glycosylate dolichyl phosphate in rat liver, preliminary kinetic studies on the transfer of mannose, glucose, and N-acetylglucosamine-1-phosphate from the respective (14)C- labeled nucleotide sugars to exogenous dolichyl phosphate were conducted in liver microsomes. Mannosyltransferase, glucosyltransferase, and, to a lesser extent, N- acetylglucosamine-phosphotransferase were found to be very unstable at 37 degrees C in the presence of Triton X-100, which was nevertheless required to disperse the membranes and the lipid acceptor in the aqueous reaction medium. The enzymes became fairly stable in the range of 10-17 degrees C and the reactions then proceeded at a constant velocity for at least 15 min. Conditions under which the reaction products are formed in amount proportional to that of microsomes added are described. For N- acetylglucosaminephosphotransferase it was necessary to supplement the incubation medium with microsomal lipids. Subsequently, liver homogenates were fractionated by differential centrifugation, and the microsome fraction, which contained the bulk of the enzymes glycosylating dolichyl phosphate, was analyzed by isopycnic centrifugation in a sucrose gradient without any previous treatment, or after addition of digitonin. The centrifugation behavior of these enzymes was compared to that of a number of reference enzymes for the endoplasmic reticulum, the golgi complex, the plasma membranes, and mitochondria. It was very simily to that of enzymes of the endoplasmic reticulum, especially glucose-6-phosphatase. Subcellular preparations enriched in golgi complex elements, plasma membranes, outer membranes of mitochondira, or mitoplasts showed for the transferases acting on dolichyl phosphate relative activities similar to that of glucose- 6-phosphatase. It is concluded that glycosylations of dolichyl phosphate into mannose, glucose, and N-acetylglucosamine-1-phosphate derivatives is restricted to the endoplasmic reticulum in liver cells, and that the enzymes involved are similarly active in the smooth and in the rough elements.
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PMID:Quantitative assay and subcellular distribution of enzymes acting on dolichyl phosphate in rat liver. 646 36

Studies suggest that liver regeneration is delayed in insulin-deficient animals, but defining a role of insulin as a growth factor in hepatic regeneration has remained elusive. By examining gene expression of hepatectomized liver in type 1 diabetic BB rats, we have identified dramatic changes in the expression of primary or immediate-early growth response genes compared with normal animals. These include altered expression of insulin-regulated genes such as glucose-6-phosphatase (G-6-Pase), phosphoenolpyruvate carboxykinase (PEPCK), and beta-actin, and genes such as CL-6 and map kinase phosphatase-1 (MKP-1) that were previously unlinked to insulin action in animals. Abnormal elevation of mRNAs encoding G-6-Pase, MKP-1, and PEPCK in the time 0 diabetic liver results in decreased induction after partial hepatectomy. Other genes, such as CL-6 and beta-actin, are induced at a lower level in the hepatectomized diabetic animals. The net effect is a blunting of the immediate-early gene response after partial hepatectomy in diabetic animals. As determined by DNA synthesis assays, the regenerative capacity of insulin-deficient BB diabetic livers is reduced, and this defect is corrected at least in part by insulin therapy. These findings suggest that because of insulin deficiency, common intracellular signaling pathways that are required for both metabolism and mitogenesis are aberrant in the type 1 diabetic liver and, as a result, the regenerative response is deficient.
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PMID:Blunting of the immediate-early gene and mitogenic response in hepatectomized type 1 diabetic animals. 748 83

The effect of histone II-A on glucose-6-phosphatase and mannose-6-phosphatase activities was investigated in relation to microsomal membrane permeability. It was found that glucose-6-phosphatase activity in histone II-A-pretreated liver microsomes was stimulated to the same extent as in detergent-permeabilized microsomes, and that the substrate specificity of the enzyme for glucose 6-phosphate was lost in histone II-A-pretreated microsomes, as [U-14C]glucose-6-phosphate hydrolysis was inhibited by mannose 6-phosphate and [U-14C]mannose 6-phosphate hydrolysis was increased. The accumulation of [U-14C]glucose from [U-14C]glucose 6-phosphate into untreated microsomes was completely abolished in detergent-treated vesicles, but was increased in histone II-A-treated microsomes, accounting for the increased glucose-6-phosphatase activity, and demonstrating that the microsomal membrane was still intact. The stimulation of glucose-6-phosphatase and mannose-6-phosphatase activities by histone II-A was found to be reversed by EGTA. It is concluded that the effects of histone II-A on glucose-6-phosphatase and mannose-6-phosphatase are not caused by the permeabilization of the microsomal membrane. The measurement of mannose-6-phosphatase latency to evaluate the intactness of the vesicles is therefore inappropriate.
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PMID:Histone II-A stimulates glucose-6-phosphatase and reveals mannose-6-phosphatase activities without permeabilization of liver microsomes. 764 48

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.
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PMID:Fatty acyl-CoA esters inhibit glucose-6-phosphatase in rat liver microsomes. 773 74

Some biochemical parameters were investigated on gossypol consumption, and were correlated to the level of cholesterol and residual glucose in rat liver. Two groups of animals were used, one group was fed with normal protein diet and the other set was fed with low protein diet. The results show that gossypol produced no apparent biochemical aberration in the liver of normal protein fed and low protein fed rats. It also had no effect on glucose-6-phosphatase and prostate phosphatase. Gossypol consumption had a significant effect on alcohol dehydrogenase. These results indicate no direct involvement of gossypol in sugar uptake but profound influence on the regulation of cholesterol level in the liver.
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PMID:Biochemical studies of the effect of gossypol consumption on cholesterol and residual glucose in fed and protein-energy deficient rats. 781 27


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