Mitochondrial and cytosolic hexokinase from rat brain: one and the same enzyme?

Impaired glucose-1,6-biphosphate production due to bi-allelic PGM2L1 mutations is associated with a neurodevelopmental disorder

We describe a genetic syndrome due to PGM2L1 deficiency. PGM2 and PGM2L1 make hexose-bisphosphates, like glucose-1,6-bisphosphate, which are indispensable cofactors for sugar phosphomutases. These enzymes form the hexose-1-phosphates crucial for NDP-sugars synthesis and ensuing glycosylation reactions. While PGM2 has a wide tissue distribution, PGM2L1 is highly expressed in the brain, accounting for the elevated concentrations of glucose-1,6-bisphosphate found there. Four individuals (three females and one male aged between 2 and 7.5 years) with bi-allelic inactivating mutations of PGM2L1 were identified by exome sequencing. All four had severe developmental and speech delay, dysmorphic facial features, ear anomalies, high arched palate, strabismus, hypotonia, and keratosis pilaris. Early obesity and seizures were present in three individuals. Analysis of the children's fibroblasts showed that glucose-1,6-bisphosphate and other sugar bisphosphates were markedly reduced but still present at concentrations able to stimulate phosphomutases maximally. Hence, the concentrations of NDP-sugars and glycosylation of the heavily glycosylated protein LAMP2 were normal. Consistent with this, serum transferrin was normally glycosylated in affected individuals. PGM2L1 deficiency does not appear to be a glycosylation defect, but the clinical features observed in this neurodevelopmental disorder point toward an important but still unknown role of glucose-1,6-bisphosphate or other sugar bisphosphates in brain metabolism.

Lactate preserves neuronal metabolism and function following antecedent recurrent hypoglycemia

Hypoglycemia occurs frequently during intensive insulin therapy in patients with both type 1 and type 2 diabetes and remains the single most important obstacle in achieving tight glycemic control. Using a rodent model of hypoglycemia, we demonstrated that exposure to antecedent recurrent hypoglycemia leads to adaptations of brain metabolism so that modest increments in circulating lactate allow the brain to function normally under acute hypoglycemic conditions. We characterized 3 major factors underlying this effect. First, we measured enhanced transport of lactate both into as well as out of the brain that resulted in only a small increase of its contribution to total brain oxidative capacity, suggesting that it was not the major fuel. Second, we observed a doubling of the glucose contribution to brain metabolism under hypoglycemic conditions that restored metabolic activity to levels otherwise only observed at euglycemia. Third, we determined that elevated lactate is critical for maintaining glucose metabolism under hypoglycemia, which preserves neuronal function. These unexpected findings suggest that while lactate uptake was enhanced, it is insufficient to support metabolism as an alternate substrate to replace glucose. Lactate is, however, able to modulate metabolic and neuronal activity, serving as a "metabolic regulator" instead.

Kinetic characterization of hexokinase isoenzymes from glioma cells: implications for FDG imaging of human brain tumors

Quantitative imaging of glucose metabolism of human brain tumors with PET utilizes 2-[(18)F]-fluorodeoxy-D-glucose (FDG) and a conversion factor called the lumped constant (LC), which relates the metabolic rate of FDG to glucose. Since tumors have greater uptake of FDG than would be predicted by the metabolism of native glucose, the characteristic of tumors that governs the uptake of FDG must be part of the LC. The LC is chiefly determined by the phosphorylation ratio (PR), which is comprised of the kinetic parameters (Km and Vmax) of hexokinase (HK) for glucose as well as for FDG (LC proportional to (Km(glc) x Vmax(FDG))/(Km(FDG) x Vmax(glc)). The value of the LC has been estimated from imaging studies, but not validated in vitro from HK kinetic parameters. In this study we measured the kinetic constants of bovine and 36B-10 rat glioma HK I (predominant in normal brain) and 36B-10 glioma HK II (increased in brain tumors) for the hexose substrates glucose, 2-deoxy-D-glucose (2DG) and FDG. Our principal results show that the KmGlc < KmFDG << Km2DG and that PR2DG < PRFDG. The FDG LC calculated from our kinetic parameters for normal brain, possessing predominantly HK I, would be higher than the normal brain LC predicted from animal studies using 2DG or human PET studies using FDG or 2DG. These results also suggest that a shift from HK I to HK II, which has been observed to increase in brain tumors, would have little effect on the value of the tumor LC.

High glycolysis in gliomas despite low hexokinase transcription and activity correlated to chromosome 10 loss

Loss of chromosome 10 was observed in 10 out of 12 xenografted glioblastomas studied. Chromosome 10 carries the gene coding the hexokinase type 1 isoenzyme (HK-I), which catalyses the first step of glycolysis, which is essential in brain tissue and glioblastomas. We investigated the relationships between the relative chromosome 10 number, the amount of HK-I mRNA, HK-I activity and its intracellular distribution, and glycolysis-related parameters such as the lactate-pyruvate ratio, lactate dehydrogenase (LDH) and ATP contents. Individual tumour HK-I mRNA amounts were 23-65% lower than that of normal human brain and reflected the relative decrease of chromosome 10 number (alpha < 0.01). Total HK activities of individual glioblastomas varied considerably but were constantly (a mean of seven times) lower than that of normal brain tissue. The mitochondria-bound HK-I fraction of individual tumours was generally over 50%, compared with that of normal brain tissue. As shown by lactate - pyruvate ratios, in all the gliomas, glycolysis was elevated to an average of 3-fold that measured in normal brain. An elevated ATP content was also constantly noted. Adaptation of glioblastoma metabolism to the chromosome 10 loss and to the HK-I transcription unit emphasises the critical role of glycolysis in their survival. We hypothesise that HK-I, the enzyme responsible for initiating glycolysis necessary for brain function, may approach its lowest limit in gliomas, thereby opening therapeutic access to pharmacological anti-metabolites affecting energy metabolism and tumour growth.

Intracellular pH governs the subcellular distribution of hexokinase in a glioma cell line

Hexokinase plays a key role in regulating cell energy metabolism. Hexokinase is mainly particulate, bound to the mitochondrial outer membrane in brain and tumour cells. We hypothesized that the intracellular pH (pH1) controls the intracellular distribution of hexokinase. Using the SNB-19 glioma cell line, pH1 variations were imposed by incubating cells in a high-K+ medium at different pH values containing specific ionophores (nigericin and valinomycin), without affecting cell viability. Subcellular fractions of cell homogenates were analysed for hexokinase activity. Imposed pH1 changes were verified microspectrofluorimetrically by using the pH1-sensitive probe SNARF-1-AM (seminaphtho-rhodafluor-1-acetoxymethyl ester). Imposition of an acidic pH1 for 30 min strongly decreased the particulate/total hexokinase ratio, from 63% in the control sample to 31%. Conversely, when a basic pH1, was imposed, the particulate/total hexokinase ratio increased to 80%. The glycolytic parameters, namely lactate/pyruvate ratio, glucose 6-phosphate and ATP levels, were measured concomitantly. Lactate/pyruvate ratio and ATP level were both markedly decreased by acidic pH1 and increased by basic pH1. Conversely, the glucose 6-phosphate level was increased by acidic pH1 and decreased by basic pH1. To demonstrate that the change of hexokinase distribution was not due to altered metabolite levels of glycolysis, a pH1 was imposed for a 5 min incubation time. Modification of the hexokinase distribution was similar to that noted after a 30 min incubation, whereas metabolite levels of glycolysis were not affected. These results provide evidence that the intracellular distribution of hexokinase is highly sensitive to variations of the pH1, and regulates hexokinase activity.

Mitochondria-bound hexokinase as target for therapy of malignant gliomas

Hexokinase plays an important role in glucose-utilizing tissues like normal brain and cancers. In these tissues, hexokinase (HK) is mainly bound to mitochondria (mHK). Our objectives were to evaluate total HK (tHK) activity and mHK fraction in gliomas and to determine whether mHK binding could be targeted for therapy. Tumors were obtained from 26 patients and 13 were xenografted. HK, lactate and ATP were measured in cytosol and mitochondria extracts. The tHK expressed in mU/mg protein were 147 +/- 19 and 78 +/- 12, in fresh gliomas and xenografts, respectively, and of 489 in the normal brain. The mHK fraction was 76% in normal brain, 74 +/- 4% in fresh tumors and 53 +/- 6% in xenografts. Lactate/mHK ratios were higher in gliomas than in normal brain. The ATP was 10, 52 +/- 31 and 19 +/- 8 nmol/mg protein in normal brain, xenografts and fresh gliomas respectively. Loss of one copy of chromosome 10 which carries the HK1 gene, was evidenced in 11 of the 13 xenografted gliomas. The anti-tumor effect of lonidamine (LND), which affects glycolysis in interfering with mHK activity, was tested in nude mice bearing 4 gliomas. LND (125 mg/kg, given i.p., twice daily for 5 days) led to a growth inhibition of TG-7-RO of 72%, with 2-fold growth retardation, and had no effect for TG-8-OZ. Intermediate LND-sensitivities for TG-11-DU and TG-10-PY were noted. The LND-sensitivity was correlated with the mHK activity (R2 = 0.73) and mHK fraction (R2 = 0.88). HK binding to mitochondria is a key of glycolysis in malignant gliomas, and targetting this binding with appropriate agents could be an effective therapeutic approach.

Mitochondrial hexokinase from differentiated and undifferentiated HT29 colon cancer cells: effect of some metabolites on the bound/soluble equilibrium

1. Solubilization of mitochondrial bound hexokinase (HK), which represents 75-80% of the total enzyme activity in the cells, was investigated in freshly isolated mitochondria from undifferentiated (Glc+) or differentiated (Glc-) HT29 adenocarcinoma cells. In both models, the bound HK is almost completely released in vitro by 100 microM glucose 6-P (G 6-P). 2. Free ATP (5 mM) or palmitate (800 microM) produce a partial solubilization of bound HK, more markedly in the case of Glc- mitochondria. 3. Glucose or glucose 1-P are found unable to solubilize bound HK. Glucose 1,6-P2, 2-deoxyglucose 6-P or glucosamine 6-P can solubilize the enzyme but are less efficient than G 6-P. 4. Mg2+ and Pi are found to counteract the glucose 6-P induced solubilization of HK in both types of mitochondria. Taking into account the intracellular concentrations of these ions, this could in part explain why, in HT29 cells, HK is predominantly bound to the mitochondria.

Intractable unphysiologically low adenylate energy charge values in synaptosome fractions: an explanatory hypothesis based on the fraction's heterogeneity

Synaptosomes prepared and incubated in a variety of ways from rat cerebra exhibited intractable, unphysiologically low adenylate energy charge values (approximately 0.37-0.60), low total adenine nucleotide contents (approximately 8-10 nmol/mg protein), and much higher adenylate kinase apparent Keq values (approximately 3-8) as compared to intact brain tissue (values of approximately 0.90, 25 nmol/mg, and 0.74, respectively). Synaptosomes prepared from mouse, dog, and chicken cerebra had values essentially identical to those from rat. When incubated under oxygen in a physiological salt solution containing glucose, synaptosomes metabolized more glucose to lactic acid than to CO2, and the addition of 100 microM veratridine caused a two- to threefold stimulation of O2 uptake, lactate accumulation, and CO2 output. It is known that synaptosome fractions contain a substantial number (at least 30-45% by volume) of cytoplasm-containing particles devoid of mitochondria (henceforth termed "cytosolic particles"), and that approximately 80% of brain hexokinase is bound to the outer mitochondrial membrane. For the cytosolic particles, lacking oxidative phosphorylation, to maintain their "in vivo" ATP turnover would require about a 19-fold increase in the glycolytic rate, which is not possible due to limiting amounts of hexokinase, and thus these particles are postulated to be responsible for the high level of aerobic lactate accumulation and the intractable low energy charge values found in synaptosome fractions. The mitochondria-containing particles are postulated to have a normal energy charge, a submaximal glycolytic rate, and minimal lactate production, on the basis of the capacity of veratridine to stimulate synaptosomal O2 uptake and CO2 and lactate output. Calculations based on this "two populations of particles" hypothesis indicate that for synaptosome fractions in general, (1) the cytosolic particles contain approximately 35-64% of the total adenine nucleotides and maintain an energy charge of approximately 0.12; (2) the cytosolic particles and mitochondria-containing particles have adenylate kinase apparent Keq values of approximately 0.21-1.66 and 0.74, respectively, revealing that the higher apparent Keq values of the synaptosome fractions probably are not real departures from equilibrium: and (3) approximately 31-45% of synaptosome fraction protein is contained in debris, which, when taken into account, yields total adenine nucleotide contents in the cytosolic particles and mitochondria-containing particles of approximately 15-24 and approximately 11-19 nmol/mg of particle protein, respectively.

Modified properties of hexokinase from heart mitochondria prepared using proteolytic enzyme

Isolation of muscle mitochondria is made easier by using proteolytic treatment of the tissue before homogenization. Normally, the proteolytic enzyme is discarded with the supernatant of the first centrifugation. However, our results show that a fraction of enzyme activity remains associated with mitochondria. As shown in experiments described in this paper, mitochondrial hexokinase from tissue treated or not with the proteolytic enzyme exhibits similar properties except that the solubilized enzyme from protease treated tissue is no longer able to rebind to mitochondrial membrane. This modification of the binding ability of the enzyme results from a partial hydrolysis of hexokinase during solubilization experiments by the proteolytic enzyme. Since, as pointed out here, proteolytic enzyme can remain associated with mitochondria, [either absorbed on mitochondrial membrane or included in the mitochondrial pellet] its use for the isolation of muscle mitochondria should be avoided.

Determination and characterization of hexokinase in thyroid cancer and benign neoplasms

Hexokinase (ADP: D-hexose-6-phosphotransferase, EC 2.7.1.1) was studied in human thyroid carcinomas (n = 11), follicular adenomas (n = 32), and normal thyroid tissue (n = 21). The specific activity was significantly increased in carcinoma (0.163 +/- 0.083 U/mg protein) compared with normal tissue (0.030 +/- 0.010 U/mg protein) (P less than 0.001). Specific activities of follicular adenomas are rather heterogeneous, but when subdivided into three groups according to histopathologic criteria, a significant difference was found between follicular adenomas group I and II and follicular adenomas group III. A lesser cellular differentiation of adenomas is indicated by the lower degree or even absence of colloid production and follicle formation. A higher proliferation rate may be assumed on the grounds of the irregularities in outline, the often defective pseudocapsule, and signs of compression of the surrounding tissue. The highest specific activity in adenomas was found in the group with the highest proliferative activity, i.e., group III, whereas the lowest specific activities were found in adenomas with the lowest grade of proliferation, i.e., group I; the former was comparable with values found in carcinomas and the latter was comparable with values found in normal thyroid tissue. An interesting difference was found when the compartmentation of hexokinase was compared in carcinomas of different degree of differentiation. In papillary carcinomas a significantly lower proportion of hexokinase (HK) is present in the cytosol in comparison to follicular and undifferentiated carcinomas. In carcinomas more HK II and less HK I was found in comparison with normal thyroid tissue. In contrast hexokinase isozyme composition and compartmentation in adenomas were not different from normal thyroid tissue.

Opposite changes with age in liver and muscle in the mitochondrial and soluble glucose-1,6-bisphosphate and 6-phosphogluconate dehydrogenase

Glucose-1,6-bisphosphate (Glc-1,6-P2), the powerful regulator of carbohydrate metabolism, was markedly decreased in liver of adult rats (2 months of age) as compared to young rats (1-2 weeks of age). This regulator was found to be present in both the mitochondrial and soluble fractions of liver. Its concentration in both these fractions was decreased with age. Concomitant to the decrease in Glc-1,6-P2, which is a potent inhibitor of 6-phosphogluconate dehydrogenase, the activity of this enzyme was markedly increased with age in both the mitochondrial and soluble fractions. However, the increase in this enzyme's activity was more pronounced in the mitochondrial fraction. The mitochondrial enzyme was more susceptible to inhibition by Glc-1,6-P2 as compared to the soluble enzyme, and this may explain the greater enhancement in its activity with age in this fraction. The tibialis anterior muscle exhibited changes with age opposite to those found in liver; Glc-1,6-P2 concentration, in both the mitochondrial and soluble fractions of muscle increased with age, and this increase was accompanied by a concomitant reduction in the activity of the mitochondrial and soluble 6-phosphogluconate dehydrogenase. Similar to liver, the mitochondrial enzyme was more affected by age, as it also exhibited a greater susceptibility to inhibition by Glc-1,6-P2.

Rabbit heart mitochondrial hexokinase: solubilization and general properties

In rabbit heart, results show that two isoenzymes of hexokinase (HK) are present. The enzymatic activity associated with mitochondria consists of only one isoenzyme; according to its electrophoretic mobility and its apparent Km for glucose (0.065 mM), it has been identified as type I isoenzyme. The bound HK I exhibits a lower apparent Km for ATPMg than the solubilized enzyme, whereas the apparent Km for glucose is the same for bound and solubilized HK. Detailed studies have been performed to investigate the interactions which take place between the enzyme and the mitochondrial membrane. Neutral salts efficiently solubilize the bound enzyme. Digitonin induces only a partial release of the enzyme bound to mitochondria; this result could be explained by the existence of contacts between the outer and the inner mitochondrial membranes [C. R. Hackenbrock (1968) Proc. Natl. Acad. Sci. USA 61, 598-605]. Furthermore, low concentrations (0.1 mM) of glucose 6-phosphate (G6P) or ATP4- specifically solubilize hexokinase. The solubilizing effect of G6P and ATP4-, which are potent inhibitors of the enzyme, can be prevented by incubation of mitochondria with Pi or Mg2+. In addition, enzyme solubilization by G6P can be reversed by Mg2+ only when the proteolytic treatment of the heart homogenate is omitted during the course of the isolation of mitochondria. These results concerning the interaction of rabbit heart hexokinase with the outer mitochondrial membrane agree with the schematic model proposed by Wilson [(1982) Biophys. J. 37, 18-19] for the brain enzyme. This model involves the existence of two kinds of interactions between HK and mitochondria; a very specific one with the hexokinase-binding protein of the outer mitochondrial membrane, which is suppressed by glucose 6-phosphate, and a less specific, cation-mediated one.

Lactate dehydrogenase associated with the mitochondrial fraction and with a mitochondrial inhibitor--I. Enzyme binding to the mitochondrial fraction

Rabbit skeletal muscle mitochondrial fraction shows LDH activity (212 +/- 43 U/g pellet). The majority of the mitochondrial enzyme was solubilized by washing with 0.15 M NaCl, pH 6, or by ultrasonic treatment in the same medium. It was also solubilized on increasing the ionic strength and the pH of the medium. Cytosoluble LDH was observed to bind in vitro to the particulate fraction and the enzyme bound was a sigmoidal function of the amount of soluble enzyme added. The bound enzyme is less active than the soluble one. Kinetically, active mitochondrial fraction or in vitro bound enzyme showed non-hyperbolic behavior which is different from the bi-bi sequential-ordered type mechanism of the soluble enzyme.

Lactate dehydrogenase associated with the mitochondrial fraction and with a mitochondrial inhibitor--II. Enzyme interaction with a mitochondrial inhibitor

A LDH inhibitor has been isolated from the LDH-free crude mitochondrial fraction of rabbit skeletal muscle. The inhibitor is only released after solubilization of the particle bound enzyme. It only interacts with soluble LDH, since the enzyme bound to the mitochondrial fraction was not inhibited. The inhibitor molecular weight is above 10,000 dalton, it is precipitated by 7.5% trichloroacetic acid or 80% (NH4)2SO4 saturation. It is highly stable to heat and pH variations. The inhibitor only interacts with the enzyme at ionic strengths below 20 mM and at pH 6.0 or less. The kinetic behavior of the inhibited enzyme is non-hyperbolic and is similar to the mitochondrial fraction bound enzyme.

Inhibition of mitochondrial and soluble hexokinase from various rat tissues by glucose 1,6-bisphosphate

Mitochondrial and soluble Type I and Type II hexokinase from various rat tissues differed in their susceptibility to inhibition by glucose-1,6-bisphosphate (Glc-1,6-P2). In tissues where Type I is the predominant form, the mitochondrial enzyme was less susceptible to inhibition by Glc-1,6-P2 than the soluble enzyme, especially at high Mg2+ concentration. In tissues where Type II is the predominant form, the mitochondrial enzyme was more susceptible to inhibition by Glc-1,6-P2 than the soluble enzyme, especially at low Mg2+ concentration. The results suggest that changes in the intracellular concentrations of Glc-1,6-P2 and Mg2+ under various conditions would affect the activity of the bound and soluble hexokinase from different tissues in a different manner.