Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:2.7.1.1 (hexokinase)
 5,274 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A sensitive and specific GTP-activated Ca2+ translocation process induces rapid Ca2+ movements within cells and appears to reflect G protein-induced membrane fusion or junctional communication between discrete subpopulations of Ca(2+)-pumping organelles (Ghosh, T. K., Mullaney, J. M., Tarazi, F. I., and Gill, D. L. (1989) Nature 340, 236-239). Since fatty acylation can modify G protein action, modification of GTP-induced Ca2+ translocation by fatty acyl-CoA was investigated to throw light on the mechanism underlying Ca2+ transfer. Using permeabilized DDT1MF-2 smooth muscle cells, 2 microM palmitoyl-CoA completely blocked Ca2+ release activated by 20 microM GTP, while having no effect on inositol 1,4,5-trisphosphate-induced Ca2+ release. The IC50 (50% inhibitory concentration) for palmitoyl-CoA was 0.5 microM. Above 3 microM, palmitoyl-CoA inhibited Ca2+ accumulation. Fatty acyl chain length was important, C-13 to C-16 fatty acyl-CoA esters all fully blocking the action of GTP; the IC50 for myristoyl-CoA was also 0.5 microM. C-18 or larger acyl groups had diminished effectiveness as did C-8 or smaller acyl groups. Acetyl-CoA had no blocking effect. In contrast, 10 microM CoA itself blocked GTP-induced Ca2+ release. CoA required a free sulfhydryl group to block, desulfo-CoA having no effect. Removal of ATP by hexokinase and glucose prevented the action of CoA but not palmitoyl-CoA. The free sulfhydryl and ATP requirements indicated CoA was being acylated by endogenous fatty-acyl-CoA synthetase to be effective. The nonhydrolyzable myristoyl-CoA analog, S-(2-oxopentadecyl)-CoA, blocked the GTP effect identically to myristoyl- and palmitoyl-CoA (IC50 = 0.5 microM); thus, fatty acyl transfer is not required, indicating that blockade is due to a direct allosteric modification of a component of the GTP-activated process by acyl-CoA esters. Palmitoyl-CoA not only inhibited but completely reversed GTP-activated Ca2+ release, resulting in the released Ca2+ being taken back up into pools. In the presence of oxalate, GTP-activated Ca2+ transfer results in a substantial increase in Ca2+ accumulation; palmitoyl-CoA also completely reversed this effect resulting in rapid termination of Ca2+ uptake. This reversal provides strong evidence that GTP-activated Ca2+ translocation does not reflect a membrane fusion event. Instead, it likely represents formation of a reversible junction or pore between organelles which may be a required prefusion event.
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PMID:Modification of GTP-activated calcium translocation by fatty acyl-CoA esters. Evidence for a GTP-induced prefusion event. 798 31

Prolonged exposure of islets to fatty acids results in a lowered glucose set-point for insulin secretion. We examined the mechanism in islets cultured for 24 h with 0.25 mmol/l palmitate. As expected, insulin secretion at 2.8 and 8.3 mmol/l glucose was increased in the palmitate-treated islets as opposed to no change at 27.7 mmol/l glucose. Co-culturing with 0.05 microgram/ml Triacsin C, an inhibitor of long chain acyl-CoA synthetase, blocked this effect. Glucose utilization and oxidation showed the same pattern as insulin secretion, with the step-up for both measurements being fully manifest at 2.8 mmol/l glucose. Glucokinase Km and Vmax measured in islet extracts were unaffected by the palmitate. In contrast, hexokinase Vmax was increased by 25-35% in both the cytoplasmic and mitochondrial-bound pools. Our data suggest prolonged exposure to fatty acids increased beta-cell hexokinase activity, thereby modifying the kinetics of glucose entry into the metabolic pathway and glucose-induced insulin secretion. The cellular mediator is likely an increased level of long chain fatty acyl-CoA esters.
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PMID:Beta-cell hypersensitivity to glucose following 24-h exposure of rat islets to fatty acids. 911 15

Diabetic states are characterized by a raised serum/islet level of long chain fatty acids and a lowered ED50 for glucose-induced insulin secretion. Prolonged culture (> 6 h) of islets with long chain fatty acids replicates the basal insulin hypersecretion. We examined this effect in rat islets cultured for 24 h with 0.25 mM oleate. Insulin secretion at 2.8 mM glucose was doubled in combination with a 60% lowered islet content of glucose-6-phosphate (G6P). Investigation of the lowered G6P showed: (a) increased glucose usage from 0.5 to 100 mM glucose with identical values measured by [2-3H]glucose and [5-3H]glucose, (c) indicating little glucose- 6-phosphatase activity, (b) unchanged low pentose phosphate shunt activity, (c) 50% increased phosphofructokinase (PFK) Vmax, (d) a normal ATP/ADP ratio, and (e) unchanged fructose 2,6 bisphosphate content. Triacsin C, an inhibitor of fatty acyl-CoA synthetase, prevented the increase in PFK activity and the lowered G6P content. These results suggest that long chain acyl-CoA mediates the rise in PFK activity, which in turn lowers the G6P level. We speculate that the inhibition of hexokinase by G6P is thus attenuated, thereby causing the basal insulin hypersecretion.
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PMID:Fatty acid-induced beta cell hypersensitivity to glucose. Increased phosphofructokinase activity and lowered glucose-6-phosphate content. 957 50

Diabetic states are characterized by a raised serum/islet level of triglycerides and a lowered EC50 (concentration at half-maximal stimulation) for glucose-induced insulin secretion. Culturing islets with long-chain fatty acids (FAs) replicates the basal insulin hypersecretion. In a previous study, we showed that the mechanism involved deinhibition of hexokinase by a 60% decrease in glucose-6-phosphate (G-6-P). The key event was proposed to be an increased phosphofructokinase (PFK) Vmax secondary to an upregulatory effect of the FA metabolite, long-chain acyl-coenzyme A (LC-CoA). We now show another contributory factor, a lowered content of the PFK inhibitor citrate. Citrate synthase Vmax and citrate levels were lowered 45% in rat islets cultured with 250 micromol/l oleate for 24 h. Both effects were reversed by triacsin C, an inhibitor of fatty acyl-CoA synthetase, the enzyme that generates LC-CoA. Culturing islets with high doses of glucose (16.7 mmol/l) for 48 h should also raise cytosolic LC-CoA. As predicted, citrate synthase Vmax was lowered and PFK Vmax was increased, both in a triacsin C-reversible fashion. These results show shared selected functional and biochemical properties in beta-cells of so-called glucotoxicity and lipotoxicity.
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PMID:Shared biochemical properties of glucotoxicity and lipotoxicity in islets decrease citrate synthase activity and increase phosphofructokinase activity. 983 20