Binding and function of mitochondrial glycerol kinase in comparison with those of mitochondrial hexokinase

Expression and characterisation of human glycerol kinase: the role of solubilising agents and molecular chaperones

BACKGROUND

Glycerol kinase (GK; EC 2.7.1.30) facilitates the entry of glycerol into pathways of glucose and triglyceride metabolism and may play a potential role in Type 2 diabetes mellitus (T2DM). However, the detailed regulatory mechanisms and structure of the human GK are unknown.

METHODS

The human GK gene was cloned into the pET-24a(+) vector and over-expressed in Escherichia coli BL21 (DE3). Since the protein was expressed as inclusion bodies (IBs), various culture parameters and solubilising agents were used but they did not produce bioactive His-GK; however, co-expression of His-GK with molecular chaperones, specifically pKJE7, achieved expression of bioactive His-GK. The overexpressed bioactive His-GK was purified using coloumn chromatography and characterised using enzyme kinetics.

RESULTS

The overexpressed bioactive His-GK was purified apparently to homogeneity (∼295-fold) and characterised. The native His-GK was a dimer with a monomeric molecular weight of ∼55 kDa. Optimal enzyme activity was observed in TEA buffer (50 mM) at 7.5 pH. K+ (40 mM) and Mg2+ (2.0 mM) emerged as prefered metal ions for His-GK activity with specific activity 0.780 U/mg protein. The purified His-GK obeyed standard Michaelis-Menten kinetics with Km value of 5.022 µM (R2=0.927) for its substrate glycerol; whereas, that for ATP and PEP was 0.767 mM (R2=0.928) and 0.223 mM (R2=0.967), respectively. Other optimal parameters for the substrate and co-factors were also determined.

CONCLUSION

The present study demonstrates that co-expression of molecular chaperones assists with the expression of bioactive human GK for its characterisation.

A Novel Clinical Six-Flavoprotein-Gene Signature Predicts Prognosis in Esophageal Squamous Cell Carcinoma

Flavoproteins and their interacting proteins play important roles in mitochondrial electron transport, fatty acid degradation, and redox regulation. However, their clinical significance and function in esophageal squamous cell carcinoma (ESCC) are little known. Here, using survival analysis and machine learning, we mined 179 patient expression profiles with ESCC in GSE53625 from the Gene Expression Omnibus (GEO) database and constructed a signature consisting of two flavoprotein genes (GPD2 and PYROXD2) and four flavoprotein interacting protein genes (CTTN, GGH, SRC, and SYNJ2BP). Kaplan-Meier analysis revealed the signature was significantly associated with the survival of ESCC patients (mean survival time: 26.77 months in the high-risk group vs. 54.97 months in the low-risk group, P < 0.001, n = 179), and time-dependent ROC analysis demonstrated that the six-gene signature had good predictive ability for six-year survival for ESCC (AUC = 0.86, 95% CI: 0.81-0.90). We then validated its prediction performance in an independent set by RT-PCR (mean survival: 15.73 months in the high-risk group vs. 21.1 months in the low-risk group, P=0.032, n = 121). Furthermore, RNAi-mediated knockdown of genes in the flavoprotein signature led to decreased proliferation and migration of ESCC cells. Taken together, CTTN, GGH, GPD2, PYROXD2, SRC, and SYNJ2BP have an important clinical significance for prognosis of ESCC patients, suggesting they are efficient prognostic markers and potential targets for ESCC therapy.

Activity of nucleoside diphosphate kinase α (NDPK α) capable of binding to outer mitochondrial membrane accounts for less than 10% of total NDPK activity present in cytoplasm of liver cells

During incubation of a constant volume of rat liver cytosol with an increasing quantity of mitochondrial protein in the presence of 3.3 mM MgCl(2), the binding of nucleoside diphosphate kinase (NDPK) from the cytosol to mitochondrial membranes is described by a saturation curve. The highest bound NDPK activity accounts for less than 9% of the added activity. Analysis of the results suggests that only one NDPK isozyme is bound to the membranes. Western blotting showed it to be NDPK α, a homolog of human NDPK-B. Substrates of NDPK, hexokinase, and glycerol kinase, as well as N,N'-dicyclohexylcarbodiimide and palmitate, did not influence the association of NDPK with mitochondrial membranes. We conclude that the sites of NDPK binding to the outer mitochondrial membrane are not identical to those of hexokinase and glycerol kinase.

Reversibility of nucleoside diphosphate kinase solubilization from the surface of the outer mitochondrial membrane

It was found that in medium with low ionic strength nucleoside diphosphate kinase (NDPK) solubilization from the outer membrane of liver mitochondria could be partially reversed by the addition of 3.3 mM MgCl2. Complete rebinding of the enzyme after the addition of MgCl2 was observed when the mitochondrial washing and storage medium contained leupeptin, an inhibitor of cathepsins. It was demonstrated that leupeptin and another inhibitor of cysteine proteinases, E-64, do not influence the rate of NDPK solubilization as well as its solubilized and membrane-associated activity. We conclude that NDPK becomes sensitive to proteolysis only after its solubilization; proteolysis does not affect the part of the enzyme molecule that is responsible for catalysis. After solubilization of NDPK in the absence of leupeptin, cathepsins damage sites of its binding on the membranes. The rate of the enzyme solubilization is dependent on the pH of the storage medium (pH 6.0-8.0); it decreases with increase in pH. It was shown that in the medium with high ionic strength, MgCl2 does not reverse pH-dependent NDPK solubilization, but solubilization could be reversed by increase in medium pH in the presence of E-64 and BSA. The physiological importance of these results is discussed.

Conserved family of glycerol kinase loci in Drosophila melanogaster

Glycerol kinase (GK) is an enzyme that catalyzes the formation of glycerol 3-phosphate from ATP and glycerol, the rate-limiting step in glycerol utilization. We analyzed the genome of the model organism Drosophila melanogaster and identified five GK orthologs, including two loci with sequence homology to the mammalian Xp21 GK protein. Using a combination of sequence analysis and evolutionary comparisons of orthologs between species, we characterized functional domains in the protein required for GK activity. Our findings include additional conserved domains that suggest novel nuclear and mitochondrial functions for glycerol kinase in apoptosis and transcriptional regulation. Investigation of GK function in Drosophila will inform us about the role of this enzyme in development and will provide us with a tool to examine genetic modifiers of human metabolic disorders.

Functional coupling between nucleoside diphosphate kinase of the outer mitochondrial compartment and oxidative phosphorylation

In rat liver mitochondria all nucleoside diphosphate kinase of the outer compartment is associated with the outer surface of the outer membrane (Lipskaya, T. Yu., and Plakida, K. N. (2003) Biochemistry (Moscow), 68, 1136-1144). In the present study, three systems operating as ADP donors for oxidative phosphorylation have been investigated. The outer membrane bound nucleoside diphosphate kinase was the first system tested. Two others employed yeast hexokinase and yeast nucleoside diphosphate kinase. The two enzymes exhibited the same activity but could not bind to mitochondrial membranes. In all three systems, muscle creatine phosphokinase was the external agent competing with the oxidative phosphorylation system for ADP. Determination of mitochondrial respiration rate in the presence of increasing quantities of creatine phosphokinase revealed that at large excess of creatine phosphokinase activity over other kinase activities (of the three systems tested) and oxidative phosphorylation the creatine phosphokinase reaction reached a quasi-equilibrium state. Under these conditions equilibrium concentrations of all creatine phosphokinase substrates were determined and K(eq)app of this reaction was calculated for the system with yeast hexokinase. In samples containing active mitochondrial nucleoside diphosphate kinase the concentrations of ATP, creatine, and phosphocreatine were determined and the quasi-equilibrium concentration of ADP was calculated using the K(eq)app value. At balance of quasi-equilibrium concentrations of ADP and ATP/ADP ratio the mitochondrial respiration rate in the system containing nucleoside diphosphate kinase was 21% of the respiration rate assayed in the absence of creatine phosphokinase; in the system containing yeast hexokinase this parameter was only 7% of the respiration rate assayed in the absence of creatine phosphokinase. Substitution of mitochondrial nucleoside diphosphate kinase with yeast nucleoside diphosphate kinase abolished this difference. It is concluded that oxidative phosphorylation is accompanied by appearance of functional coupling between mitochondrial nucleoside diphosphate kinase and the oxidative phosphorylation system. Possible mechanisms of this coupling are discussed.

Neuronal uptake and metabolism of glycerol and the neuronal expression of mitochondrial glycerol-3-phosphate dehydrogenase

Glycerol is effective in the treatment of brain oedema but it is unclear if this is due solely to osmotic effects of glycerol or whether the brain may metabolize glycerol. We found that intracerebral injection of [14C]glycerol in rat gave a higher specific activity of glutamate than of glutamine, indicating neuronal metabolism of glycerol. Interestingly, the specific activity of GABA became higher than that of glutamate. NMR spectroscopy of brains of mice given 150 micromol [U-13C]glycerol (0.5 m i.v.) confirmed this predominant labelling of GABA, indicating avid glycerol metabolism in GABAergic neurones. Uptake of [14C]glycerol into cultured cerebellar granule cells was inhibited by Hg2+, suggesting uptake through aquaporins, whereas Hg2+ stimulated glycerol uptake into cultured astrocytes. The neuronal metabolism of glycerol, which was confirmed in experiments with purified synaptosomes and cultured cerebellar granule cells, suggested neuronal expression of glycerol kinase and some isoform of glycerol-3-phosphate dehydrogenase. Histochemically, we demonstrated mitochondrial glycerol-3-phosphate dehydrogenase in neurones, whereas cytosolic glycerol-3-phosphate dehydrogenase was three to four times more active in white matter than in grey matter, reflecting its selective expression in oligodendroglia. The localization of mitochondrial and cytosolic glycerol-3-phosphate dehydrogenases in different cell types implies that the glycerol-3-phosphate shuttle is of little importance in the brain.

Characterization of glycerol uptake and glycerol kinase activity in rat hepatocytes cultured under different hormonal conditions

Glycerol uptake and glycerol kinase activity were studied in primary cultures of rat hepatocytes in the presence of either 1 nM insulin, 1 nM glucagon, or 100 nM dexamethasone, alone or in combination in the culture medium. Glycerol uptake exhibited saturation kinetic with K(m) values (microM) and Vmax (nmol/min x mg protein) ranging from 250-402, and 7.9-10.1, respectively. The corresponding K(m) and Vmax values for glycerol kinase activity were 36-46 and 8.7-12.7. Using the metabolic uncoupler 2,4-dinitrophenol, glycerol uptake and the cellular content of glycerol phosphorylated metabolites were reduced 33% and 43%, respectively, whereas no decrease in the cellular content of glycerol was seen. The glycerol analogues monoacetin, monobutyrin and dihydroxypropyl dichloroacetate were able in a concentration-dependent manner to inhibit glycerol uptake into hepatocytes with the two latter having IC50 values of approximately 1 mM. Moreover, it was demonstrated that the three glycerol analogues were substrates for glycerol kinase, which indicates a competitive mode of inhibition. The kinetic parameters for these substrates were calculated by using glycerol kinase from Candida Mycoderma. Monobutyrin was found to be 4 times lees efficient as substrate compared to the other substrates. Overall, these results indicate that independently of the culture conditions, glycerol uptake is the rate-limiting step in glycerol metabolism, and that the investigated glycerol analogues are metabolized via the same route as glycerol.

Biochemistry and toxicology of the diterpenoid glycoside atractyloside

Atractyloside (Atr) is a diterpenoid glycoside that occurs naturally in plants (many of which are used in ethnomedicines) found in Europe, Africa, South America, Asia and the far East. It is also present in animal grazing forage. Atr (and its analogues) may be present at levels as high as 600 mg/kg dried plant material. Consumption of the plants containing Atr or carboxyatractyloside (carboxyAtr) has caused fatal renal proximal tubule necrosis and/or centrilobular hepatic necrosis in man and farm animals. Although pure Atr and crude plant extracts disrupt carbohydrate homeostasis and induce similar pathophysiological lesions in the kidney and liver, it is also possible that the toxicity of Atr may be confounded by the presence of other natural constituents in plants. Atr competitively inhibits the adenine nucleoside carrier in isolated mitochondria and thus blocks oxidative phosphorylation. This has been assumed to explain changes in carbohydrate metabolism and the toxic effects in liver and kidney. Although the acute toxicity of Atr is well described, many aspects of Atr toxicity (subchronic and chronic toxicity, reproductive toxicity, mutagenicity and carcinogenicity) have not been investigated and pharmacokinetic and metabolism data are limited. In vitro proximal tubular cells are selectively sensitive to Atr, whereas other renal cell types are quite resistant. There are also differences in the response of liver and renal tissue to Atr. Thus, not all of the clinical, biochemical and morphological changes caused by Atr can simply be explained on the basis of inhibition of mitochondrial phosphorylation. The relevance to a wider human risk is shown by the presence of Atr analogues in dried roasted Coffea arabica beans (17.5 32 mg/kg). There are no data to help identify the risk of low dose chronic exposure in human coffee consumers, nor is there information on the levels of Atr or its analogues in other commonly consumed human foodstuffs.

X-linked glycerol kinase deficiency in the mouse leads to growth retardation, altered fat metabolism, autonomous glucocorticoid secretion and neonatal death

Glycerol kinase is an X chromosome-encoded enzyme involved in the metabolism of endogenous and dietary glycerolipids. The physiological significance of its activity in mammals is not well understood. Glycerol kinase deficiency in humans occurs as an isolated enzyme deficiency or as part of a contiguous gene deletion syndrome in variable association with Duchenne muscular dystrophy and adrenal hypoplasia congenita. Isolated glycerol kinase deficiency has an inconstant phenotype, ranging from asymptomatic hyperglycerolemia to a severe metabolic disorder with growth and psychomotor retardation. Although intragenic mutations were reported recently, the pathophysiological basis for the phenotypic variability remains unknown. To understand better the physiological significance of glycerol kinase and the pathophysiology of its deficiency, we generated glycerol kinase-deficient mice by gene targeting. Mutant male mice appear normal at birth, but exhibit postnatal growth retardation, altered fat metabolism with profound hyperglycerolemia and elevated free fatty acids, autonomous glucocorticoid synthesis and death by 3-4 days of age. Heterozygous females are healthy and biochemically normal. The biochemical features observed in glycerol kinase-deficient mice provide the basis for further investigations into the pathogenesis of the human disorder.

Menadione-dependent alpha glycerophosphate and succinate dehydrogenases in the developing canine retina

Reducing equivalents for the electron transport chain are generated within the mitochondria by the Krebs cycle and in cytoplasm by processes like lipid metabolism. Two mitochondrial enzymes, succinate dehydrogenase (SDH), a prominent enzyme in the Krebs cycle, and alpha-glycerophosphate dehydrogenase (alpha-GPDH), half of the glycerophosphate shuttle system for bringing reducing equivalents from cytoplasm to mitochondria, were examined enzyme histochemically to assess the contribution of each to metabolism of the developing canine retina. SDH activity, a common marker for oxidative metabolism, was insignificant at birth. By 4 days of age, activity was observed only in developing photoreceptor inner segments. By 21 days of age SDH activity was present throughout the retina, especially in photoreceptor inner segments and plexiform layers, and approached the level observed in the adult dog. Menadione-linked alpha-GPDH (M-alpha-GPDH) activity, however, was prominent in developing vasculature and outermost portion of the neuroblastic layer of the 1 day-old retina. Most notable was localization in vascular precursors, angioblasts, found distant from formed vessels in the peripheral nerve fiber layer. Retinal dependence on an oxidative metabolism in neuronal elements, as represented by SDH activity, occurs only when the vasculature is well established.

The importance of the outer mitochondrial compartment in regulation of energy metabolism

Substitution of physiologically present macromolecules during isolation of mitochondria and investigation of their functions led to a significant change in regulation of oxidative phosphorylation. The differences compared to conventionally isolated mitochondria were that stimulation of oxidative phosphorylation appeared to rather depend on the activity of peripheral kinases than on the addition of free ADP. The localisation of peripheral kinases such as hexokinase and mitochondrial creatine kinase are described as well as the effects of macromolecules on the regulation of bound hexokinase and of oxidative phosphorylation via this enzyme.

Glycerol kinase of Escherichia coli is activated by interaction with the glycerol facilitator

Glycerol transport is commonly cited as the only example of facilitated diffusion across the Escherichia coli cytoplasmic membrane. Two proteins, the glycerol facilitator and glycerol kinase, are involved in the entry of external glycerol into cellular metabolism. The glycerol facilitator is thought to act as a carrier or to form a selective pore in the cytoplasmic membrane, whereas the kinase traps the glycerol inside the cell as sn-glycerol-3-phosphate. We found that the kinetics of glycerol uptake in a facilitator-minus strain are significantly different from the kinetics of glycerol uptake in the wild type. Free glycerol was not observed inside wild-type cells transporting glycerol, and diffusion of glycerol across the cytoplasmic membrane was not the rate-limiting step for phosphorylation in facilitator-minus mutants. Therefore, the kinetics of glycerol phosphorylation are different, depending on the presence or absence of the facilitator protein. We conclude that there is an interaction between the glycerol facilitator protein and glycerol kinase that stimulates kinase activity, analogous to the hexokinase- and glycerol kinase-porin interactions in mitochondria.

Location of dinucleoside triphosphatase in the matrix space of rat liver mitochondria

The submitochondrial location of dinucleoside triphosphatase (EC 3.6.1.29), previously shown to be in part associated with mitochondria, has been studied in rat liver. The precipitability and latency of activity in organelle suspensions, and the profile of solubilization by digitonin, were like those of the matrix space marker glutamate dehydrogenase, and differed from those of other submitochondrial fractions. This, and the synthesis of diadenosine polyphosphates by mitochondrial aminoacyl-tRNA synthetases, suggest the occurrence of a pathway for the intramitochondrial turnover of diadenosine 5',5'''-P1,P3-triphosphate (Ap3A).

Porin interaction with hexokinase and glycerol kinase: metabolic microcompartmentation at the outer mitochondrial membrane

Porin is the pore-forming protein involved in the movement of adenine nucleotides across the outer mitochondrial membrane (OMM). Hexokinase and glycerol kinase interact with porin on the outer surface of the OMM in a manner which provides these enzymes with preferred access to the ATP generated in the mitochondrion. We review recent evidence which permits refinement of our knowledge of these proteins and their interactions at the OMM. The involvement of this system in metabolic microcompartmentation is discussed, as well as possible pathological consequences of its disruption in malignancy and genetic deficiencies of hexokinase, glycerol kinase, and porin.

Hexokinase of rat brain mitochondria: relative importance of adenylate kinase and oxidative phosphorylation as sources of substrate ATP, and interaction with intramitochondrial compartments of ATP and ADP

Interactions between intramitochondrial ATP-generating, ADP-requiring processes and ATP-requiring, ADP-generating phosphorylation of glucose by mitochondrially bound hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) have been investigated using well-coupled mitochondria isolated from rat brain. ADP generated by mitochondrially bound hexokinase was more effective at stimulating respiration than was ADP generated by hexokinase dissociated from the mitochondria, and pyruvate kinase was less effective as a scavenger of ADP generated by the mitochondrially bound hexokinase than was the case with ADP generated by the dissociated enzyme. These results indicate that ADP generated by the mitochondrially bound enzyme is at least partially sequestered and directed toward the mitochondrial oxidative phosphorylation apparatus. Under the conditions of these experiments, the maximum rate of ATP production by oxidative phosphorylation was approximately 10-fold greater than the maximum rate of ATP generation by the adenylate kinase reaction. Moreover, during periods of active oxidative phosphorylation, adenylate kinase made no detectable contribution to ATP production. Thus, adenylate kinase does not represent a major source of ATP for hexokinase bound to actively phosphorylating brain mitochondria. With adenylate kinase as the sole source of ATP, a steady state was attained in which ATP formation was balanced by utilization in the hexokinase reaction. In contrast, when oxidative phosphorylation was the source of ATP, a steady state rate of Glc phosphorylation was attained, but it was equivalent to only about 40-50% of the rate of ATP production and thus there was a continued net increase in ATP concentration in the system. Rates of Glc phosphorylation with ATP generated by oxidative phosphorylation exceeded those seen with equivalent levels of exogenously added ATP. Moreover, at total ATP concentrations greater than approximately 0.2 mM, hexokinase bound to actively phosphorylating mitochondria was unresponsive to continued slow increases in ATP levels; acute increase in ATP (by addition of exogenous nucleotide) did, however, result in increased hexokinase activity. The relative insensitivity of mitochondrially bound hexokinase to extramitochondrial ATP suggested dependence on an intramitochondrial pool (or pools) of ATP during active oxidative phosphorylation. Two intramitochondrial compartments of ATP were identified based on their selective release by inhibitors of electron transport or oxidative phosphorylation. These compartments were distinguished by their sensitivity to inhibitors and the kinetics with which they were filled with ATP generated by oxidative phosphorylation. Exogenous glycerol kinase competed effectively with mitochondrially bound hexokinase for extramitochondrial ATP, with relatively low levels of glycerol kinase completely inhibiting phosphorylation of Glc.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of mitochondrial porin with cytosolic proteins

Intracellular phosphorylation is an important step in active uptake and utilization of carbohydrates. For example glucose and glycerol enter the liver cell along the extra intracellular gradient by facilitated diffusion through specific carriers and are concentrated inside the cell by phosphorylation via hexokinase or glycerol kinase. Depending on the function of the respective tissue the uptake of carbohydrates serves different metabolic purposes. In brain and kidney medulla cells which depend on carbohydrates, glucose and glycerol are taken up according to the energy demand. However, in tissues such as muscle which synthesize glycogen or like liver which additionally produce fat from glucose, the uptake of carbohydrates has to be regulated according to the availability of glucose and glycerol. How the reversible coupling of the kinases to the outer membrane pore and the mitochondrial ATP serves to fulfil these specific requirements will be explained as well as how this regulates the carbohydrate uptake in brain according to the activity of the oxidative phosphorylation and how this allows glucose uptake in liver and muscle to persist in the presence of high glucose 6-phosphate without activating the rate of glycolysis.

Demonstration and characterization of human cardiac porin: a voltage-dependent channel involved in adenine nucleotide movement across the outer mitochondrial membrane

The porins are a class of voltage-dependent, anion-selective, channel-forming proteins located in the outer mitochondrial membrane (OMM). The porins are responsible for passage of adenine nucleotides across the OMM, as well as for specific binding of hexokinase and glycerol kinase. This porin-kinase complex has direct access to ATP generated by mitochondrial oxidative phosphorylation and may be important in the regulation of glycolysis. Porin had not been described previously in humans but, due to its importance in bioenergetics, would be expected to be present, especially in organs requiring a large and constant supply of energy. We therefore postulated that porin would occur in human myocardium where it would be important in cardiac function. Polyclonal antibodies to bovine myocardial and rat liver porins were utilized in transblotting experiments after polyacrylamide gel electrophoresis of human heart preparations from atria, ventricles, papillary muscles, and interventricular septum. These immunoblots demonstrated selective staining of a 34-kDa band. This was identical to the results obtained with purified porin and the antibodies. Also notable was the finding that the vast majority of this staining was found in the homogenate pellet after high speed centrifugation (20,000g), as would be expected for a mitochondrial protein. The demonstration of human cardiac porin by immunoblotting with rat liver and bovine myocardial porin antibodies is the first demonstration of cross-species identification of the porins. The success of this approach undoubtedly occurred because of strong homology between porins from a variety of species.

Stimulation by D-glucose of mitochondrial respiration

D-glucose increases O2 uptake by cerebellum mitochondria. This effect is abolished by D-glucose-6-phosphate and D-mannoheptulose. It is proposed that the phosphorylation of D-glucose as catalyzed by bound hexokinase directly affects mitochondrial respiration.

Glycerol oxidation in discrete areas of rat brain from young, adolescent, and adult rats

We have reported previously that the oxidation rate of [1,3-14C] glycerol to 14CO2 is lower in whole brain homogenates from neonatal rats and increases about 30% during the suckling period to adult levels. To determine whether there are developmental changes in glycerol oxidation in discrete regions of brain, we examined the oxidation of glycerol by homogenates of hypothalamus, cerebellum, brain stem, hippocampus, and cerebral cortex of young (4-6 days) and older (18-20 days) pups and adult (greater than 90 day) rats. The oxidation was measured at both low (0.2 mM) and high (2.0 mM) concentrations of glycerol, since the oxidation of glycerol by brain tissue has been shown to exhibit biphasic kinetics. At each age, and with both concentrations of glycerol, there were significant differences among the discrete brain regions. Although the rate of glycerol oxidation increased with age in most areas of brain, each brain region had a distinct developmental profile. In the hypothalamus, the oxidation of glycerol increased significantly between 4-6 days and adult levels at 18-20 days. The oxidation of glycerol was essentially the same in homogenates of cortex from young and older pups, but it was significantly increased in adults. In contrast with other brain regions, the oxidation of glycerol by brain stem was highest at 4-6 days and significantly decreased with age. The developmental profile of glycerol oxidation in hippocampus and cerebellum was particularly complex, since it increased with age at low, but not at high, concentrations of glycerol. This developmental pattern in hippocampus and cerebellum could be related to changes in the biphasic kinetics of glycerol oxidation and suggests that glycerol metabolism is different in these two areas of brain compared with other areas of brain.

Glycerol phosphorylation and oxidation in pancreatic islets

Homogenates of rat pancreatic islets and tumoral islet cells (RINm5F line) were found to display glycerokinase activity. In the islets like in the liver, about one-sixth of the enzyme appears bound to mitochondria. The enzymatic activities in liver and islets differ from one another, however, by their response to increasing concentrations of either glycerol or ATP and sensitivity to inhibition by D-glyceraldehyde. In intact islets, [U-14C]glycerol is efficiently oxidized, albeit at a much lower rate than that found for its phosphorylation by islet homogenates. These findings are relevant to the role played by glycerol liberated from endogenous triglycerides in the basal respiration of islet cells.

Hexose metabolism in pancreatic islets: compartmentation of hexokinase in islet cells

Hexokinase activity was found in both soluble (cytosolic) and particulate subcellular fractions prepared from rat pancreatic islet homogenates. The bound enzyme was associated with mitochondria rather than secretory granules. Relative to the total hexokinase activity, the amount of bound enzyme was higher in islet homogenates prepared at pH 6.0 (72 +/- 7%) than in islets homogenized at pH 7.4 (38 +/- 1%). The affinity of hexokinase for equilibrated D-glucose was not different in the cytosolic and mitochondrial fractions. In both fractions, hexokinase displayed a greater affinity for alpha- than beta-D-glucose, but a higher maximal velocity with the beta- than alpha-anomer. Glucose 6-phosphate inhibited to a greater extent cytosolic than mitochondrial hexokinase. A high Km glucokinase-like enzymic activity was also present in both subcellular fractions. It is proposed that the ambiguity of hexokinase plays a propitious role in the glucose-sensing function of pancreatic islet cells.

Pore formation by the mitochondrial porin of rat brain in lipid bilayer membranes

The porin of the outer membrane of rat-brain mitochondria was isolated and purified. The protein showed a single band of apparent Mr 35,500 on dodecyl sulfate-containing polyacrylamide gels. The incorporation of rat-brain porin into artificial lipid bilayer membranes showed that it is able to form pores with an average single-channel conductance of 400 pS in 0.1 M KCI. The pores were found to be voltage-dependent and switched to substrates at higher transmembrane potentials. The voltage-dependence of the rat brain pore was considerably smaller than that of the other known eukaryotic porins. The possible role of the rat-brain porin in the regulation of transport process across the outer mitochondrial membrane is discussed.

The role of the mitochondrial outer membrane in energy metabolism of tumor cells

The outer mitochondrial membrane contains a pore structure which is composed of a 30,000 Da protein, porin. The pore has an internal diameter of 2 nm and exhibits a molecular-sieving exclusion limit between 3000 and 6000 Da. These pores, therefore, provide the exit/entrance port for metabolites moving between mitochondria and the cytosol. Hexokinase binds to porin on the outer surface of mitochondria. The location of hexokinase has evoked a number of theories in which bound hexokinase is given a central role in regulating glycolysis, and, perhaps, the metabolic communication between oxidative and glycolytic metabolism. This is of particular importance in rapidly growing tumor cells in which the aerobic production of lactate and hexokinase activity are highly induced. In the present paper, we summarize the suggested roles of the outer membrane and bound hexokinase in regulation glycolysis of tumor cells. Experiments attempting to elucidate the role of hexokinase binding in the regulation of tumor cell metabolism are presented.