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)

The aluminum present as a contaminant in ATP preparations can cause strong inhibition of yeast hexokinase P-II activity at pH 7.0 or below but has little or no inhibitory effect at a pH of 7.5 or greater. The inhibition is reversed by citrate, 3-phosphoglycerate, malate, phosphate, and catecholamines, all of which have previously been described as activators of hexokinase at low pH. We suggest that these agents activate the enzyme only by virtue of their ability to coordinate with aluminum present in the assay system. The presence of aluminum is also responsible for the "negative cooperativity" observed at low pH with respect to Mg . ATP concentration--i.e., the inhibition by aluminum is uncompetitive at low Mg . ATP concentrations but becomes competitive at high Mg . ATP concentrations. The inhibition is thought to be due to formation of a complex of Al . ATP with the enzyme, with a dissociation constant (Ki) of 0.1 microM. Yeast hexokinase P-I is somewhat less sensitive to A1 than is hexokinase P-II, and yeast glucokinase is not detectably affected. The hexokinase in rat brain (type I) shows a pH-dependent inhibition by Al similar to that observed with the yeast hexokinases, whereas the rat muscle (type II) enzyme is less sensitive, suggesting a possible relationship to aluminum encephalopathy in man.
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PMID:Proton-dependent inhibition of yeast and brain hexokinases by aluminum in ATP preparations. 11 25

1. Aluminum is an established neurotoxin. Prolonged exposure to even low levels of aluminum permit its chelation and subsequent transport to brain where it is non-uniformly distributed. 2. Available evidence suggests that (i) aluminum interferes with glucose metabolism by inhibiting hexokinase and glucose-6-phosphate dehydrogenase; (ii) it binds to calmodulin and affects numerous phosphorylation-dephosphorylation reactions; (iii) it binds to transferrin and ferritin, affects the function of these proteins which in turn affect iron metabolism. 3. Thus accumulation of aluminum-induced metabolic errors colocalized in specific areas of the brain may lead to neurological disorders.
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PMID:Neurochemical hypothesis: participation by aluminum in producing critical mass of colocalized errors in brain leads to neurological disease. 167 37

Aluminum, an abundant element in the earth's crust, has been implicated in various pathological disorders and low concentrations of this element have recently been shown to inhibit brain glycolysis. However, despite the fact that aluminum accumulates in high concentrations in the liver, potential effects of this metal on hepatic intermediary metabolism have not been explored. In perfused livers from untreated rats, maximal rates of production of lactate plus pyruvate (glycolysis) were 93 +/- 15 mumols/g/hr. Glycolysis was severely inhibited in livers from aluminum-treated rats (0.5 mg/kg, 6 hr before experiment) with maximal rates of only 23 +/- 4 mumols/g/hr. In contrast, glucose production (glycogenolysis) and hepatic oxygen uptake were not altered significantly by prior treatment with aluminum. In livers from fasted rats, pretreatment with aluminum did not influence gluconeogenesis or production of lactate and pyruvate from fructose (5 mM). This finding indicates that pyruvate kinase is not inhibited by aluminum and implicates phosphofructokinase, hexokinase and/or glucokinase as sites for the inhibitory effect of aluminum on glycolysis. In liver homogenates from untreated rats, increasing concentrations of aluminum did not show any appreciable effect on hexokinase or glucokinase activity but did cause progressive decreases in phosphofructokinase activity. Therefore, aluminum-induced inhibition of liver phosphofructokinase, an important control site in the glycolytic pathway, is most likely responsible for aluminum-induced inhibition of hepatic glycolysis.
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PMID:Mechanism of aluminum-induced inhibition of hepatic glycolysis: inactivation of phosphofructokinase. 214 21

To what extent can damage to the central and peripheral nervous systems be ascribed to chronic aluminum (Al) intoxication taken as a chelating agent for phosphorus, to limit hyperphosphatemia in uremic patients? Since Al is normally eliminated by the renal route, its accumulation in uremia has to be ascribed to a reduced or abolished renal clearance of the metal, which results in preferential toxicity for certain tissues, especially nervous tissue, which shows difficulty in eliminating Al, even after intake has been stopped. This review discusses, on the basis of toxicologic, experimental and clinical data, the possible pathogenic steps of Al neurotoxicity in uremia, considering: the damage to axonal transport in which Al intoxication tends to affect the components of the cytoskeleton, the polymerization phase of the alpha and beta tubulin constituents of neurotubules, and the normal translocation of neurofilaments from the perikaryon to more distal positions of the axon; the abnormalities in the brain pool of adrenergic, cholinergic and GABA neurotransmitters; the increase in permeability and changes in perm-selectivity of the blood-brain-barrier, with further loss of neurotransmitters and with acquisition, from the systemic circulation, of neurotransmitter-like substances such as hormones, monoamines and peptides, which may adversely modulate synaptic and membrane functions; the cerebral energy metabolism and particularly the hexokinase reaction, by Al replacement of the Mg-ion in the Mg-ATP complex, so that phosphorylation of glucose to G6P is blocked; the interaction of Al with calmodulin by displacement of the Ca-ion and subsequent formation of a stable Al-calmodulin complex with a cytotoxic effect due to the increase in the intracellular calcium concentration.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:[The physiopathologic bases of the neurotoxicity of phosphorus chelating agents containing soluble aluminum salts in patients with renal insufficiency]. 266 59

Prolonged intake of low levels of aluminum from the drinking water has been found to increase the aluminum content in rat brain homogenates and to reduce the activity of hexokinase and glucose-6-phosphate dehydrogenase (G6PD). To determine the interaction of G6PD with aluminum in the brain, we have recently purified two isozymes of G6PD (isozymes I and II) from human and pig brain. Unlike isozyme I, isozyme II also had 6-phosphogluconate dehydrogenase (6-PGD) activity. We report here that G6PD isozymes I and II from human and pig brain purified to apparent homogeneity are inactivated by aluminum. Aluminum did not affect the 6-PGD activity of isozyme II. The aluminum-inactivated enzyme contained 1 mol of aluminum/mol of enzyme subunit. The protein-bound metal ion was not dissociated by exhaustive dialysis at 4 degrees C against 10 mM Tris-HCl (pH 7.0) containing 0.2 mM EDTA. Preincubation of aluminum with citrate, NADP+, EDTA, NaF, ATP, and apotransferrin protected the G6PD isozymes against aluminum inactivation. However, when the G6PD isozymes were completely inactivated by aluminum, only citrate, NaF, and apotransferrin restored the enzyme activity. The dissociation constants for the enzyme-aluminum complex of the isozymes varied from 2 to 4 microM, as measured by using NaF, a known chelator for aluminum. Inhibition of G6PD by low levels of aluminum further strengthens the suggested role of aluminum toxicity in the energy metabolism of the brain.
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PMID:Inactivation of glucose-6-phosphate dehydrogenase isozymes from human and pig brain by aluminum. 274 39

Previous analyses of glycolytic metabolites in Artemia embryos indicate that an acute inhibition of glucose phosphorylation occurs during pHi-mediated metabolic arrest under anoxia. We describe here kinetic features of hexokinase purified from brine shrimp embryos in an attempt to explain the molecular basis for this inhibition. At saturating concentrations of cosubstrate, ADP is an uncompetitive inhibitor toward glucose and a partial noncompetitive inhibitor toward ATP (Kis = 0.86 mM, Kii = 1.0 mM, Kid = 1.9 mM). With cosubstrates at subsaturating concentrations, the uncompetitive inhibition versus glucose becomes noncompetitive, while inhibition versus ATP remains partial noncompetitive. The partial noncompetitive inhibition of ADP versus ATP is characterized by a hyperbolic intercept replot. These product inhibition patterns are consistent with a random mechanism of enzyme action that follows the preferred order of glucose binding first and glucose-6-P dissociating last. We propose that inhibition by glucose-6-P (Kis = 65 microM) occurs primarily by competing with ATP at the active site, resulting in the formation of the dead-end complex, enzyme-glucose-glucose-6-P. Versus glucose, inhibition by glucose-6-P is uncompetitive at pH 8.0 and noncompetitive at pH 6.8. Over a physiologically relevant pH range of 8.0 to 6.8 alterations in Km and Ki values do not account for the reduction in glucose phosphorylation, and no evidence suggests that Artemia hexokinase activity is modulated by reversible binding to intracellular structures. Total aluminum in the embryos is 4.01 +/- 0.36 micrograms/g dry weight, or, based upon tissue hydration, 72 microM. This concentration of aluminum dramatically reduces enzyme activity at pH values less than 7.2, even in the presence of physiological metal ion chelators (citrate, phosphate). When pH, aluminum, citrate, phosphate, substrates, and products were maintained at cellular levels measured under anoxia, we can account for a 90% inhibition of hexokinase relative to activity under control (aerobic) conditions.
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PMID:Kinetic properties of hexokinase under near-physiological conditions. Relation to metabolic arrest in Artemia embryos during anoxia. 276 70

Aluminum inhibited both the cytosolic and mitochondrial hexokinase activities in rat brain. The IC50 values were between 4 and 9 microM. Aluminum was effective at mildly acidic (pH 6.8) or slightly alkaline (pH 7.2-7.5) pH, in the presence of a physiological level of magnesium (0.5 mM). However, saturating (8 mM) magnesium antagonized the effect of aluminum on both forms of hexokinase activity. Other enzymes examined were considerably less sensitive to inhibition by aluminum. The IC50 of aluminum for phosphofructokinase was 1.8 mM and for lactate dehydrogenase 0.4 mM. At 10-600 microM, aluminum actually stimulated pyruvate kinase. Aluminum also inhibited lactate production by rat brain extracts: this effect was much more marked with glucose as substrate than with glucose-6-phosphate. However, the IC50 for inhibiting lactate production using glucose as substrate was 280 microM, higher than that required to inhibit hexokinase. This concentration of aluminum is comparable to those reportedly found in the brains of patients who had died with dialysis dementia and in the brains of some of the patients who had died with Alzheimer disease. Inhibition of carbohydrate utilization may be one of the mechanisms by which aluminum can act as a neurotoxin.
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PMID:Inhibition of brain glycolysis by aluminum. 622 6

In the presence of glucose, yeast hexokinase is specifically and strongly inhibited by all MIIIATP (M = metal) complexes that do not hydrolyze at neutral pH, as long as the ionic radius of the metal is less than 0.89 A. Ki values vary from the micromolar range (0.16 microM for AlATP at pH 7, for example) to as low as 13 nM for LuATP. With glucose and fructose, the tightly bound complexes also show reversible, slow binding behavior, but with poor substrates, little or no change in inhibition constant with time is observed. The kinetics of citrate as an activator of the hexokinase reaction are consistent with its reaction with AlATP present as a contaminant in commercial ATP to form Al citrate. The complex of Al(III) with citrate is 5 orders of magnitude more stable than AlATP, whose Kd is 0.7 microM at pH 7. ATP that has been treated with excess EDTA and adsorbed on and eluted from charcoal is free of aluminum, and citrate no longer affects the kinetics of the hexokinase reaction. Glycerokinase is also specifically inhibited by trivalent metal ATP complexes (Ki = 4 microM at pH 7 for AlATP).
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PMID:Interaction of metal(III)-adenosine 5'-triphosphate complexes with yeast hexokinase. 699 99

The variation of kinetic parameters with pH was examined for bovine brain hexokinase with glucose and MgATP as substrates. The -log V1 and -log (V1/Km) profiles for both substrates were examined and seen to decrease below pH 6.5. All profiles asymptotically approached slopes of -1, indicating that the loss of activity in each instance was due to the protonation of a single group on the enzyme. Analysis of the data indicated two ionizable groups were involved in the reaction. One functions in the binding of ATP and in catalysis while the other participates in the binding of glucose. The -log V1 profiles both showed a "hump" attributed to a loss of activity in the pH region 7.5-5.5. Addition of aluminum ions to the reaction mixture increased the magnitude of the hump, but the inhibition was abolished by the addition of citrate. Kinetic studies carried out at pH 7 indicated that aluminum was a competitive inhibitor with respect to ATP and noncompetitive with respect to glucose. However, secondary plots of the kinetic data were nonlinear, concave downward, indicating that the inhibition is not of a simple type. Possible explanations for this phenomenon are presented.
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PMID:pH kinetic studies of bovine brain hexokinase. 747 Apr 51

The aluminum and yeast hexokinase interaction was studied. Structural changes were correlated with variations in protein functionality. Results show two different behaviors: At low metal concentrations preferential adsorption of metal (and water exclusion) induces aggregate formation. No significant changes in the protein structure occur, but there is a continuous loss of activity (from the first concentration). At large salt concentrations a monomerization process and a conformational change in the secondary structure as well as in the three-dimensional structure take place. This change reduces the percentage of alpha-helix conformation, gives thermal stability to the protein, and allows the exposure of some tryptophan residue and hydrophobic regions. The protein inhibition increases. Conformational change and monomerization may allow access of the metal to the substrate site, mainly the ATP site. The inhibition in any case is of mixed type with a competitive component.
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PMID:Analysis of aluminum-yeast hexokinase interaction: modifications on protein structure and functionality. 1098 12


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