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PubMed1994

Use of Yarrowia lipolytica hexokinase for the quantitative determination of trehalose 6-phosphate.

This paper describes a procedure for the quantitative determination of trehalose 6-phosphate (T6P) based on its ability to inhibit hexokinase from Yarrowia lipolytica. The assay is linear between 1 nmol and at least 8 nmol. The concentration of T6P in wild-type Saccharomyces cerevisiae (0.15 mM) and in ras2 mutants (0.25 mM) remained unchanged in the exponential or stationary phase of growth or after heat shock. A tps1 mutant affected in T6P synthase did not show detectable T6P. Heat shock increased the concentration of T6P in Schizosaccharomyces pombe from 0.43 to 0.75 mM.

Hexokinase↗

PubMed2002

The hexokinase 2-dependent glucose signal transduction pathway of Saccharomyces cerevisiae.

Sugars, predominantly glucose, evoke a variety of responses in Saccharomyces cerevisiae. These responses are elicited through a complex network of regulatory mechanisms that transduce the signal of presence of external glucose to their final intracellular targets. The HXK2 gene, encoding hexokinase 2 (Hxk2), the enzyme that initiates glucose metabolism, is highly expressed during growth in glucose and plays a pivotal role in the control of the expression of numerous genes, including itself. The mechanism of this autocontrol of expression is not completely understood. Hxk2 is found both in the nucleus and in the cytoplasm of S. cerevisiae; the nuclear localization is dependent on the presence of a stretch of amino acids located from lysine-6 to methionine-15. Although serine-14, within this stretch, can be phosphorylated in the absence of glucose, it is still unsettled whether this phosphorylation plays a role in the cellular localization of Hxk2. The elucidation of the mechanism of transport of Hxk2 to and from the nucleus, the influence of the oligomeric state of the protein on the nuclear transport and the fine mechanism of regulation of transcription of HXK2 are among the important unanswered questions in relation with the regulatory role of Hxk2.

Base Sequence↗

PubMed1979

Hexokinase deficiency in erythrocytes: a new variant in 5 members of a Finnish family.

15 cases of congenital haemolytic anaemia have thus far been attributed to hexokinase (HK) deficiency in erythrocytes. We report some clinical, biochemical and genetic findings from 5 members of a Finnish family with this deficiency. The proband, a 1-year-old girl, was the only patient with anaemia. All subjects had either mild or marked reticulocytosis. Red cell ATP levels were at the lower range of normal in all subjects and 2,3-DPG was abnormally low in one. The activities of red cell enzymes, other than HK, were within or above the normal range, respectively. The Km-values for glucose and fructose were elevated (ATP normal) in the subjects with HK deficiency. We speculate that the family represents heterozygosity of a mutant allele and that there is phenotypic variation associated with the HK mutant. The locus might be subject to mutations which lead to a variety of HK variants and to a spectrum of diseases. This point of view is in accordance with the overwhelming variation of reaction kinetics and metabolic effects of this and other reported cases.

Adenosine Triphosphate↗

PubMed2005

Hexokinase II protein content is a determinant of exercise endurance capacity in the mouse.

Hexokinase (HK) II content is elevated in fatigue resistant muscle fibres and exercise trained muscle. The aim of this study was to determine if exercise capacity is dependent on muscle HK protein content. C57Bl/6 mice with a 50% HK knockout (HK+/-), no genetic manipulation (wild-type, WT) and an approximately 3-fold HK overexpression (HKTg) were tested. Mice (n = 12/group) completed both a maximal oxygen consumption test(VO2max) test and an endurance capacity test (run at approximately 75% VO2max) on an enclosed treadmill equipped to measure gas exchange. Arterial and venous catheters were surgically implanted into separate groups of mice (n = 9-11/group) in order to measure an index of muscle glucose uptake Rg during 30 min of treadmill exercise. Maximum work rate (0.95 +/- 0.05, 1.00 +/- 0.04 and 1.06 +/- 0.07 kg m min-1), (137 +/- 3, 141 +/- 4 and 141 +/- 5 ml kg-1 min-1) and maximal respiratory exchange ratio (1.04 +/- 0.02, 1.00 +/- 0.03 and 1.04 +/- 0.04) were similar in HK+/-, WT and HKTg, respectively. Exercise endurance capacity (measured as time to exhaustion) increased as HK content increased (55 +/- 11, 77 +/- 5 and 98 +/- 9 min) and this was related to Rg measured in mice during 30 min of exercise (13 +/- 2, 24 +/- 5 and 42 +/- 5 micromol (100 g)-1 min-1). Muscle glycogen in sedentary HK+/-mice and HK+/- mice following 30 min of exercise were significantly lower than in HKTg and WT mice. However, the net exercise-induced muscle glycogen breakdown was equal in the three genotypes. In summary, HK protein content within the range studied (a) was not associated with a difference in the capacity to perform maximal intensity exercise, (b) was a powerful determinant of the ability to sustain moderate intensity exercise, as reducing HK content impaired endurance and increasing HK content enhanced endurance, and (c) although directly related to exercise endurance, was not a determinant of net muscle glycogen usage during exercise. In conclusion, adaptations that increase HK protein content and/or functional activity such as regular exercise contribute to increased muscular endurance.

Anaerobic Threshold↗

PubMed1967

Hexokinase isoenzymes in liver and adipose tissue of man and dog.

A hexokinase, with a low Michaelis constant, not previously described, has been found in extracts of human and dog liver but not of rat liver. Earlier reports are contradicted in that glucokinase occurs in extracts of liver from well-nourished humans and dogs; it is absent, or almost so, during states of poor nutrition.

Adipose Tissue↗

PubMed2000

Hexokinase regulates kinetics of glucose transport and expression of genes encoding hexose transporters in Saccharomyces cerevisiae.

Glucose transport kinetics and mRNA levels of different glucose transporters were determined in Saccharomyces cerevisiae strains expressing different sugar kinases. During exponential growth on glucose, a hxk2 null strain exhibited high-affinity hexose transport associated with an elevated transcription of the genes HXT2 and HXT7, encoding high-affinity transporters, and a diminished expression of the HXT1 and HXT3 genes, encoding low-affinity transporters. Deletion of HXT7 revealed that the high-affinity component is mostly due to HXT7; however, a previously unidentified very-high-affinity component (K(m) = 0.19 mM) appeared to be due to other factors. Expression of genes encoding hexokinases from Schizosaccharomyces pombe or Yarrowia lipolytica in a hxk1 hxk2 glk1 strain prevented derepression of the high-affinity transport system at high concentrations of glucose.

Biological Transport↗

PubMed2006

Identification and characterization of an ATP-dependent hexokinase with broad substrate specificity from the hyperthermophilic archaeon Sulfolobus tokodaii.

As a new member of the glucose-phosphorylating enzymes, the ATP-dependent hexokinase from the hyperthermophilic crenarchaeon Sulfolobus tokodaii was purified, identified, and characterized. Our results revealed that the enzyme differs from other known enzymes in primary structure and its broad substrate specificity for both phosphoryl donors and acceptors.

Adenosine Triphosphate↗

PubMed2003

Hexokinase II partial knockout impairs exercise-stimulated glucose uptake in oxidative muscles of mice.

Muscle glucose uptake (MGU) is distributively controlled by three serial steps: delivery of glucose to the muscle membrane, transport across the muscle membrane, and intracellular phosphorylation to glucose 6-phosphate by hexokinase (HK). During states of high glucose fluxes such as moderate exercise, the HK activity is of increased importance, since augmented muscle perfusion increases glucose delivery, and increased GLUT4 at the cell membrane increases glucose transport. Because HK II overexpression augments exercise-stimulated MGU, it was hypothesized that a reduction in HK II activity would impair exercise-stimulated MGU and that the magnitude of this impairment would be greatest in tissues with the largest glucose requirement. To this end, mice with a HK II partial knockout (HK+/-) were compared with their wild-type control (WT) littermates during either sedentary or moderate exercise periods. Rg, an index of glucose metabolism, was measured using 2-deoxy-[3H]glucose. No differences in glucose metabolism were detected between sedentary groups. The increase in Rg due to exercise was impaired in the highly oxidative heart and soleus muscles of HK+/- compared with WT mice (7 +/- 10 vs. 29 +/- 9 and 8 +/- 3 vs. 25 +/- 7 micromol. 100 g-1. min-1, respectively). However, the increase in Rg due to exercise was not altered in gastrocnemius and superficial vastus lateralis muscles in HK+/- and WT mice (8 +/- 2 vs. 12 +/- 3 and 5 +/- 2 vs. 8 +/- 2 micromol. 100 g-1. min-1, respectively). In conclusion, MGU is impaired by reductions in HK activity during exercise, a physiological condition characterized by high glucose flux. This impairment is critically dependent on the tissue's glucose metabolic rate and correlates with tissue oxidative capacity.

Animals↗

PubMed2006

Role of AMPKalpha2 in basal, training-, and AICAR-induced GLUT4, hexokinase II, and mitochondrial protein expression in mouse muscle.

We investigated the role of AMPKalpha2in basal, exercise training-, and AICAR-induced protein expression of GLUT4, hexokinase II (HKII), mitochondrial markers, and AMPK subunits. This was conducted in red (RG) and white gastrocnemius (WG) muscle from wild-type (WT) and alpha2-knockout (KO) mice after 28 days of activity wheel running or daily AICAR injection. Additional experiments were conducted to measure acute activation of AMPK by exercise and AICAR. At basal, mitochondrial markers were reduced by approximately 20% in alpha2-KO muscles compared with WT. In both muscle types, AMPKalpha2 activity was increased in response to both stimuli, whereas AMPKalpha1 activity was increased only in response to exercise. Furthermore, AMPK signaling was estimated to be 60-70% lower in alpha2-KO compared with WT muscles. In WG, AICAR treatment increased HKII, GLUT4, cytochrome c, COX-1, and CS, and the alpha2-KO abolished the AICAR-induced increases, whereas no AICAR responses were observed in RG. Exercise training increased GLUT4, HKII, COX-1, CS, and HAD protein in WG, but the alpha2-KO did not affect training-induced increases. Furthermore, AMPKalpha1, -alpha2, -beta1, -beta2, and -gamma3 subunits were reduced in RG, but not in WG, by 30-60% in response to exercise training. In conclusion, the alpha2-KO was associated with an approximately 20% reduction in mitochondrial markers in both muscle types and abolished AICAR-induced increases in protein expression in WG. However, the alpha2-KO did not reduce training-induced increases in HKII, GLUT4, COX-1, HAD, or CS protein in WG, suggesting that AMPKalpha2 may not be essential for metabolic adaptations of skeletal muscles to exercise training.

AMP-Activated Protein Kinases↗

PubMed1993

Regulation of glucose transporter and hexokinase II expression in tissues of diabetic rats.

Glucose transport and phosphorylation are decreased in muscle and adipose tissue in diabetes mellitus. The glucose transporter GLUT-4 and hexokinase II (HK II) are the main isoforms of proteins involved in glucose transport and phosphorylation in insulin-sensitive tissues, adipose tissue, skeletal muscle, and heart. The molecular mechanisms responsible for the decrease of glucose transport and phosphorylation have been studied during the first 3 days after streptozotocin (STZ) administration in adult male Wistar rats. GLUT-4 mRNA and protein and HK II mRNA and enzyme activity were measured. After the injection of STZ (30 h), GLUT-4 and HK II mRNAs were decreased to 10 +/- 1 and 20 +/- 3% that found in nondiabetic rats, respectively; they remained at these low levels for 72 h. Normalization of the blood glucose level by phlorizin infusion did not restore GLUT-4 and HK II mRNA concentrations to normal. In contrast, normalization of the blood glucose level by physiological infusion of insulin resulted in a total normalization of GLUT-4 and HK II mRNA concentrations. When insulin therapy was stopped, GLUT-4 and HK II mRNA and protein concentrations fell in 6 h to 40 and 20% of control levels, respectively. Minimal changes of GLUT-4 and HK II mRNA, and of HK II activity, were observed in skeletal muscle and heart of diabetic rats. We conclude that GLUT-4 and HK II mRNA are coordinately expressed in white adipose tissue. They are rapidly affected by an acute decrease of the plasma insulin concentrations but are not modified by hyperglycemia. In contrast, skeletal muscle and heart GLUT-4 and HK II mRNA are not greatly affected by short-term diabetes.

Adipose Tissue↗

PubMed1994

Rat skeletal muscle hexokinase II mRNA and activity are increased by a single bout of acute exercise.

This study addresses the potential role of skeletal muscle hexokinase (HK) II in the regulation of glucose uptake and metabolism in vivo. Male rats undertook a single bout of treadmill exercise and were then killed immediately or after a predetermined recovery period. Three muscles [soleus (Sol), gastrocnemius/plantaris (Gc), and white vastus] were excised, and HK II mRNA, GLUT-4 mRNA, total HK (HK I and HK II) and heat-stable HK (predominantly HK I) activities were assessed. Three hours after the cessation of a single bout of exhaustive exercise, HK II mRNA was significantly increased in all three muscles. Ninety or thirty minutes of exercise, with a 3-h recovery, increased Gc HK II mRNA to the same extent as exhaustive exercise, but 15 min of exercise had no effect. Gc HK II mRNA continued to increase up to 8 h after the cessation of 90 min of exercise but returned to basal by 24 h postexercise. In contrast to HK II mRNA, Gc GLUT-4 mRNA was unchanged at 0, 3, 8, and 24 h after the cessation of 90 min of exercise. Total HK activity was significantly increased in Sol and Gc, 8 and 24 h after the cessation of 90 min of exercise. Heat-stable HK activity was unchanged in all three muscles. The increase in total HK activity, inferred to be an increase of HK II, may be important in the persistence of the postexercise increase in insulin action.

Animals↗

PubMed1994

Development and regulation of glucose transporter and hexokinase expression in rat.

The ontogenesis of the glucose transporters GLUT-1, GLUT-2, and GLUT-4 and the hexokinases HK-I, HK-II, and HK-IV (glucokinase) was studied in rat tissues. In brown adipose tissue, high levels of GLUT-4 and HK-II were observed during fetal life; both decreased at birth and then increased throughout development. At birth, cold exposure increased GLUT-4 and HK-II expression in brown adipose tissue, whereas fasting decreased it. GLUT-1 and HK-I were present in fetal muscle, but GLUT-4 and HK-II were absent. The coordinate appearance of GLUT-4 and HK-II in skeletal muscle was concomitant with the acquisition of insulin sensitivity after weaning. In the heart, the glucose transporter isoform switched from GLUT-1 to GLUT-4 during the suckling period. The coordinate expression of GLUT-4 and HK-II in heart was observed after weaning. GLUT-2, detected in fetal liver, increased throughout development. GLUT-1 and HK-I were detectable in fetal liver, whereas glucokinase appeared after weaning. Consumption of a high-carbohydrate diet after weaning increased GLUT-4 and HK-II in muscle and GLUT-2 in liver, whereas consumption of a high-fat diet prevented these changes. These results showed that 1) GLUT-1 and HK-I are abundant in most fetal rat tissues, 2) GLUT-4 and HK-II expression is associated with the appearance of tissue insulin sensitivity, and 3) GLUT-2 is expressed early in liver, before the appearance of glucokinase.

Adipose Tissue, Brown↗

PubMed1995

Regulation of hexokinase II and glycogen synthase mRNA, protein, and activity in human muscle.

Insulin regulates the activity of key enzymes of glucose metabolism in skeletal muscle by altering transcription or translation or by producing activity-altering modifications of preexisting enzyme molecules. Because of the small size of percutaneous muscle biopsies, these phenomena have been difficult to study in humans. This study was performed to determine how physiological hyperinsulinemia regulates the activities of hexokinase (HK), glycogen synthase (GS), and GLUT-4 in human skeletal muscle in vivo. We determined mRNA abundance, protein content, and activities for these proteins in muscle biopsies before and after a hyperinsulinemic clamp in normal subjects. HK I, HK II, GS, and GLUT-4 were expressed in muscle. HK II accounted for 80% of total HK activity and was increased by insulin from a basal value of 2.11 +/- 0.26 to 3.35 +/- 0.47 pmol.min-1.mg protein-1 (P < 0.05); HK I activity was unaffected. Insulin increased GS activity from 3.85 +/- 0.82 to 6.06 +/- 0.49 nmol.min-1.mg-1 (P < 0.01). HK II mRNA was increased 3.3 +/- 1.3-fold (P < 0.05) by insulin infusion. HK I, GS, and GLUT-4 mRNA and protein were unaffected. Because insulin infusion increased HK II but not GS mRNA, we conclude that HK II and GS may be regulated by insulin by different mechanisms in human skeletal muscle.

Adult↗

PubMed1997

Regulation of glucose transporter GLUT-4 and hexokinase II gene transcription by insulin and epinephrine.

Transport of glucose across the plasma membrane by GLUT-4 and subsequent phosphorylation of glucose by hexokinase II (HKII) constitute the first two steps of glucose utilization in skeletal muscle. This study was undertaken to determine whether epinephrine and/or insulin regulates in vivo GLUT-4 and HKII gene transcription in rat skeletal muscle. In the first experiment, adrenodemedullated male rats were fasted 24 h and killed in the control condition or after being infused for 1.5 h with epinephrine (30 microg/ml at 1.68 ml/h). In the second experiment, male rats were fasted 24 h and killed after being infused for 2.5 h at 1.68 ml/h with saline or glucose (625 mg/ml) or insulin (39.9 microg/ml) plus glucose (625 mg/ml). Nuclei were isolated from pooled quadriceps, tibialis anterior, and gastrocnemius muscles. Transcriptional run-on analysis indicated that epinephrine infusion decreased GLUT-4 and increased HKII transcription compared with fasted controls. Both glucose and insulin plus glucose infusion induced increases in GLUT-4 and HKII transcription of twofold and three- to fourfold, respectively, compared with saline-infused rats. In conclusion, epinephrine and insulin may regulate GLUT-4 and HKII genes at the level of transcription in rat skeletal muscle.

Adrenal Medulla↗

PubMed1999

Overexpression of hexokinase II increases insulinand exercise-stimulated muscle glucose uptake in vivo.

The hypothesis of this investigation was that glucose uptake would be increased in skeletal muscle of transgenic mice (TG) overexpressing hexokinase II (HK II) compared with their nontransgenic littermates (NTG) during euglycemic hyperinsulinemia and treadmill exercise. For insulin experiments, catheters were surgically implanted in the jugular vein and carotid artery for infusions and sampling, respectively. Conscious mice underwent experiments approximately 5 days later in which 4 mU. kg-1. min-1 insulin and variable glucose (n = 7 TG and n = 7 NTG) or saline (n = 5 TG and n = 4 NTG) was infused for 140 min. Over the last 40 min of the experiments, 2-deoxy-[3H]glucose ([2-3H]DG) was infused, after which muscles were removed. For the exercise experiments, jugular vein catheters were surgically implanted. Five days later, mice received a bolus of [2-3H]DG and then remained sedentary (n = 6 TG and n = 8 NTG) or ran on a motorized treadmill (n = 12 TG and n = 8 NTG) for 30 min. TG and NTG had similar muscle [2-3H]DG 6-phosphate ([2-3H]DGP) accumulation in the basal state (P > 0.05). In the hyperinsulinemic experiments, TG required approximately 25% more glucose to maintain euglycemia (P < 0.05), and muscle [2-3H]DGP accumulation normalized to infusate [2-3H]DG was similarly increased (P < 0.05). In the exercise experiments, muscle [2-3H]DGP accumulation was significantly greater in TG than NTG (P < 0.05). In conclusion, we did not detect an effect of HK II overexpression on muscle [2-3H]DGP accumulation under basal conditions. Hyperinsulinemia and exercise shift the control of muscle glucose uptake so that phosphorylation is a more important determinant of the rate of this process.

Animals↗

PubMed1997

GLUT2 and hexokinase control proximodistal gradient of intestinal glucose metabolism in the newborn pig.

We have reported previously that a high glycolytic capacity develops soon after birth in enterocytes isolated from suckling newborn pigs. In the present work, we investigated whether such metabolic changes could affect intestinal glucose utilization in vivo and examined possible variations in glucose metabolism along the small intestine. Glucose utilization by individual tissues was assessed using the 2-deoxyglucose technique. The overall glucose utilization rate was doubled in suckling vs. fasting 2-day-old pigs because of significantly higher rates in all tissues studied, except for the brain. In parallel, enterocytes were isolated from the proximal, medium, or distal jejunoileum of newborn vs. 2-day-old pigs and assessed for their capacity to utilize, transport, and phosphorylate glucose. Intestinal glucose consumption accounted for approximately 15% of glucose turnover rate in suckling vs. 8% in fasting pigs. Moreover, there was a proximal-to-distal gradient of glucose utilization in the intestinal mucosa of suckling pigs. Such a gradient was also evidenced on isolated enterocytes. The stimulation of both hexokinase activity (HK2 isoform) and basolateral glucose transporter (GLUT2), as observed in the proximal jejunum, could account for such a site-specific effect of suckling.

Animals↗

PubMed2005

Ischemic preconditioning, insulin, and morphine all cause hexokinase redistribution.

Association of hexokinase (HK) with mitochondria preserves mitochondrial integrity and is an important mechanism by which cancer cells are protected against hypoxic conditions. Maintenance of mitochondrial integrity also figures prominently as a major characteristic of many cardioprotective manipulations. In this study, we provide evidence that cardioprotective interventions may promote HK redistribution from the cytosol to the mitochondria in the heart. Isolated Langendorff-perfused rat hearts (n = 6/group) were subjected to normoxic perfusion (control, Con), three 5-min ischemia-reperfusion periods (ischemic preconditioning, IPC), 1 U/l insulin (Ins), or 1 microM morphine (Mor). Hearts were immediately homogenized and centrifuged to obtain whole cell, cytosolic, and mitochondrial fractions. HK, lactate dehydrogenase (LDH), and citrate synthase (CS) enzyme activities were determined. No change in LDH or CS present in the cytosol fraction relative to whole cell activity was observed with any of the cardioprotective interventions. By contrast, HK present in the cytosol fraction relative to whole cell activity decreased significantly (P < 0.05) with all cardioprotective interventions, from 0.58 +/- 0.03 (Con) to 0.46 +/- 0.04 (IPC), 0.41 +/- 0.01 (Ins), and 0.45 +/- 0.02 (Mor). In addition, HK relative to CS activity in the mitochondrial fraction increased significantly with cardioprotection, from 0.15 +/- 0.001 (Con) to 0.21 +/- 0.002 (IPC), 0.18 +/- 0.003 (Ins), and 0.21 +/- 0.005 (Mor). Our novel data suggest that well-known cardioprotective interventions share a common end-effector mechanism of cytosolic HK translocation. Association of HK with mitochondria may promote inhibition of the mitochondrial permeability transition pore and thereby reduce cell death and apoptosis.

Animals↗

PubMed2002

Elevated expression of hexokinase II protects human lung epithelial-like A549 cells against oxidative injury.

Increased glucose utilization and hexokinase (HK)-II expression are adaptive features of lung cells exposed to hypoxia or hyperoxia. HK-II is the most regulated isoform of HK. Whether its overexpression could be protective against oxidative stress was explored in human lung epithelial-like (A549) cells. HK-II was overexpressed in A549 cells in a tetracycline-repressible retroviral vector system. Elevated expression of HK-II was confirmed by Western blot and activity measurements. Cell death caused by exposure to hyperoxia was decreased in HK-II-overexpressing cells. This effect was reversed when HK-II expression was suppressed with doxycycline. A similar protective effect was observed in HK-II-overexpressing cells after treatment with 1 mM hydrogen peroxide for 48 h. At baseline, fluorescence microscopy showed that overexpressed HK-II was localized to mitochondria. Electron microscopic studies showed that hyperoxia-exposed HK-II overexpressors had better-preserved and quantitatively smaller mitochondria than those in which the HK-II expression was suppressed or in the nontransduced A549 cells. Mitochondrial membrane potential was increased in HK-II-overexpressing cells exposed to hyperoxia compared with the nontransduced control cells under similar conditions. The present study demonstrates that HK-II protects human lung epithelial-like A549 cells against oxidative insults by protecting the mitochondria.

Cell Line↗