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D-glucose anomeric preference of hexokinases from animals and yeast.

The D-glucose anomeric preference of hexokinases partially purified from animals (rat, mouse, and chicken) and baker's yeast (Saccharomyces cerevisiae) were investigated by the assay system with glucose-6-phosphate dehydrogenase as a coupling enzyme. With low Km hexokinases in animal tissues and cells, the ratios of Vmax for the beta-anomer to Vmax for the alpha-anomer (V beta/V alpha) were within a range from 1.3 to 1.5. In yeast, the V beta/V alpha value was 1.1 for hexokinase A, 0.8 for hexokinase B, and 1.4 for glucokinase. The possible explanation for D-glucose anomeric preference of hexokinase is discussed.

Animals↗

Compartmentation of hexokinase in rat heart. A critical factor for tracer kinetic analysis of myocardial glucose metabolism.

Radiolabeled analogues of 2-deoxyglucose are widely used to trace glucose metabolism in cell cultures, whole organs, and intact animals, although kinetic differences in transport and phosphorylation between these compounds and glucose exist. The present studies were undertaken to determine the effects of insulin stimulation on the phosphorylation of 2-deoxyglucose compared to glucose in the intact, saline-perfused working rat heart. Rates of glucose utilization determined from tritiated glucose differed from rates estimated from the accumulation of [14C]2-deoxyglucose in a nonconstant manner when comparing rates in the absence or presence of physiologic levels of insulin (13 microU/ml). The fraction of monophosphorylated hexoses that was accounted for by [14C]2-deoxyglucose 6-phosphate was dramatically decreased in hearts perfused in the presence of insulin. Additionally, hexokinase activity associated with the mitochondrial fraction of tissue extracts was increased in hearts stimulated by insulin. While this redistribution of hexokinase to the mitochondria did not affect the apparent affinity constant for glucose, hexokinase bound to mitochondria exhibited an 8.5-fold decrease in the affinity for 2-deoxyglucose when compared with hexokinase present in the cytosolic fraction. The findings are consistent with an insulin-mediated preferential uptake and phosphorylation of glucose compared to deoxyglucose. The results also imply that the redistribution of hexokinase and the differential effect of insulin on its affinity for tracer and tracee are responsible for changes in the "lumped constant" (i.e., the correction factor used to equate 2-deoxyglucose to glucose uptake). These changes must be taken into account when regional myocardial glucose metabolism is assessed by the 2-deoxyglucose method.

Animals↗

Isozymes of mammalian hexokinase: structure, subcellular localization and metabolic function.

The first step in metabolism of glucose (Glc) is usually phosphorylation, catalyzed by hexokinase. However, the Glc-6-P produced can then enter one or more of several alternative pathways. Selective expression of isozymic forms of hexokinase, differing in catalytic and regulatory properties as well as subcellular localization, is likely to be an important factor in determining the pattern of Glc metabolism in mammalian tissues/cells. Despite their overall structural similarity, the Type I, Type II and Type III isozymes differ in important respects. All three isozymes are inhibited by the product, Glc-6-P, but with the Type I isozyme, this inhibition is antagonized by P(I), whereas with the Type II and Type III isozymes, P(i) actually causes additional inhibition. Reciprocal changes in intracellular levels of Glc-6-P and P(i) are closely associated with cellular energy status, and it is proposed that the response of the Type I isozyme to these effectors adapts it for catabolic function, introducing Glc into glycolytic metabolism for energy production. In contrast, the Type II, and probably the Type III, isozymes are suggested to serve primarily anabolic functions, e.g. to provide Glc-6-P for glycogen synthesis or metabolism via the pentose phosphate pathway for lipid synthesis. Type I hexokinase binds to mitochondria through interaction with porin, the protein that forms channels through which metabolites traverse the outer mitochondrial membrane. Several experimental approaches have led to the conclusion that the Type I isozyme, bound to actively phosphorylating mitochondria, selectively uses intramitochondrial ATP as substrate. Such interactions are thought to facilitate coordination of the introduction of Glc into glycolysis, via the hexokinase reaction, with the terminal oxidative stages of Glc metabolism occurring in the mitochondria, thus ensuring an overall rate of Glc metabolism commensurate with cellular energy demands and avoiding excessive production of lactate. The Type II isozyme also binds to mitochondria. Whether such coupling occurs with mitochondrially bound Type II hexokinase in normal tissues, and how it might be related to the proposed anabolic role of this isozyme, remain to be determined. The Type III isozyme lacks the hydrophobic N-terminal sequence known to be critical for binding of the Type I and Type II isozymes to mitochondria. Immunolocalization studies have indicated that, in many cell types, the Type III has a perinuclear localization, the possible metabolic consequences of which remain unclear.

Animals↗

Hexokinase II: the integration of energy metabolism and control of apoptosis.

Hexokinase II is often highly expressed in poorly differentiated and rapidly growing tumors that exhibit a high rate of aerobic glycolysis. Hexokinase II binds to the mitochondrial membrane through its interaction with the outer membrane voltage-dependent anion channel (VDAC), preferentially at contact sites between the outer and inner mitochondrial membrane. This location is thought to be important for the integration of glycolysis with mitochondrial energy metabolism. VDAC is a critical component of the mitochondrial phase of apoptosis and its interaction with Bcl-2 family proteins controls the rate of release of mitochondrial intermembrane space proteins that activate the execution phase of apoptosis. The proteins involved in the contact sites also constitute the mitochondrial permeability transition, one of the mechanisms by which mitochondrial protein release can be mediated. Hexokinase II binding to VDAC suppresses the release of intermembrane space proteins and inhibits apoptosis, thereby contributing to the survival advantage of tumor cells. This interaction places hexokinase II in a position to integrate glycolytic metabolism of the tumor cell with the control of apoptosis at the mitochondrial level. Mitochondrial binding of hexokinase II may constitute an attractive target for therapeutic intervention to suppress tumor growth.

Adenosine Triphosphate↗

Control of glucose metabolism in pancreatic beta-cells by glucokinase, hexokinase, and phosphofructokinase. Model study with cell lines derived from beta-cells.

Glucose usage by soluble fractions of cell extracts from two insulin-producing cell lines, RINm5F and HIT, was investigated. Analysis of enzyme activities indicated that glucose phosphorylation and phosphofructokinase are likely to be the rate-limiting steps of glycolysis in both RINm5F and HIT cell extracts. RINm5F extracts, which lack glucokinase, exhibited relatively flat concentration-dependency curves of glucose usage and showed substantial inhibition of hexokinase. HIT cell extracts, which contain glucokinase but lack hexokinase, exhibited sigmoidal concentration-dependency curves of glucose usage, reflecting almost fully expressed glucokinase activity. A reconstituted system prepared from RINm5F and HIT cell extracts exhibited a composite concentration-dependency curve of glucose usage and showed substantial inhibition of hexokinase and almost fully expressed glucokinase. However, conditions that activate phosphofructokinase, such as addition of ammonium sulfate or fructose 2,6-bisphosphate or alkalization, removed the inhibition of hexokinase without noticeably affecting the glucokinase component of usage. Results obtained with a reconstituted system containing RINm5F cell extract and purified glucokinase were consistent with these findings. The data presented here indicate that this reconstituted cell-free system serves as a valid model for the study of aspects of glycolytic control in the islet. This model illustrates the preeminent role of glucokinase in the control of glycolysis, consistent with its glucose-sensor function in the islet. In addition, these studies help to define the contribution of phosphofructokinase to the control of glycolysis and the mechanism whereby changes in phosphofructokinase activity could modulate, via changes in the glucose 6-phosphate concentration, the activity of hexokinase and hence the net glycolytic flux.

Ammonium Sulfate↗

Hexokinase shift to mitochondria is associated with an increased sensitivity to glucose in rat pancreatic islets.

When rat pancreatic islets are incubated in 5.5 or 16.7 mmol/l glucose for 3 h, an increased sensitivity is observed in islets pre-exposed to high glucose, as indicated by a shift to the left of the glucose dose-response curve (EC50 7.1 +/- 0.9 and 11.5 +/- 1.2 in high- and low-glucose-exposed islets, respectively; n = 5, P < 0.05). To investigate the mechanism(s) responsible for this effect, we measured hexokinase and glucokinase activity both in the cytosolic fraction and in a mitochondrion-enriched fraction, since binding to the outer mitochondrial membrane has been reported to result in an increased enzyme activity. In islets cultured at 16.7 mmol/l glucose, the cytosolic hexokinase activity was similar to control islets, but mitochondrial enzyme activity was significantly increased (124 +/- 7 vs. 51 +/- 9 nmol x microg(-1) x 90 min(-1), P < 0.01). As a consequence, the cytosolic:mitochondrial fraction ratio was altered in comparison with control islets. In contrast, glucokinase activity in the two groups of islets was similar in the cytosolic fraction and undetectable in the mitochondrial fraction. Hexokinase I quantitation by Western blot confirmed the enzyme translocation from the free cytosolic to the mitochondria-bound form in islets cultured at 16.7 mmol/l glucose. Glucose-induced alterations were reversible after 1 h exposure to 5.5 mmol/l glucose. Moreover, in islets exposed to 16.7 mmol/l glucose, inhibition of hexokinase binding to mitochondria by the addition of 20 nmol/l dicyclohexylcarbodiimide resulted in no increase of glucose sensitivity (EC50 10.9 +/- 0.4, n = 3, similar to that of control islets). These data indicate that after chronic exposure to high glucose, the beta-cell becomes more sensitive to glucose before eventually getting desensitized. This increased sensitivity is associated with (and may be due to) an increased hexokinase activity secondary to a subcellular shift of the enzyme from the free cytosolic to the mitochondria-bound, more active form.

Animals↗

The protective effect of glucose on soluble rat lens hexokinase in the presence of oxidative stress.

An in vitro animal model was used to characterize the protective effect of glucose on lenses subjected to oxidative stress. Paired rat lenses were incubated in TC-199 medium for six hours in the presence of an oxidant (0.06 mM H2O2, superoxide produced from 5 mM purine, or hydroxyl radical) and 2 mM glucose (control) or no glucose (experimental). Soluble hexokinase (HK) specific activity and lactate production were measured. 0.06 mM H2O2 inactivates 48% of the hexokinase in the absence of glucose; with glucose present hexokinase activity is reduced only 26%. Control experiments without oxidants show a statistically insignificant difference between hexokinase activities in the 0 and 2 mM groups, suggesting that the changes observed are not simply due to the presence or absence of glucose. Hexosemonophosphate shunt activity increases nearly 2.5-fold in the presence of 0.06 mM H2O2 and 2.0, 4.0 or 5.5 mM glucose. This suggests that the loss of hexokinase (a -SH enzyme) in the presence of H2O2 and 0 mM glucose is due to NADPH production inadequate to offset the oxidative stress on enzyme -SH groups. FPLC analysis suggests that type II HK is more susceptible to oxidative inactivation than type I, and further studies have shown that this inactivation is localized to the capsule/epithelium. Lactate levels were measured and controls (without oxidants) were run, to obtain a baseline value for fresh lenses and assess the contribution of endogenous glucose to lactate production. H2O2 levels in superoxide and hydroxyl radical media were measured, and the protective effects of mannitol and catalase were also determined.

Animals↗

Levels of expression of hexokinase, aldose reductase and sorbitol dehydrogenase genes in lens of mouse and rat.

The level of expression of the genes for hexokinase, aldose reductase and sorbitol dehydrogenase was investigated in lenses of mice and rats. These genes represent two separate but interrelated pathways for the metabolism of glucose in the cell. It is hypothesized that the extent of expression of the hexokinase gene may play an important role in the regulation of the levels of glucose in the lens. It is known that if there occurs a build up of intracellular glucose, such as in diabetes mellitus, activation of the aldose reductase/sorbitol dehydrogenase pathway may lead to various diabetic complications, including a lessening of lens clarity. We have therefore determined the levels of expression of the genes for these three enzymes in the lens of both mice and rats. Mice are known to be more resistant than rats to the development of lens opacification during hyperglycemia. By Northern blot hybridization analysis, and by quantitation of the resulting hexokinase, aldose reductase and sorbitol dehydrogenase mRNA hybrids, we found that in the mouse lens the expression of the hexokinase gene exceeded that of the aldose reductase gene by a factor of three, while in the rat it only approached about 1/4 that of the aldose reductase gene. The extent of expression of the SDH gene, however, was equal between the mouse and rat lenses. These results were calculated relative to the level of expression of the alpha A-crystallin gene in those two types of lenses, in order to account for the generally higher genetic expression found in the rat relative to the mouse lens due to its higher content of DNA, henceforth larger mass. The presence of high levels of hexokinase mRNAs relative to aldose reductase mRNAs in the lens would be expected to favor metabolism of glucose via the glycolytic pathway rather than the sorbitol pathway, leading to retardation of development of sugar cataracts in the mouse lens; while the opposite is true for the rat lens.

Aldehyde Reductase↗

Mechanism of "hypoglycemic" cataract formation in the rat lens. I. The role of hexokinase instability.

Lenses from 100 gram albino rats remain clear and possess normal levels of Na+, K+, ATP, and hexokinase activity for 20 hours incubated in medium containing 12 mM glucose. Below 2.0 mM glucose, a cataract forms and there is an abrupt rise in lens Na+ and wet weight, a fall in lens K+, ATP, and hexokinase activity. The cataract is a thin lamellar opacity involving the anterior and posterior surfaces of the lens. If the lens is deprived of glucose for 48 hours, a nuclear cataract forms; the cortex between the superficial lamellar opacity and the nucleus being clear. This experimental cataract bears a striking resemblance to the hypoglycemic cataract seen in children. The thermal deactivation of hexokinase follows rapidly upon the depletion of its substrates (ATP and glucose) and is a primary factor leading to cataract formation. This was established by incubating the lens with 2-deoxyglucose, a competitive inhibitor of lens hexokinase. This compound blocks the entry of glucose into the glycolytic sequence. The cataract formed in its presence is identical morphologically and biochemically to that observed in a glucose-free medium. The effects of 2-deoxyglucose are prevented by increasing the glucose level; this rules out a direct toxic influence of 2-deoxyglucose and further supports the primary role of hexokinase thermolability in the etiology of this experimental cataract. This in vitro system appears to be an excellent experimental model for the study of the human hypoglycemic cataract.

Adenosine Triphosphate↗

Further studies on the role of phospholipids in determining the characteristics of mitochondrial binding sites for type I hexokinase.

Previous work has indicated that two types (A and B) of binding sites for hexokinase exist, but in different proportions, on brain mitochondria from various species. Hexokinase is readily solubilized from Type A sites by glucose 6-phosphate (Glc-6-P), while hexokinase bound to Type B sites remains bound even in the presence of Glc-6-P. Type A:Type B ratios are approximately 90:10, 60:40, 40:60, and 20:80 for brain mitochondria from rat, rabbit, bovine and human brain, respectively. The present study has indicated that MgCl2-dependent partitioning of mitochondrially bound hexokinase into a hydrophobic (Triton X-114) phase is generally correlated with the proportion of Type B sites. This partitioning behavior is sensitive to phospholipase C, implying that the factor(s) responsible for conferring hydrophobic character is(are) phospholipid(s). Substantial differences were also seen in the resistance of hexokinase, bound to brain mitochondria from various species, to solubilization by Triton X-100, Triton X-114, or digitonin. This resistance increased with proportion of Type B sites. Enrichment of bovine brain mitochondria in acidic phospholipids (phosphatidylserine or phosphatidylinositol), but not phosphatidylcholine or phosphatidylethanolamine, substantially increased solubilization of the enzyme after incubation at 37 degrees C. Collectively, the results imply that the Type A and Type B sites are located in membrane domains of different lipid composition, the Type A sites being in domains enriched in acidic phospholipids which lead to greater susceptibility to solubilisation by Glc-6-P.

Animals↗

[Preparation of hexokinase isoenzyme II with expressed adsorption properties].

The isolation of the native hexokinase isozyme II possessing a high adsorptive capacity is described. This property underlies the adsorption mechanism responsible for the control of the hexokinase activity in the cell and is realized only under conditions of the structural integrity of the enzyme. The latter is due, primarily, to the functional state of the specific adsorption domain which provides the specific interaction of hexokinase isozyme II with biological membranes. The criteria of nativity of skeletal muscle hexokinase were elaborated. A procedure for obtaining highly purified native hexokinase isozyme II from rat skeletal muscle was developed.

Animals↗

[Interaction of hexokinase II isoenzyme from rat skeletal muscles with lecithin liposomes].

It was found that in the presence of Mg2+ (pH 7.5) rat skeletal muscle hexokinase isozyme II is firmly adsorbed on mitochondrial and artificial phospholipid membranes (lecithin liposomes). In both cases the adsorption isotherm has similar quantitative and qualitative characteristics, which points to the absence of specific binding sites on the membranes. Under these conditions, immobilization of hexokinase on various membranes is concomitant with similar changes in the enzyme stability upon storage as well as with the pH-dependence of the enzyme activity. It was demonstrated that the bound hexokinase form has a greater value of V, an increased affinity for glucose and a decreased sensitivity to the inhibitory action of glucose-6-phosphate as compared to the free form. Besides, this form is in a greater degree subjected to the inhibitory influence of ADP with respect to glucose. In this case, the enzyme affinity for ATP and the Ki value for ADP with respect to ATP is practically the same both for the free and membrane-bound forms. The data obtained suggest that the phospholipid component of mitochondrial membranes participates in the enzyme binding in the presence of Mg2+. It was assumed that the model system used in the present study, i.e., hexokinase-Mg2+-liposomes, may be successfully used for the analysis of an adsorption mechanism of regulation of hexokinase activity in the cell.

Adsorption↗

Hereditary nonspherocytic hemolytic anemia and hexokinase deficiency.

An 11-yr-old child with mild chronic hemolytic anemia was found to have decreased red cell hexokinase activity in spite of the reduced mean age of her red cell population. Similar decreases in red cell hexokinase activity were documented in the patient's parents and in one sib. The red cells were morphologically normal. Red cell 2,3-DPG levels were normal and ATP and glucose-6-phosphate levels were diminished. The kinetic properties, electrophoretic mobility, and thermal stability of the residual red cell hexokinase were normal or nearly so. Glucose consumption of the hexokinase-deficient cells was not appreciably decreased, probably because less of the potent inhibitor glucose-6-phosphate was present in the erythrocytes. It is likely, although not certain, that in this patient nonspherocytic hemolytic anemia resulted from hexokinase deficiency.

Adenosine Triphosphate↗

[Hexokinase and sucrose phosphorylase activity of cell-free preparations of haploid and diploid strains of Pullularia pullulans].

The autodiploid strain of Pullularia pullulans 1125(4)(13) that differs from the haploid strain 1125(4) in the rate of synthesis of the polysaccharide pullulan has a higher activity of hexokinase in cell-free preparations. A change in the activity of hexokinase in the diploid strain coincides in time with a change in the rate of the polysaccharide synthesis: the activity of hexokinase in cell-free preparations is highest when the rate of pullulan synthesis is maximal. In the haploid strain, the activity of hexokinase in preparations from the cultures grown for 24, 38 and 48 hours remains at the same level, and then decreases by the 72nd hour. The activity of sucrose phosphorylase in cell-free preparations of the haploid strain increases with aging of the culture; in preparations of the diploid strain, the activity decreases by the 38th hour and then increases like in the haploid strain. These data make it possible to suggest that the activity of hexokinase, rather than that of sucrose phosphorylase, influences the rate of pullulan synthesis.

Cell-Free System↗

[Changes in hexokinase activity during muscle alteration].

Changes in extractability and activity of hexokinase (HK) were studied under the action of heating and of urea on skeletal muscles of Rana temporaria L., and besides the stability of this enzyme in muscle extract to those agents in vitro was examined. Under a 15 minutes heating of muscle, a decrease in extractability (the activity calculated for 1 g of tissue) and activity (the activity calculated for 1 mg of protein) of hexokinase is first revealed at 37 degrees C. Then the enzyme extractability decreases gradually in accordance to the decrease in extractability of the total water-soluble protein; the level of hexokinase activity attained at 37 degrees does not change up to 40 degrees. At 42 degrees the activity of the enzyme is completely inhibited. Under the heating of the muscle extract, the decrease of enzyme activity takes place at 36 degrees, the level achieved being stable up to 42 degrees C. Under the action of urea on the muscle at the reversible phase of alteration (1 M urea from 5 minutes to 2 hours at room temperature, 1 M urea for 9 hours at + 4 degrees C), hexokinase activity increases, calculated for 1 g of tissue and for 1 mg of protein. Under the irreversible disappearance of muscle excitability (1 M urea during 9 hours, 2 M urea during 2 hours at room temperature) no hexokinase activity was revealed. The activation of the enzyme is discussed in connection with the data on the increase of ATP content in muscle under the urea alteration. The treatment of the enzyme in muscle extract with 1 M urea decreases its activity in 30 minutes down to 67%; the level achieved does not change during 20 hours.

Aerobiosis↗

Rabbit red blood cell hexokinase. Mechanism of decay during cell life-span.

Rabbit red blood cells contain hexokinase type I whereas in the reticulocyte two distinct molecular forms (HK Ia and Ib) are present. One (HK Ia) corresponds to hexokinase type I from other tissues, while the other differs from any previously reported isozyme. Rabbit bone marrow cells contain hexokinase type I and II. However, when the erythroid precursor cells become predominant over the non-erythroid cells (during phenylhydrazine anemia) a great increase of HK Ia can be observed concomitant with the appearance of HK Ib. Fractionation of the bone marrow cells on density gradients provides evidence that basophil erythroblasts and proerythroblasts contain only HK Ia while HK Ib appears at the reticulocyte stage. Maturation and ageing of circulating reticulocytes are associated with the decrease of hexokinase activity. Since the decay rate of HK Ib is about three times higher than the decay rate of HK Ia, the mature erythrocytes do not contain appreciable amounts of HK Ib. Furthermore, in vitro, HK Ia and Ib possess similar stabilities so that a cellular mechanism must be responsible of their in vivo different decay rates. This mechanism, as reported in this paper, is ATP-dependent, could be found in the soluble fraction, and is active only at the reticulocyte stage. These properties are similar to those of the ATP-dependent proteolytic system. Pure ubiquitin, an essential polypeptide of the ATP-dependent proteolytic system, is also able to catalyze the decay of hexokinase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

[Kinetics behavior of isozymes I and II of rat skeletal muscle hexokinase after their binding to mitochondrial membranes].

Hexokinase from rat skeletal muscle hyaloplasm is represented by two isozymic forms I and II. The values of apparent Km (glucose) for these forms were calculated according to the Reiner method. After binding to mitochondrial membranes under effects of Mg2+ the value of V for hexokinase isozyme II is increased 3-fold, while its Km value is decreased 5 times; the properties of isozyme I remain thereby unchanged. After cessation of interaction with the membrane, under effect of ATP the free hexokinase reveals the same kinetic properties (V, Km, sensitivity to the inhibiting effect of glucose 6-phosphate) as does the native enzyme from hyaloplasm. The ability of hexokinase to exist in a bound or free state is also reversible and depends on the Mg2+/ATP ratio. The conditions for a reversible transfer of muscle hexokinase (isozymes I and II) from the hyaloplasm to the mitochondria-bound state completely coincide with those for cardiac enzyme (isozyme I). The data obtained are discussed in terms of a concept on an adsorption mechanism controlling the enzyme activity.

Animals↗

[Anaesthesia and intracellular distribution of cerebral hexokinase (author's transl)].

The purpose of the present investigation was to demonstrate how the intracellular distribution of hexokinase was influenced by anaesthetics. The experiments were carried out using mice in vivo and the isolated perfused rat brain. Thiopentone (100 mg/kg i.p.) produced an increase of soluble hexokinase activity in brains of mice which were removed from the skull in 45 s to 120 s. A solubilization of hexokinase activity was demonstrable in the surgical stage of anaesthesia, when the animals awoke redistribution to control values was measurable. A dose dependent influence of thiopentone on hexokinase distribution was demonstrated in the isolated perfused rat brain. When the thiopentone concentration in the perfusion medium was increased (0.05-0.8 mM) the soluble hexokinase activity was elevated while the EEG activity was reduced up to isoelectricity.

Anesthetics↗