Studies on heart phosphofructokinase. Decavanadate as a potent allosteric inhibitor at alkaline and acidic pH

Pascoite Minerals and Potential Application of NMR Spectroscopy

The 20 minerals encompassing the pascoite family of decavanadate isopolyanion-containing [V10O28]6− minerals include a few minerals, such as rakovanite, that have been described as containing a protonated decavanadate anion. Rakovanite was originally assigned the formula Na3[H3V10O28]•15H2O and now is redefined with an ideal formula (NH4)3Na3[V10O28]•12H2O. Nuclear magnetic resonance (NMR) and particularly 51V NMR spectroscopy is an informative method used to describe the protonation state and speciation in both solid and solution states of materials in the chemical and life sciences. However, 51V NMR spectroscopy has not yet been used experimentally to distinguish the protonation state of the decavanadate ion of leaching solutions and thus contributing to the discussion regarding the controversial protonation states of decavanadate ions in gunterite, rakovanite, and nashite. In contrast, the morphology and crystal structure for apatites, vanadinite, pyromorphite, and mimetite was related to 207Pb NMR chemical shifts, assisting in describing the local environments of these minerals. NMR spectroscopy could be a useful method if used in the future for decavanadate-containing minerals. Currently, partial reduction of two Pascoite minerals (caseyite and nashite) is proposed and accordingly could now effectively be investigated using a different magnetic resonance technique, EPR spectroscopy.

The fructose-dependent acceleration of ethanol metabolism

The aim of the present study was to elucidate how fructose is able to increase the rate of ethanol metabolism in the liver, an observation previously termed the fructose effect. Previous studies suggest that an increase in ATP consumption driven by glucose synthesis from fructose stimulates the oxidation of NADH in the mitochondrial respiratory chain, allowing faster oxidation of ethanol by alcohol dehydrogenase; however, this idea has been frequently challenged. We tested the effects of fructose, sorbose and tagatose both in vitro and in vivo. Both ethanol and each sugar were either added to isolated hepatocytes or injected intraperitoneally in the rat. In the in vitro experiments, samples were taken from the hepatocyte suspension in a time-dependent manner and deproteinized with perchloric acid. In the in vivo experiments, blood samples were taken every 15 min and the metabolites were determined in the plasma. These metabolites include ethanol, glucose, glycerol, sorbitol, lactate, fructose and sorbose. Ethanol oxidation by rat hepatocytes was increased by more than 50% with the addition of fructose. The stimulation was accompanied by increased glucose, glycerol, lactate and sorbitol production. A similar effect was observed with sorbose, while tagatose had no effect. The same pattern was observed in the in vivo experiments. This effect was abolished by inhibiting alcohol dehydrogenase with 4-methylpyrazole, whereas inhibition of the respiratory chain with cyanide did not affect the fructose effect. In conclusion, present results provide evidence that, by reducing glyceraldehyde and glycerol and fructose to sorbitol, respectively, NADH is consumed, allowing an increase in the elimination of ethanol. Hence, this effect is not linked to a stimulation of mitochondrial re-oxidation of NADH driven by ATP consumption.

Semimetal-functionalised polyoxovanadates

Polyoxovanadates (POVs), known for their wide applicability and relevance in chemical, physical and biological sciences, are a subclass of polyoxometalates and usually self-assemble in aqueous-phase, pH-controlled condensation reactions. Archetypical POVs such as the robust [VO42](12-) polyoxoanion can be structurally, electronically and magnetically altered by heavier group 14 and 15 elements to afford Si-, Ge-, As- or Sb-decorated POV structures (heteroPOVs). These main-group semimetals introduce specific chemically engineered functionalities which cause the generally hydrophilic heteroPOV compounds to exhibit interesting reactivity towards organic molecules, late transition metal and lanthanoid ions. The fully-oxidised (V(V)), mixed-valent (V(V)/V(IV) and V(IV)/V(III)), "fully-reduced" (V(IV)) and "highly-reduced" (V(III)) heteroPOVs possess a number of intriguing properties, ranging from catalytic to molecular magnet characteristics. Herein, we review key developments in the synthetic and structural chemistry as well as the reactivity of POVs functionalised with Si-, Ge-, As- or Sb-based heterogroups.

Decavanadate (V10 O28 6-) and oxovanadates: oxometalates with many biological activities

The decameric vanadate species V(10)O(28)(6-), also referred to as decavanadate, impact proteins, lipid structures and cellular function, and show some effects in vivo on oxidative stress processes and other biological properties. The mode of action of decavanadate in many biochemical systems depends, at least in part, on the charge and size of the species and in some cases competes with the simpler oxovanadate species. The orange decavanadate that contains 10 vanadium atoms is a stable species for several days at neutral pH, but at higher pH immediately converts to the structurally and functionally distinct lower oxovanadates such as the monomer, dimer or tetramer. Although the biological effects of vanadium are generally assumed to derive from monomeric vanadate or the vanadyl cation, we show in this review that not all effects can be attributed to these simple oxovanadate forms. This topic has not previously been reviewed although background information is available [D.C. Crans, Comments Inorg. Chem. 16 (1994) 35-76; M. Aureliano (Ed.), Vanadium Biochemistry, Research Signpost Publs., Kerala, India, 2007]. In addition to pumps, channels and metabotropic receptors, lipid structures represent potential biological targets for decavanadate and some examples have been reported. Decavanadate interact with enzymes, polyphosphate, nucleotide and inositol 3-phosphate binding sites in the substrate domain or in an allosteric site, in a complex manner. In mitochondria, where vanadium was shown to accumulate following decavanadate in vivo administration, nM concentration of decavanadate induces membrane depolarization in addition to inhibiting oxygen consumption, suggesting that mitochondria may be potential targets for decameric toxicity. In vivo effects of decavanadate in piscine models demonstrated that antioxidant stress markers, lipid peroxidation and vanadium subcellular distribution is dependent upon whether or not the solutions administered contain decavanadate. The present review summarizes the reports on biological effects of decavanadate and highlights the importance of considering decavanadate in evaluations of the biological effects of vanadium.

Decavanadate, a P2X receptor antagonist, and its use to study ligand interactions with P2X7 receptors

In this study we have studied decavanadate effects at P2X receptors. Decavanadate competitively blocked 2'- and 3'-O-(4benzoylbenzoyl) ATP (BzATP) stimulated ethidium accumulation in HEK293 cells expressing human recombinant P2X7 receptors (pK(B) 7.5). The effects of decavanadate were rapid (minutes) in both onset and offset and contrasted with the much slower kinetics of pyridoxal 5-phosphate (P5P), Coomassie brilliant blue (CBB) and 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN62). Decavanadate competitively blocked the slowly reversible, or irreversible, blockade of the P2X7 receptor produced by P5P and oxidised ATP suggesting competition for a common binding site. However, the interaction between decavanadate and KN62 was non-competitive. Decavanadate also blocked P2X2 and P2X4 receptors but with slightly lower potency. These data demonstrate that decavanadate is the first reversible and competitive antagonist of the P2X7 receptor and is a useful tool for studying the mechanism of interaction of ligands with the P2X7 receptor.

Antagonistic regulation of native Ca2+- and ATP-sensitive cation channels in brain capillaries by nucleotides and decavanadate

Regulation by cytosolic nucleotides of Ca²⁺- and ATP-sensitive nonselective cation channels (CA-NSCs) in rat brain capillary endothelial cells was studied in excised inside-out patches. Open probability (P_o) was suppressed by cytosolic nucleotides with apparent K_I values of 17, 9, and 2 μM for ATP, ADP, and AMP, as a consequence of high-affinity inhibition of channel opening rate and low-affinity stimulation of closing rate. Cytosolic [Ca²⁺] and voltage affected inhibition of P_o, but not of opening rate, by ATP, suggesting that the conformation of the nucleotide binding site is influenced only by the state of the channel gate, not by that of the Ca²⁺ and voltage sensors. ATP inhibition was unaltered by channel rundown. Nucleotide structure affected inhibitory potency that was little sensitive to base substitutions, but was greatly diminished by 3′-5′ cyclization, removal of all phosphates, or complete omission of the base. In contrast, decavanadate potently (K_1/2 = 90 nM) and robustly stimulated P_o, and functionally competed with inhibitory nucleotides. From kinetic analyses we conclude that (a) ATP, ADP, and AMP bind to a common site; (b) inhibition by nucleotides occurs through simple reversible binding, as a consequence of tighter binding to the closed-channel relative to the open-channel conformation; (c) the conformation of the nucleotide binding site is not directly modulated by Ca²⁺ and voltage; (d) the differences in inhibitory potency of ATP, ADP, and AMP reflect their different affinities for the closed channel; and (e) though decavanadate is the only example found to date of a compound that stimulates P_o with high affinity even in the presence of millimolar nucleotides, apparently by competing for the nucleotide binding site, a comparable mechanism might allow CA-NSC channels to open in living cells despite physiological levels of nucleotides. Decavanadate now provides a valuable tool for studying native CA-NSC channels and for screening cloned channels.

NADH-dependent decavanadate reductase, an alternative activity of NADP-specific isocitrate dehydrogenase protein

The well known NADP-specific isocitrate dehydrogenase (IDH) obtained from pig heart was found to oxidize NADH with accompanying consumption of oxygen (NADH:O(2)=1:1) in presence of polyvanadate. This activity of the soluble IDH-protein has the following features common with the previously described membrane-enzymes: heat-sensitive, active only with NADH but not NADPH, increased rates in acidic pH, dependence on concentrations of the enzyme, NADH, decavanadate and metavanadate (the two constituents of polyvanadate), and sensitivity to SOD and EDTA. Utilizing NADH as the electron source the IDH protein was able to reduce decavanadate but not metavanadate. This reduced form of vanadyl (V(IV)) was similar in its eight-band electron spin resonance spectrum to vanadyl sulfate but had a 20-fold higher absorbance at its 700 nm peak. This decavanadate reductase activity of the protein was sensitive to heat and was not inhibited by SOD and EDTA. The IDH protein has the additional enzymic activity of NADH-dependent decavanadate reductase and is an example of "one protein--many functions".

Stimulation of mutases and isomerases by vanadium

The effects of vanadate and vanadate complexes on the rates of exchange of phosphoryl groups in the reactions catalyzed by the enzymes phosphoglucomutase and the coupled system formed by phosphoglycerate mutase and enolase, and the effects of vanadyl complexes on the interconversion of aldehyde and keto groups catalyzed by the enzymes phosphomannose isomerase, phosphoribose isomerase, and phosphoglucose isomerase, were measured using one-dimensional 31P nuclear magnetic resonance spectroscopy. Chemical exchange was investigated by observing the transfer of magnetization achieved by selective irradiation of resonances using the DANTE pulse sequence. The presence of vanadium stimulated the catalytic activity of the enzymes in vitro, with the exception of enolase whose activity was not affected. Addition of vanadate also increased the rate constants of the interconversion of glucose 6-phosphate and fructose 6-phosphate in hemolysates. 51V nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy were employed to investigate the interactions between ammonium vanadate and sugar phosphates and the formation of vanadium--sugar phosphate complexes that may be involved in the stimulation of the catalytic activity of the isomerases.

Vanadate promotes photooxidative cleavage and inactivation of muscle phosphofructokinase

During irradiation in the presence of decavanadate, the subunits of phosphofructokinase underwent progressive degradation to a fragment of about 78,000 daltons. This cleavage pattern was altered when the photoirradiation was performed in the presence of monomeric vanadate with formation of several smaller peptides. The specificity of the decavanadate induced cleavage was proved by the resistance of other enzymes to the treatment and by the effects of phosphofructokinase ligands. During irradiation, the activity of the enzyme declined. Differences between the rate of inactivation and of cleavage of enzyme subunits suggest the occurrence of multiple processes.

Electrostatic analysis of effects of ions on the inhibition of corn root plasma membrane Mg2+-ATPase by the bivalent orthovanadate

The microelectrophoretic mobility of corn root plasma membranes and the inhibition of the Mg+2-ATPase by vanadate were investigated under different ionic conditions. The Mg2+-ATPase was uncompetitively inhibited and a 10-fold variation of the apparent inhibition constant was observed, depending on the addition of K+ and Mg2+. The determination of the zeta potential indicated that a 5-fold decrease of the apparent inhibition constant was due to aspecific electrostatic interactions of the vanadate anion and the negative charge of the membrane. The screening and masking effects of 6 mM free Mg2+ totally abolished electrostatic interactions and allowed the direct determination of the intrinsic vanadate inhibition constant (KIi). On the other hand, a specific, non-electrostatic, effect of K+ caused a 2-fold decrease of the inhibition constant in addition to the electrostatic effect. Finally, the electrostatic analysis indicates that the Mg2+-ATPase is inhibited by the monomeric bivalent anion HVO4(2-).

Interaction of vanadium(V) oxoanions with ovalbumin and transfusion gelatin

Vanadate(V) anions have been found to modulate the activity of a large number of enzymes. To elucidate these interactions, a physico-chemical study of the binding of meta-vanadate(V) with two typical proteins ovalbumin and transfusion gelatin has been taken up. The binding has been investigated by polarographic method at pHs 7.5 and 9.3, and at 30 degrees C. Values of intrinsic association constant, standard Gibb's free energy change, and the number of amino acids involved in these bindings are reported. Analysis of the binding sites has been done.

Phosphofructokinase from Fasciola hepatica: activation by phosphorylation and other regulatory properties distinct from the mammalian enzyme

Phosphofructokinase from the liver fluke, Fasciola hepatica, was phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase isolated from this organism. Phosphorylated fluke phosphofructokinase had a sevenfold lower apparent Km for its substrate, Fru-6-P, and an eightfold higher 0.5 Vopt for ATP, the enzyme's primary inhibitor, than native phosphofructokinase. Activation of fluke phosphofructokinase following phorphorylation by a mammalian protein kinase catalytic subunit was previously reported (E. S. Kamemoto and T. E. Mansour (1986) J. Biol. Chem. 261, 4346-4351). The catalytic subunit of protein kinase isolated from the liver fluke phosphorylated sites on fluke phosphofructokinase similar to those phosphorylated by the mammalian enzyme. Maximal phosphate incorporation was 0.3 mol P/mol of protomer. The native enzyme was found to contain 1.3 mol P/mol of protomer. In contrast to fluke phosphofructokinase, activity of the mammalian heart enzyme was slightly decreased following phosphorylation. The dependence of allosteric interaction on an acidic pH observed with the mammalian phosphofructokinase was not observed with the fluke enzyme. Unlike mammalian phosphofructokinase, allosteric kinetics of the fluke enzyme was observed at alkaline pH (8.0). Fluke phosphofructokinase was found to be relatively insensitive to inhibition by citrate, a known potent inhibitor of the mammalian enzyme. Fru-2,6-P2, a potent modifier of phosphofructokinase from a variety of sources, was found to activate both native and phosphorylated fluke phosphofructokinase. The most potent activators of fluke phosphofructokinase were found to be Fru-2,6-P2, AMP, and phosphorylation. The endogenous level of Fru-2,6-P2 in the flukes was determined to be 29 +/- 1.3 nmol/g wet wt, a level that may well modulate enzyme activity. Fru-6-P,2-kinase, the enzyme responsible for synthesis of Fru-2,6-P2, was found to be present in the flukes. Our results suggest physiological roles for phosphorylation and Fru-2,6-P2 in regulation of fluke phosphofructokinase.

Effect of chemical modification on the crystallization of Ca2+-ATPase in sarcoplasmic reticulum

The influence of chemical modification on the morphology of crystalline ATPase aggregates was analyzed in sarcoplasmic reticulum (SR) vesicles. The Ca2+-ATPase forms monomer-type (P1) type crystals in the E1 and dimer-type (P2) crystals in the E2 conformation. The P1 type crystals are induced by Ca2+ or lanthanides; P2 type crystals are observed in Ca2+-free media in the presence of vanadate or inorganic phosphate. P1- and P2-type Ca2+-ATPase crystals do not coexist in significant amounts in native sarcoplasmic reticulum membrane. The crystallization of Ca2+-ATPase in the E2 conformation is inhibited by guanidino-group reagents (2,3-butanedione and phenylglyoxal), SH-group reagents, phospholipases C or A2, and detergents, together with inhibition of ATPase activity. Amino-group reagents (fluorescein 5'-isothiocyanate, pyridoxal phosphate and fluorescamine) inhibit ATPase activity but do not interfere with the crystallization of Ca2+-ATPase induced by vanadate. In fluorescamine-treated sarcoplasmic reticulum the vanadate-induced crystals contain significant P1-type regions in addition to the dominant P2 form.

Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: phosphate dependence and effects of other oxyanions

The effects of various oxyanions on the activities of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (EC 2.7.1.105/3.1.3.46) were examined. No evidence could be found for an absolute dependence of the kinase activity on inorganic phosphate as was recently reported by M. Laloux, E. Van Schaftingen, and H.-G. Hers ((1985) Eur. J. Biochem. 148, 155-159). Rather, phosphate and arsenate activated the kinase by decreasing the enzyme's Km for fructose 6-phosphate without affecting its Km for ATP or Vmax. The Km of the kinase for fructose 6-phosphate in the presence of inorganic phosphate was found to be significantly lower (6 microM) than previously reported (30 microM) when the hydrolysis of fructose 2,6-bisphosphate by the concomitant bisphosphatase activity at low Fru 6-P concentrations was taken into account. The KA's for phosphate and arsenate activation of the kinase were 0.2 and 0.3 mM, respectively. A number of other oxyanions, including pyrophosphate, sulfate, tungstate, selenate, and molybdate all inhibited the kinase by increasing the Km for fructose 6-phosphate. The apparent Ki's for inhibition of the kinase were in the 0.5-1 mM range. In contrast, all of these oxyanions activated the bisphosphatase, with half-maximal effects requiring millimolar concentrations. Inorganic phosphate was the most potent activator with a KA of 1 mM. In contrast to the other oxyanions, vanadate and meta-periodate inhibited the kinase but had no effect on the bisphosphatase. Vanadate appeared to be a noncompetitive inhibitor since its effects were not overcome by Pi, ATP, or fructose 6-phosphate, and the species responsible was shown to be decavanadate. Like vanadate, meta-periodate had no effect on the bisphosphatase, though it was a potent inhibitor (I0.5 = 30 microM) of the kinase. Its effects were shown to be time-dependent and reversed by dithiothreitol, suggesting that it acted by an oxidative mechanism. These results augment the mounting body of evidence that the enzyme's two reactions are catalyzed at discrete active sites.

The effects of vanadate on rabbit ventricular muscle adenylate cyclase and sodium pump activities

Vanadate in the +5 oxidation state has been reported to have a positive inotropic action on cardiac ventricular muscle. We have investigated the biochemical actions of vanadate on ventricular muscle adenylate cyclase and sodium pump activities in both intact or disrupted cell systems in an attempt to elucidate the mechanism(s) responsible for the physiological response. Vanadate at concentrations up to 100 microM (Ka = 2 microM) stimulated adenylate cyclase activity in sarcolemmal membrane preparations or disrupted myocytes isolated from rabbit ventricular muscle by 2-3-fold. Increasing the vanadate concentrations above 100 microM resulted in a progressive inhibition of basal or hormone-stimulated adenylate cyclase activity (Ki = 5 mM) which was similar to that found by the reaction product, pyrophosphate (Ki = 0.5 mM). Both activation and inhibition by vanadate was fully reversible. Maximum activation of adenylate cyclase by vanadate and isoprenaline were not additive whereas maximum fluoride activation was decreased (18%) and the forskolin-stimulated response was slightly potentiated. Vanadate reversibly inhibited ouabain-sensitive p-nitrophenylphosphatase activity (Ki = 60 nM) in sarcolemmal membrane preparations and disrupted myocytes. Complete inactivation was found at 1 microM vanadate. Acute or chronic incubation of intact myocytes with vanadate at concentrations up to 0.5 mM had no measurable affect on ouabain-sensitive 86Rb influx or isobutylmethylxanthine, isoprenaline or forskolin-stimulated accumulation of intracellular cAMP concentration. Inhibition of 86Rb influx and cAMP accumulation was found at higher concentrations of vanadate; however, this accompanied the progressive decrease in cell viability as measured by the decrease in percentage of rod-shaped cells. It is concluded that vanadate, at concentrations which have been reported to induce a positive inotropic action on mammalian ventricular muscle, does not increase adenylate cyclase activity or inhibit the sodium pump activity in intact myocytes. These results show that caution must be applied when extrapolating the actions found with vanadate in broken cell systems to intact tissues.

The binding of vanadium (V) oligoanions to sarcoplasmic reticulum

The binding of monovanadate and decavanadate anions to sarcoplasmic reticulum vesicles was measured by equilibrium sedimentation. The affinity of vanadate binding and the molar amount of vanadium (V) bound at equilibrium is much greater with decavanadate than with monovanadate. The binding data can be rationalized in terms of one binding site per ATPase molecule for monovanadate and two sites per ATPase for decavanadate. The Ca-ATPase crystals formed with monovanadate and with decavanadate are similar in appearance, but decavanadate is particularly effective in promoting the crystallization of Ca2+-ATPase at low V concentration (10-100 microM) in a Ca2+-free medium.

Do vanadate polyanions inhibit phosphotransferase enzymes?

Decavanadate inhibits hexokinase, adenylate kinase and phosphofructokinase; neither mono-, tri nor tetrameric vanadate anion is an inhibitor. Decavanadate inhibits phosphofructokinase obtained from bacterial and protistic sources. No form of vanadium(V) anion inhibits galacto-, glycero-, pyruvate and creatine kinase, or inorganic pyrophosphatase. Decavanadate appears to be a non-competitive inhibitor of both hexokinase substrates.

Metal ion release from titanium-based prosthetic segmental replacements of long bones in baboons: a long-term study

Forty-five baboons that had received titanium-based fiber metal composite segmental bone replacements were studied along with 13 controls without implants. Thirty-eight baboons with implants were sacrificed, and titanium, aluminum, and vanadium levels were assayed in homogenized lung, kidney, spleen, liver, adjacent muscle, and regional lymph nodes. In seven living baboons with implants, blood and urine samples were obtained for trace metal analysis as well as for biochemical and hematological profiles. In the 38 sacrificed baboons with implants, increased titanium levels were noted in the lungs, spleen, adjacent muscle (quadriceps, soleus, and triceps), and regional lymph nodes (inguinal, axillary, and popliteal) in comparison to those of six sacrificed controls without implants. In addition, vanadium was significantly elevated in the lungs of some animals, while aluminum increases were noted in adjacent muscle (quadriceps, soleus, and triceps), lung, and regional lymph nodes (inguinal, axillary, and popliteal). In the seven living baboons with implants, a sixfold increase (p less than 0.0005) in the urine titanium level was noted in comparison to that of seven living controls without implants. Additionally, elevated aluminum levels were found in the serum (p less than 0.0005) of the group with implants. Biochemical and hematological studies did not indicate statistically significant differences in serum electrolytes, in liver and renal function tests, or in complete blood counts between the seven living baboons with implants and their controls.