Acidic phospholipids may inhibit rat brain hexokinase by interaction at the nucleotide binding site

Rat brain hexokinase (ATP:D-hexose 6-phosphotransferase; EC 2.7.1.1) is inhibited by acidic phospholipids such as phosphatidylinositol, phosphatidylserine, and cardiolipin. Several aspects of this inhibition are atypical when compared to inhibition by established reversible inhibitors of this enzyme such as the product, Glc-6-P. Maximal inhibition is attained rather slowly (approximately 30 min at 22 degrees C), and is not reversed by simple dilution of the enzyme-lipid mixture. Ligands such as ATP or Glc-6-P can protect the enzyme against inhibition by acidic phospholipids; addition of protective ligands after mixing of enzyme and lipids does not, however, reverse inhibition that occurred prior to ligand addition. Inhibition can be prevented but not reversed by elevated (0.1-0.2 M) [NaCl], indicating a probable role for electrostatic forces in the interaction of lipid with enzyme. Greater inhibition is seen at 22 degrees C than at 3-4 degrees C, suggesting that hydrophobic interactions may also be involved. It is suggested that acidic phospholipids inhibit brain hexokinase by binding at the nucleotide-binding site of the enzyme. The effectiveness of ATP (or the ATP analog, Cibacron Blue) in protecting against inhibition by acidic phospholipids is attributed to direct competition between ATP and the phospholipid for a common binding site. The effectiveness of Glc-6-P (or analogs) in preventing the inhibition is attributed to a conformational change, induced by the binding of this ligand, which prevents binding of ATP or acidic phospholipids to the enzyme. The pH dependency of the inhibition has suggested involvement of the protonated form of a dissociable group (pK approximately 7) on the enzyme in the interaction with acidic phospholipids; this may be the histidyl residue implicated by Solheim and Fromm [Biochemistry 19, 6074-6080 (1984)] in the binding of ATP to brain hexokinase. Structural similarities in the nucleotide-binding sites of several nucleotide-binding enzymes suggest that similar inhibition by acidic phospholipids may be seen with other enzymes of this type; there are already some reports to this effect.

Interaction with protein SH groups could be involved in adriamycin cardiotoxicity

The use of the antineoplastic agent adriamycin is limited by its cardiotoxicity. The mechanism of cardiotoxicity has been investigated through the study of adriamycin effects on a number of heart enzymes. Adriamycin inhibited the activity of NADP-isocitrate dehydrogenase, malic enzyme, and 6-phosphogluconate dehydrogenase, three enzymes that have in common the presence of reactive SH groups involved in activity. Adriamycin action was prevented by the presence of proteins or dithioerythrol and mimicked by dithiobis dinitrobenzoate. It is suggested that adriamycin effects are due to interaction with enzyme SH groups by a product of adriamycin metabolism.

Neurotoxic effects of copper: inhibition of glycolysis and glycolytic enzymes

The effects of Cu2+ on glycolysis and several glycolytic enzymes were studied in rat brain extracts in vitro. At concentrations reportedly found in Wilson's disease, Cu2+ significantly inhibited lactate production from glucose or glucose-6-phosphate in rat brain postnuclear supernatant with an IC50 of about 3 microM. Cu2+ also inhibited several glycolytic enzymes. Amongst the latter, Cu2+ was most effective in inhibiting hexokinase (IC50 for Cu2+ = 7 microM), moderately effective in inhibiting pyruvate kinase (IC50 for Cu2+ = 56 microM), but least effective in inhibiting lactate dehydrogenase (IC50 for Cu2+ = 300 microM). These results suggest that inhibition of brain glycolysis may have pathophysiological importance in copper poisoning and in Wilson's disease.

Suppression of kinetic cooperativity of hexokinase D (glucokinase) by competitive inhibitors. A slow transition model

Hexokinase D ('glucokinase') displays positive cooperativity with mannose with the same h values (1.5-1.6) as with glucose but with higher K0.5 values (8 mM at pH 8.0 and 12 mM at pH 7.5). In contrast, fructose and 2-deoxyglucose exhibit Michaelian kinetics [Cárdenas, M. L., Rabajille, E., and Niemeyer, H. (1979) Arch. Biol. Med. Exp. 12, 571-580; Cárdenas, M. L., Rabajille, E., and Niemeyer, H. (1984) Biochem. J. 222, 363-370]. Mannose, fructose, 2-deoxyglucose and N-acetylglucosamine acted as competitive inhibitors of glucose phosphorylation and decreased the cooperativity with glucose. Their relative efficiency for reducing the value of h to 1.0 was: fructose greater than mannose greater than 2-deoxyglucose greater than N-acetylglucosamine. Galactose, which is not a substrate nor an inhibitor, was unable to change the cooperativity. The competitive inhibition of glucose phosphorylation by N-acetylglucosamine or mannose was cooperative at very low glucose concentrations (less than 0.5 K0.5), suggesting the interaction of the inhibitors with more than one enzyme form. These and previously reported results are discussed on the basis of a slow transition model, which assumes that hexokinase D exists mainly in one conformation state (E1) in the absence of ligands and that the binding of glucose (or mannose) induces a conformational transition to EII. This new conformation would have a higher affinity for the sugar substrates and a higher catalytic activity than EI. Cooperativity would emerge from shifts of the steady-state distribution between the two enzyme forms as the sugar concentration increase. The inhibitors would suppress cooperativity with glucose by inducing or trapping the EII conformation. In addition, the model postulates that the different kinetic behaviour of hexokinase D with the different sugar substrates, cooperative with glucose and mannose and Michaelian with 2-deoxyglucose and fructose, is the consequence of differences in the velocities of the conformational transitions induced by the sugar substrates.

Proteolytic dissection of rat brain hexokinase: determination of the cleavage pattern during limited digestion with trypsin

Limited treatment of rat brain hexokinase (ATP: D-hexose-6-phosphotransferase; EC 2.7.1.1) with trypsin causes cleavage of the Mr 98K enzyme into three major fragments having molecular weights of 10K, 40K, and 50K, with intermediates of Mr 60K and 90K being detected. This information, in conjunction with N- and C-terminal analysis of the intact enzyme and tryptic cleavage products, has established the tryptic cleavage pattern as where T1 and T2 indicate tryptic cleavage sites; cleavage at only T1 or T2 gives rise to the 90K or 60K intermediate, respectively. Confirmation of this cleavage pattern has been provided by two-dimensional peptide mapping using Staphylococcus aureus V8 protease, and epitope mapping with two monoclonal antibodies directed against rat brain hexokinase. The epitopes recognized by one of the monoclonal antibodies is located within the 40K C-terminal fragment while the epitope for the other monoclonal antibody lies within the 50K fragment. A two-dimensional peptide mapping-immunoblotting technique has permitted a more defined localization of these epitopes to specific regions within these major tryptic cleavage fragments. Complete tryptic cleavage of the enzyme occurs with only modest (approximately 20%) loss of catalytic activity, and the cleaved enzyme retains many of the properties of intact hexokinase. Specifically, there was no effect of cleavage on the Km for Glc or the Ki for Glc-6-P, though a slight decrease in Km for ATP was consistently noted to result from cleavage. Furthermore, like the intact enzyme, cleaved hexokinase retained the ability to bind to outer mitochondrial membranes in a Glc-6-P-sensitive manner. Under nondenaturing conditions, the cleaved fragments remain associated by noncovalent forces. Thus, the cleaved enzyme sedimented at a rate comparable to intact enzyme during centrifugation on sucrose density gradients, and migrated only slightly faster when electrophoresed on gradient acrylamide gels under nondenaturing conditions.

Hexose metabolism in pancreatic islets. Inhibition of hexokinase

In islet homogenates, hexokinase-like activity (Km 0.05 mM; Vmax. 1.5 pmol/min per islet) accounts for the major fraction of glucose phosphorylation. Yet the rate of glycolysis in intact islets incubated at low glucose concentrations (e.g. 1.7 mM) sufficient to saturate hexokinase only represents a minor fraction of the glycolytic rate observed at higher glucose concentrations. This apparent discrepancy between enzymic and metabolic data may be attributable, in part at least, to inhibition of hexokinase in intact islets. Hexokinase, which is present in both islet and purified B-cell homogenates, is indeed inhibited by glucose 6-phosphate (Ki 0.13 mM) and glucose 1,6-bisphosphate (Ki approx. 0.2 mM), but not by fructose 2,6-bisphosphate. In intact islets, the steady-state content of glucose 6-phosphate (0.26-0.79 pmol/islet) and glucose 1,6-bisphosphate (5-48 fmol/islet) increases, in a biphasic manner, at increasing concentrations of extracellular glucose (up to 27.8 mM). From these measurements and the intracellular space of the islets, it was estimated that the rate of glucose phosphorylation as catalysed by hexokinase represents, in intact islets, no more than 12-24% of its value in islet homogenates.

Hexokinase, phosphofructokinase and pyruvate kinase isoenzymes in lymphocyte subpopulations

In order to study the three regulator enzymes of glycolysis, hexokinase (HK), phosphofructokinase (PFK) and pyruvate kinase (PK), in relation to lymphocyte maturation, lymphocytes of different origin were investigated. Lymphocytes from bone marrow, thymus, cord blood, adult peripheral blood and mitogen-stimulated lymphocytes were investigated. The enzyme activities were determined and the isozyme patterns were studied by means of electrophoresis, kinetic measurements and immunoprecipitation. The young lymphocytes from bone marrow and the mitogen-stimulated lymphocytes could be distinguished from the other lymphocytes by a higher residual HK activity in the presence of the inhibitor glucose-1,6-diphosphate. Peripheral blood T lymphocytes differed from non-T lymphocytes in the PK isozymes distribution. All the cells contained PK type K4 and the hybrid K3M. In T cells a smaller amount of the K isozyme was seen than in non-T cells. The PK residual activity in the presence of alanine was significantly higher in peripheral blood T cells than in non-T cells. Thymocytes are characterised by a larger amount of PFK M-subunits than peripheral blood T and non-T lymphocytes. The stimulation of PFK by the positive effector glucose-1,6-diphosphate was higher in thymocytes than in the peripheral blood lymphocytes.

Inhibition of 6-phosphogluconate dehydrogenase by glucose 1,6-diphosphate in human normal and malignant colon extracts

Increased activity of 6-phosphogluconate dehydrogenase was found in human colon tumors as compared to the adjacent unaffected mucosa. Glucose 1,6-diphosphate (Glc-1,6-P2), an endogenous potent regulator of glucose metabolism, markedly inhibited the activity of 6-phosphogluconate dehydrogenase (6-PGD) in extracts of the normal and malignant human colon. Glc-1,6-P2 also inhibited the activity of hexokinase in these extracts. The endogenous levels of Glc-1,6-P2 in the colon and tumors were measured. Since the pentose cycle can be inhibited by Glc-1,6-P2, means to increase endogenous levels of Glc-1,6-P2 or to introduce it into cells, might result in antitumor effects.

Photodynamic effects of hematoporphyrin-derivative on transmembrane transport systems of murine L929 fibroblasts

Photodynamic treatment of murine L929 fibroblasts with hematoporphyrin-derivative causes deterioration of various membrane functions. Most sensitive to photodynamic inactivation are the energy-coupled transport systems for aminoisobutyric acid and for Rb+. The facilitated diffusion system for 2-deoxy-D-glucose is slightly less sensitive. After longer illumination periods also the membrane barrier function is impaired, as reflected by K+ leakage and increased passive Rb+ uptake. After still longer illumination periods intermolecular protein crosslinking can be observed. This makes it unlikely that intermolecular protein crosslinking is causally involved in the deterioration of these membrane functions.

Competitive inhibition of human brain hexokinase by metrizamide and related compounds

We have compared the competitive inhibitory effects of 2-deoxyglucose, glucosamine, N-acetylglucosamine, N-benzoylglucosamine, and the commonly used radiographic and density gradient agent metrizamide (2-[3-acetamido-2,4,6-triiodo-5-(N-methylacetamido) benzamido]-2-deoxyglucose) on the mitochondrial and soluble forms of human brain hexokinase. Metrizamide produces a classical competitive inhibition with glucose for human brain hexokinase, with Kis of 2.8 and 2.5 mM, respectively, for the mitochondrial and soluble forms. Glucosamine exhibited Kis of 0.58 and 0.29 mM, while 2-deoxyglucose exhibited Kis of 0.074 and 0.15 mM and N-acetylglucosamine 0.098 and 0.092 mM for these two forms, respectively. N-Benzoylglucosamine was by far the most effective inhibitor tested, with Ki values of 0.0086 and 0.022 mM, respectively. In order of increasing potency as a competitive inhibitor for mitochondrial hexokinase are metrizamide, glucosamine, N-acetylglucosamine, 2-deoxyglucose, and N-benzoylglucosamine. For the soluble form of the enzyme in increasing potency are metrizamide, glucosamine, 2-deoxyglucose, N-acetylglucosamine, and N-benzoylglucosamine. Since N-benzoylglucosamine was over 100 times more potent than metrizamide, some of the effects of metrizamide could be due to contamination by N-benzoylglucosamine. However, gas chromatography-mass spectrometry analysis of metrizamide did not indicate the presence of N-benzoyl-glucosamine. In addition, column chromatographic separation of commercially available metrizamide and reconstitution of freeze-dried eluate fractions localized the inhibitory effect to the metrizamide peak.

Inhibition of brain glycolysis by aluminum

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

Inhibition of brain hexokinase by a multisubstrate analog results from binding to a discrete regulatory site

P1-(adenosine-5')-P3-(glucose-6)-triphosphate (Ap3glucose) is a linear uncompetitive inhibitor vs glucose and a linear mixed inhibitor vs ATP of brain hexokinase, an inhibition pattern inconsistent with binding of Ap3glucose to the catalytic site when either the rapid equilibrium random or ordered sequential mechanism, which have been proposed for this enzyme, is considered. It is concluded that inhibition results from binding to a discrete regulatory site. The apparent ability of the regulatory site to accommodate both hexose and nucleotide moieties is consistent with suggestions by previous investigators that the regulatory site on mammalian hexokinases may have evolved from what was originally a catalytic site.

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.

Comparison of the inhibitory effects of mercuric chloride on cytosolic and mitochondrial hexokinase activities in rat brain, kidney and spleen

Hg2+ (10-20 microM), at concentrations comparable to mercury levels reportedly occurring in mercury neurotoxicity (Minamata disease), effectively inhibited both cytosolic (IC50 for Hg2+ = 4.1 microM) and mitochondrial (IC50 for Hg2+ = 1.4 microM) rat brain hexokinases. Kidney (IC50 for Hg2+ approximately equal to 3 microM) and spleen hexokinases were less susceptible to inhibition by Hg2+. IC50 values for Hg2+ in inhibiting cytosolic and mitochondrial spleen hexokinases were 8.9 and 3.1 microM, respectively. In both brain and spleen, mitochondrial hexokinases were more susceptible to inhibition by Hg2+ than cytosolic forms, suggesting that the microenvironment of the mitochondrial membranes may exert some modulatory effects on the properties of hexokinases. These results also suggest that inhibition of glucose utilization may be an important mechanism of tissue damage in mercury poisoning.

Effects of 2-formylpyridine monothiosemicarbazonato copper II on red cell components

1-Formylpyridine monothiosemicarbazonato copper II (CuL+) is readily taken up by red cells and is initially bound to glutathione and hemoglobin. Glutathione was depleted within 5 hr of incubation, presumably by oxidation mediated by CuL+ and O2 with concomitant generation of toxic oxygen species. Cupric ion was slowly transferred from CuL+ to hemoglobin within about 7 hr and hemoglobin was oxidized until the major form prevailing after 10 hr was alpha 2 beta 2+. Little increase in hemolysis due to addition of CuL+ dissolved in the radical scavenger dimethyl sulfoxide was observed with prolonged incubation. Strong inhibition of red cell hexokinase by CuL+ was observed when the enzymes in red cell lysates and hemoglobin-free red cell lysates were examined. CuL+ was also an effective inhibitor of yeast hexokinase. However, the inhibitory effect of CuL+ within the red cells was less pronounced. It is suggested that even though intracellular accumulation of CuL+ creates an oxidizing environment and is potentially capable of inhibiting thiol enzymes such as hexokinase, protective effects are exerted in the red cell by the presence of hemoglobin, of radical scavengers, and of high levels of enzymes that detoxify toxic oxygen species.

Characterization of Trypanosoma cruzi hexokinase

Properties of hexokinase (EC 2.7.1.1) from Trypanosoma cruzi epimastigote forms (Tulahuen strain) were studied and compared with enzymes from other sources. The enzyme activity was 37 units g-1 of wet cells (1.2 units mg-1 protein). Hexokinase showed Km values for glucose and ATP of 0.09 and 0.4 mM, respectively. The enzyme reacted with other nucleotides too. N-Acetylglucosamine was a competitive inhibitor with respect to glucose (Ki = 0.3 mM). ADP inhibited the enzyme competitively with respect to ATP (Ki = 1.5 mM) and noncompetitively with respect to glucose (Ki = 7 mM). The enzyme was markedly inhibited by 5-thioglucose, its Ki value was 0.4 mM. Hexokinase activity was not affected by glucose 6-phosphate.

Inactivation of yeast hexokinase B by triethyltin bromide

Triethyltin bromide was found to demonstrate temperature-dependent inactivation of yeast hexokinase B. At temperatures of 20 degrees C or lower, little or no inactivation of the enzyme was detected after 2 h of reaction with 50-300 microM concentrations of the reagent. However, incubation at 25 degrees C or higher resulted in an increased rate and extent of loss of the enzyme activity with increasing incubation temperatures. The Arrhenius plot for the inactivation process showed a sharp break at approximately 30 degrees C, with a heat of activation (delta H*) above this temperature of 55.2 kcal, indicating that a triethyltin-induced conformational change occurred at the elevated temperatures. Sugar substrates provided protection against the inactivating effect by reducing the binding of triethyltin to the enzyme. In the absence of glucose, two sites of different affinity for triethyltin exist in the hexokinase monomer. Binding of triethyltin to the enzyme shifted its monomer-dimer equilibrium toward the monomeric form in an early stage of the interaction. Inactivation of the enzyme was associated with a slower subsequent event. Comparative effects of various organotin compounds on the activity of the enzyme indicated that inhibitory potency was associated with increasing hydrophobicity of the alkyl groups attached to the tin.

Solvent isotope effects on the reaction catalyzed by yeast hexokinase

The pH dependence of the maximum velocity (V) for the phosphorylation of glucose, the V/Kglucose and the V/KMgATP have been obtained in H2O and 2H2O. In H2O, V decreases below a pK of 5.8, V/Kglucose decreases below a pK of 6.1 and V/KMgATP decreases below a pK of 6.7. In 2H2O, complex behavior is observed for these parameters as a function of pD. The ratios of the parameters in H2O and 2H2O above their respective pK values give solvent deuterium isotope effects of about 1.5-1.7 for all three parameters. When 1,5-anhydromannitol is used as an alternative substrate, an isotope effect different than unity is obtained only for V/K1,5-anhydromannitol which gives a value of about 0.7. Both the complex pH profiles and the relative magnitude of the isotope effects are interpreted in terms of a pH-dependent change in the E X glucose complex.

Isotope-exchange evidence that glucose 6-phosphate inhibits rat-muscle hexokinase II at an allosteric site

The flux ratio for hexokinase type II from rat muscle, i.e. the rate of conversion of glucose 6-phosphate molecules into ATP molecules divided by the simultaneous rate of conversion of glucose 6-phosphate molecules into glucose molecules, increases with the MgATP concentration but is independent of the glucose concentration. This behaviour requires that glucose must bind before MgATP when the reaction is proceeding in the normal physiological direction, i.e. phosphorylation of glucose. Although at low non-inhibitory glucose 6-phosphate concentrations the flux ratio increases linearly with the MgATP concentration, the dependence becomes non-linear, with a slope that increases with the MgATP concentration, at glucose 6-phosphate concentrations above 1 mM. This behaviour does not permit glucose 6-phosphate to act only as a normal product inhibitor. Instead, it seems to require glucose 6-phosphate to act as an allosteric inhibitor and for a second site for binding of MgATP to exist. Measurements of the flux from ATP to glucose 6-phosphate and to ADP showed no dependence of the flux ratio on the concentrations of either glucose 6-phosphate or ADP. This result does not permit the order of product-release steps in this direction to be determined, but shows that the second product is released virtually instantaneously after first.

Metrizamide inhibits human brain hexokinase

The myelographic contrast agent, metrizamide, causes a temporary confusional state in many patients. Since metrizamide is a 2-deoxyglucose analogue, it was tested for inhibitory effects on glucose metabolism. The Michaelis constant (Km) of human brain hexokinase for glucose rose from 0.039 to 0.24 and 0.47 mM with final metrizamide concentrations of 0, 16 and 32 mM, respectively. The maximal velocity did not change. Since metrizamide is injected into the human CSF in concentrations of up to 780 mM, impairment of brain glucose metabolism can be expected. These effects could be largely counteracted if metrizamide were injected in a 100 mM glucose solution.

Metal-ion-promoted binding of triazine dyes to proteins. The interaction of Cibacron Blue F3G-A with yeast hexokinase

Bivalent metal ions, particularly Zn2+ and other members of the first-row transition series, promote irreversible inactivation of yeast hexokinase by Cibacron Blue F3G-A at a site competitive with both ATP and D-glucose. Difference spectroscopy indicates that the protein-dye dissociation constant is decreased from 250 micrometers in the absence of metal ions to less than 100 micrometers in the presence of appropriate concentrations of metal ions, with specificity displayed in the sequence of Zn2+ greater than Cu2+ greater than Ni2+ greater than Mn2+. Quantitative inactivation of yeast hexokinase leads to the incorporation of approx. 1 mol of Cibacron Blue F3G-A/mol of subunit of mol. wt. 51 000 in both the presence and the absence of metal ion. These results suggest the formation of a highly specific ternary complex involving enzyme, dye and metal ion at the active-site region of the enzyme, and correlate well with the known effects of metal ions in promoting the binding of hexokinase to immobilized Cibacron Blue F3G-A.

Species- or isozyme-specific enzyme inhibitors. 6. Synthesis and evaluation of two-substrate condensation products as inhibitors of hexokinases and thymidine kinases

Syntheses are described of p1-(adenosine-5')-p3-(glucose-6) triphosphate (Ap3 glucose), Ap4 glucose, and p1-(adenosine-5')-P3-(thymidine-5') triphosphate (Ap3T). The compounds were not substrates of any of the enzymes used in the present studies. Ap3 glucose and Ap4 glucose were inhibitors of yeast hexokinase (HK) and the rat isozymes HK I-III; in general, they had less affinity for the enzymes than the substrates ATP and glucose. Inhibition constants (Ki values) of Ap3T with rat mitochondrial thymidine kinase (M-TK) and rat cytoplasmic TK (C-TK) were determined for variable thymidine (TdR) with a constant saturating level of ATP and for variable ATP with constant saturating TdR. Ap3T was a potent and selective inhibitor of M-TK [KM (TdR)/Ki = 1.6, KM (ATP)/Ki = 38 with variable ATP; KM (TdR) Ki = 0.06, KM (ATP)/Ki = 1.4 with variable TdR] relative to C-TK [KM (TdR)/Ki = 0.006, KM (ATP)/Ki = 0.7 with variable ATP; KM (TdR)/Ki = 0.001, KM (ATP)/Ki = 0.12 with variable TdR]. Inhibition of M-TK and C-TK by Ap3T differed qualitatively and quantitatively from inhibition under the same conditions by the metabolic feedback inhibitor TdR 5'-triphosphate.

Inhibition of hexokinase and protein kinase activities of tumor cells by a chloromethyl ketone derivative of lactic acid

A chloromethyl ketone derivative of lactic acid is a potent inhibitor of glycolysis of Ehrlich ascites tumor cells. It inhibited glycolysis of intact cells by about 50% at 200 microM (100 nmol/mg of protein) while cell-free extracts were inhibited 50% at 50 microM (50 nmol/mg of protein). N alpha-(p-Tosyl)-L-lysine chloromethyl ketone and N alpha-(p-tosyl)-L-phenylalanine chloromethyl ketone inhibited only slightly or not at all at this concentration. The inhibition was localized at the hexokinase and phosphofructokinase steps since these two enzymes added to an inactivated extract restored the glycolytic activity, whereas none of the other glycolytic enzymes did. In fact, addition of pyruvate kinase or lactate dehydrogenase, which stimulated glycolysis, resulted in a more pronounced inhibition. Glycolysis and hexokinase activities in extracts of Rous sarcoma virus transformed cells were considerably more sensitive to the inhibitor than the activities from normal chick embryo fibroblasts. Hexokinase from mouse brain required 50 times higher concentrations for inhibition than the enzyme from mouse Ehrlich ascites tumor cells. Yeast hexokinase was unaffected at all concentrations tested. Since 5,5'-dithiobis(2-nitrobenzoate) protected against the inhibition, the chloromethyl ketone appeared to inhibit by interaction with an essential SH group. A pronounced inhibition of protein kinase activity of plasma membranes of Ehrlich ascites tumor cells was observed in the presence of the chloromethyl ketone. As in the case of glycolysis, the chloromethyl ketone of lactic acid was a more potent inhibitor of protein kinase activity than several other chloromethyl ketones that were tested.