Inactivation of Tetrameric Rabbit Muscle Pyruvate Kinase by Specific Binding of 2 to 4 Moles of Pyridoxal 5'-Phosphate

Tight coupling of Na+/K+-ATPase with glycolysis demonstrated in permeabilized rat cardiomyocytes

The effective integrated organization of processes in cardiac cells is achieved, in part, by the functional compartmentation of energy transfer processes. Earlier, using permeabilized cardiomyocytes, we demonstrated the existence of tight coupling between some of cardiomyocyte ATPases and glycolysis in rat. In this work, we studied contribution of two membrane ATPases and whether they are coupled to glycolysis--sarcoplasmic reticulum Ca2+ ATPase (SERCA) and plasmalemma Na+/K+-ATPase (NKA). While SERCA activity was minor in this preparation in the absence of calcium, major role of NKA was revealed accounting to ∼30% of the total ATPase activity which demonstrates that permeabilized cell preparation can be used to study this pump. To elucidate the contribution of NKA in the pool of ATPases, a series of kinetic measurements was performed in cells where NKA had been inhibited by 2 mM ouabain. In these cells, we recorded: ADP- and ATP-kinetics of respiration, competition for ADP between mitochondria and pyruvate kinase (PK), ADP-kinetics of endogenous PK, and ATP-kinetics of total ATPases. The experimental data was analyzed using a series of mathematical models with varying compartmentation levels. The results show that NKA is tightly coupled to glycolysis with undetectable flux of ATP between mitochondria and NKA. Such tight coupling of NKA to PK is in line with its increased importance in the pathological states of the heart when the substrate preference shifts to glucose.

Functional analysis, overexpression, and kinetic characterization of pyruvate kinase from Plasmodium falciparum

The important role of pyruvate kinase during malarial infection has prompted the cloning of a cDNA encoding Plasmodium falciparum pyruvate kinase (pfPyrK), using mRNA from intraerythrocytic-stage malaria parasites. The full-length cDNA encodes a protein with a computed molecular weight of 55.6 kDa and an isoelectric point of 7.5. The purified recombinant pfPyrK is enzymatically active and exists as a homotetramer in its active form. The enzyme exhibits hyperbolic kinetics with respect to phosphoenolpyruvate and ADP, with K(m) of 0.19 and 0.12 mM, respectively. pfPyrK is not affected by fructose-1,6-bisphosphate, a general activating factor of pyruvate kinase for most species. Glucose-6-phosphate, an activator of the Toxoplasma gondii enzyme, does not affect pfPyrK activity. Similar to rabbit pyruvate kinase, pfPyrK is susceptible to inactivation by 1mM pyridoxal-5'-phosphate, but to a lesser extent. A screen for inhibitors to pfPyrK revealed that it is markedly inhibited by ATP and citrate. Detailed kinetic analysis revealed a transition from hyperbolic to sigmoidal kinetics for PEP in the presence of citrate, as well as competitive inhibitory behavior for ATP with respect to PEP. Citrate exhibits non-competitive inhibition with respect to ADP with a K(i) of 0.8mM. In conclusion, P. falciparum expresses an active pyruvate kinase during the intraerythrocytic-stage of its developmental cycle that may play important metabolic roles during infection.

Inhibition of camel lens zeta-crystallin/NADPH:quinone oxidoreductase by pyridoxal-5'-phosphate

Camel lens zeta-crystallin was inhibited by pyridoxal-5'-phosphate (PAL-P) and o-phthalaldehyde. PAL-P inactivated zeta-crystallin in a time- and concentration-dependent manner. The initial rate of inactivation followed pseudo-first-order kinetics with the second-order rate constant of 91 M-1 s-1. The modified enzyme showed the characteristic absorption peak at 325 nm indicative of the formation of phosphopyridoxallysine. Quantitative analysis suggested the incorporation of 1 mole of PAL-P/subunit of enzyme. NADPH was able to substantially protect zeta-crystallin against PAL-P inactivation, whereas the substrate 9,10-phenanthrenequinone (PQ) did not provide any protection. Inhibition of zeta-crystallin by PAL-P was uncompetitive with NADPH (Ki=37 microM) and non-competitive with respect to the substrate (Ki=57 microM). Inhibition of zeta-crystallin by o-phthalaldehyde was used to establish the location of an essential lysine residue. Incubation of zeta-crystallin with o-phthalaldehyde resulted in the formation of an isoindole derivative that had a characteristic fluorescence spectrum. This suggested that a lysine residue is located within 3 A of a cysteine residue at the NADPH binding region. SDS-PAGE showed the o-phthalaldehyde-modified enzyme remained largely monomer (approx. 80%), although bands corresponding to dimer and tetramer forms were also present. These results suggested that an essential lysine residue is located in the vicinity of the NADPH binding site. This residue may simply ensure the proper binding of NADPH to the active site of zeta-crystallin.

Pyridoxal 5'-phosphate related changes in retention of 1,25-dihydroxy vitamin D-receptor ligands in rat intestinal mucosa cell nuclei

After feeding rats a vitamin B-6-deficient diet, we observed a decrease in pyridoxal 5'-phosphate concentrations in intestinal mucosa cells to 32 and 48% of control in cytoplasm and cell nuclei, respectively. Correlation analysis suggested that there were two pyridoxal 5'-phosphate pools in the nuclei: a "mobile" pool (equivalent to about 5% the concentration of the cytoplasmic pyridoxal 5'-phosphate), and a "stable" pool, which was independent of cytoplasmic fluctuations of pyridoxal 5'-phosphate (about 9 pmol pyridoxal 5'-phosphate/mg DNA). Reduction in pyridoxal 5'-phosphate content in the cells of vitamin B-6-deficient animals was accompanied by a substantial increase in 1,25-dihydroxyvitamin D-receptor ligand concentration in the cell nuclei (76.6 +/- 19.7 vs 762 +/- 291 fmol/mg DNA, mean +/- SEM). The degree of 1,25-dihydroxyvitamin D accumulation in the nuclei appeared to be an exponential function of the "mobile" nuclear pyridoxal 5'-phosphate concentration. Semilogarithmic transformation of the data yielded a straight line, representing an inverse correlation between the cytoplasm-related nuclear pool of pyridoxal 5'-phosphate and the logarithm of the 1,25-dihydroxyvitamin D concentration in the nuclei (r = -0.95). These data suggest that pyridoxal 5'-phosphate may be related to 1,25-dihydroxyvitamin D retention in the nuclei, possibly through interaction of the pyridoxal 5'-phosphate with the vitamin D receptor protein in the nuclei.

The identification of a lysine residue reactive to pyridoxal-5-phosphate in the glycerol dehydrogenase from the thermophile Bacillus stearothermophilus

The glycerol dehydrogenase (GDH) from Bacillus stearothermophilus is inactivated by incubation with pyridoxal-5-phosphate (PALP). The complex formed between the two can be trapped by reduction with sodium borohydride to yield a protein with an absorbance band at 325 nm and a fluorescence emission band at 430 nm, typical of trapped pyridoxal-5-phosphate moieties. Total loss of catalytic activity of the enzyme is associated with the modification of approximately one equivalent of the reagent; the incorporation of the reagent and the loss of activity can be prevented by the additional presence of the oxidised or reduced coenzyme. Peptides derived from the labelled protein have been sequenced and have identified Lys-97 as the reactive residue. Site-directed mutagenesis had been used to replace Lys-97 by a His residue. This mutated enzyme has no catalytic activity and fluorescence spectroscopy studies suggest that it is unable to bind NADH.

The in vivo effect in humans of pyridoxal-5'-phosphate on platelet function and blood coagulation

Vitamin B6 has an antithrombotic effect. This, based on the results of in vitro studies, has been attributed to an antiplatelet effect. We assessed the in vivo effect of vitamin B6 by measuring the effect of long-term administration of vitamin B6 on platelet function and blood coagulation. Vitamin B6 (pyridoxine hydrochloride), 100mg twice daily p.o. for fifteen days, was administered to 10 healthy volunteers. The bleeding time was measured before the first dose and 15 days after. A baseline value, the acute effect, chronic effect, and the acute-on-chronic effect of vitamin B6 was estimated by measuring platelet function. The following tests were performed: platelet aggregation induced by collagen, ADP and epinephrine; thromboxane A2 (TxA2)-production and prostacyclin inhibition of ADP-induced aggregation. The effects on the coagulation system were monitored by measuring: the prothrombin time, activated partial thromboplastin time and levels of coagulation factor. Vitamin B6 significantly prolonged the bleeding time from 4.1 +/- 1.1 minutes to 6.8 +/- 1.0 minutes (p = 0.0063). Aggregation of platelets with collagen was slightly but not significantly inhibited. Platelet aggregation induced with the agonists ADP or epinephrine was significantly inhibited by vitamin B6, and the platelets tended to aggregate at a slightly decreased rate. The mean TxA2-production was slightly, but not significantly, decreased. Vitamin B6 had no effect on the sensitivity of platelets to prostacyclin, or on the coagulation system. Our results indicate that the antithrombotic effects of vitamin B6 is limited to inhibition of platelet function; there was no measurable influence on coagulation. The results of this in vivo study are however such that clinical trials are warranted to further assess the efficacy of vitamin B6 as an antiplatelet drug.

Reactivity of the fructose 1,6-bisphosphate-activated pyruvate kinase from Escherichia coli with pyridoxal 5'-phosphate

The allosteric fructose 1,6-bisphosphate-activated pyruvate kinase from Escherichia coli was modified with pyridoxal 5'-phosphate in the presence and in the absence of phosphoenolpyruvate, fructose 1,6-bisphosphate, MgADP and MgATP. In all cases a time-dependent inactivation was observed, but the rate and the extent of inactivation varied according to the conditions used. The kinetic properties of the partially inactivated enzyme were differently modified by addition of substrates and effectors to the modification mixture, the parameters mostly affected being those concerning fructose 1,6-bisphosphate. Tryptic peptides obtained from fully inactivated pyruvate kinase in the different conditions have been separated. In all conditions three main 6-pyridoxyllysine-containing peptides were present, the amounts of which showed significant differences in the presence of fructose 1,6-bisphosphate and MgADP. The function of the labelled peptides and the evidence supporting the physical existence of different conformational states are discussed. The main conclusion concerns the involvement of one of the above peptides in the binding of the allosteric effector fructose 1,6-bisphosphate.

Inactivation of rat brain acetylcholinesterase by pyridoxal 5'-phosphate

Effects of pyridoxal 5'-phosphate on the activity of crude and purified acetylcholinesterase from cerebral hemispheres of adult rat brain were examined. Acetylcholinesterase was completely inactivated by incubation with 0.5 mM pyridoxal 5'-phosphate. The enzyme activity remained unaltered in the presence of analogs of pyridoxal 5'-phosphate, pyridoxal, pyridoxamine and pyridoxamine 5'-phosphate. The inhibition of acetylcholinesterase activity by pyridoxal 5'-phosphate appeared to be of a noncompetitive nature, as determined by Lineweaver-Burk analysis. The inhibitory effect of pyridoxal 5'-phosphate on acetylcholinesterase appeared to be a general one, as the activity of the enzyme from the brains of immature chick and egg-laying hen, and from different tissues of the adult male rats, exhibited a similar pattern in the presence of the inhibitor. The inhibitory effects of pyridoxal 5'-phosphate could be reversed upon exhaustive dialysis of the pyridoxal 5'-phosphate-treated acetylcholinesterase preparations. We propose that the effects of pyridoxal 5'-phosphate are due to its interaction with acetylcholinesterase, and that it can be employed as a useful tool for studying biochemical aspects of this important brain enzyme.

The effect of pH on the interaction of substrates and effector to yeast and rabbit muscle pyruvate kinase

The interaction of fructose 1,6-bisphosphate, phosphoenolpyruvate and ADP with pyruvate kinase (ATP: pyruvate 2-O-phosphotransferase, EC 2.7.1.40) from yeast and rabbit muscle has been studied as a function of pH utilizing the quenching of protein fluorescence at 330 nm by these ligands. Both the muscle and the yeast pyruvate kinase interact with either ADP or phosphoenolpyruvate with similar affinity, indicating that the substrate-binding sites for these two isozymes are similar. The major difference between the yeast and muscle isozymes is their affinity with fructose 1,6-bisphosphate. Fructose 1,6-bisphosphate interacts with the yeast isozyme in orders of magnitude more strongly than with the muscle isozyme. Moreover, the affinity of fructose 1,6-bisphosphate to the yeast isozyme is strongly pH-dependent, while the interaction of fructose 1,6-bisphosphate with the muscle isozyme is independent of pH. The data indicate that yeast pyruvate kinase undergoes a conformational change as the pH is increased from 6.0 to 8.5.

The effect of pyridoxal-5-phosphate on the inhibition of platelet aggregation and adenosine-3'-5'-cyclic monophosphate accumulation in human platelets

Platelet adenosine-3'-5'-cyclic monophosphate (c-AMP) was determined in platelet rich plasma and washed platelets by a modification of the c-AMP protein binding assay. Studies were performed in the presence and absence of platelet aggregation inducers (adenosine diphosphate and thrombin) and inhibitors (pyridoxal-5-phosphate (PALP), prostaglandin E1 (PGE1), theophylline and papaverine). At 70% inhibitory concentration of platelet aggregation (PA) induced by papaverine or theophylline, a small but significant increase in platelet c-AMP level was found. With PALP however the inhibition of PA was not associated with a significant increase in the c-AMP level. PALP or theophylline potentiated greatly both the inhibition of PA and c-AMP accumulation induced by PGE1. Yet PALP potentiated the inhibition of PA induced by theophylline without increasing platelet c-AMP level. Our results indicate that the inhibition of PA by PALP is not mediated by c-AMP accumulation. However in the presence of PGE1, the increment in PA inhibition, is mediated by further indirect activation of the adenyl-cyclase.

Phosphopyridoxal complexes with histamine and histidine. (5) The kinetics of cyclic compound formation between histamine and pyridoxal-5'-phosphate in the presence of pig kidney diamine oxidase and rat intestinal histaminase

Pig kidney diamine oxidase (DAO) and rat intestinal histaminase (Hi-ase) activities are inhibited in vitro by high concentrations of both a substrate (histamine) and a coenzyme (pyridoxal-5'-phosphate). This inhibition may be at least partially associated with the formation of a cyclic compound between histamine (Hi) and pyridoxal-5'-phosphate (PLP). The dynamics of this cyclic compound formation in the presence of both enzymes has been examined. In an incubation mixture containing partially purified pig kidney DAO, the rate of cyclization decreased slightly as compared with a buffer. On the contrary, in the presence of crude rat intestinal histaminase, the rate of cyclization was inhibited significantly; this inhibition was proportional to the amount of enzyme preparation present in the incubation mixture. The possible mechanism of the influence of enzyme protein on the rate of cyclic compound formation, and its possible biological significance, are discussed.

Threonine deaminase from a nonsense mutant of Escherichia coli requiring isoleucine or pyridoxine: evidence for half-of-the-sites reactivity

The mutant IP7 of Escherichia coli B requires isoleucine or pyridoxine for growth as a consequence of a mutation in the gene coding for biosynthetic threonine deaminase. The mutation of IP7 was shown to be of the nonsense type by the following data: (1) reversion to isoleucine prototrophy involves the formation of external suppression at a high frequency, as shown by transduction experiments; and (ii) the isoleucine requirement is suppressed by lysogenization with a phage carrying the amber suppressor su-3. Cell extracts of the mutant strain contain a low activity of threonine deaminase. The possibility that this activity is biodegradative was ruled out by kinetic experiments. The mutant threonine deaminase was purified to homogeneity by conventional procedures. The enzyme is a dimer of identical subunits of an approximate molecular weight of 43,000 (Grimminger and Feldner, 1974), whereas the wild-type enzyme is a tetramer of 50,000-dalton subunits (Calhoun et al., 1973; Grimminger et al., 1973). The mutant enzyme is not inhibited by isoleucine and does not bind isoleucine, as shown by equilibrium dialysis experiments. Pyridoxal phosphate enhances the maximum catalytic activity of the mutant enzyme by a factor of five, whereas the wild-type enzyme is not affected. In wild-type and mutant threonine deaminase the ratio of protein subunits and bound pyridoxal phosphate is 2:1. The activation of threonine deaminase from strain IP7 is due to a second coenzyme binding site, as shown by (i) spectrophotometric titration of the enzyme with pyridoxal phosphate and by (ii) measurement the pyridoxal phosphate content of the enzyme after sodium borohydride reduction of the protein. The observation of one pyridoxal phosphate binding site per peptide dimer in the wild-type enzyme and of two binding sites per dimer in the mutant strongly suggests that one of the potential sites in the wild-type enzyme is masked by allosteric effects. The factors responsible for the half-of-the-sites reactivity of the coenzyme sites appear to be nonoperative in the mutant protein.

Phosphopyridoxal complexes with histamine and histidine. (4) The kinetics of complex formation between histidine and pyridoxal 5' -phosphate in the presence of bacterial histidine decarboxylase

The dynamics of the complex formation between pyridoxal 5'-phosphate (PLP) and histidine in the presence of bacterial histidine decarboxylase was examined. Since PLP is able to form a cyclic product with histidine and histamine, the possibility of complex formation between PLP and histamine formed during a decarboxylation reaction was examined too. It was found that the cyclization reaction between PLP and histidine is equimolecular and the rate of cyclic product formation is not significantly influenced by the presence of enzyme. In the presence of bacterial histidine decarboxylase both the cyclization reaction and cyclic product formation were observed. Predominance of histamine or cyclic product formation was dependent on pH and substrate concentration. In the presence of histidine and enzymatic protein, histamine formed during the decarboxylation reaction was unable to form a cyclic product with PLP.

Inhibition of horse muscle acylphosphatase by pyridoxal 5'-phosphate

It has been shown that horse muscle acylphosphatase is inhibited by pyridoxal 5'-phosphate and that the inhibition is pH dependent, reversible and competitive with respect to substrate binding. Spectral analysis on the EI complex demonstrates the presence of a Schiff base. Reduction of the pyridoxal 5'-phosphate-inhibited enzyme with sodium borohydride, followed by amino acid analysis, produces a diminution of the free lysine peak and the appearance of a new peak corresponding to epsilon-pyridoxyllysine. The results suggest that there is at least one NH2-lysyl residue of horse muscle acylphosphatase at or near the active site of the enzyme.

Phosphopyridoxal complexes with histamine and histidine. (1) The kinetics of complex formation between pyridoxal 5'-phosphate and histamine

Amines and amino acids are able to form cyclic compounds with pyridoxal 5'-phosphate (PLP). The quantitative relationship between histamine and PLP in the cyclization reaction were examined. It was found that one mole of amine reacts with one mole of coenzyme. The velocity of complex formation increased with an excess of either histamine or PLP, without any change in molar ratios. The formation of cyclic compound was confirmed by the use of isotopic method. The separation of cyclic compound from histamine, but not from PLP, was achieved by paper chromatography.

Phosphopyridoxal complexes with histamine and histidine. (3) The influence of presumed complex on activity of rat intestinal histaminase

Histamine in high concentration inhibits the rat intestinal histaminase (diamine oxidase E.C. 1.4.3.6.). The apparent Km is approximately 4.2 X 10(-5) M. This inhibition can be reversed by an addition of PLP. It was also found that excess of PLP inhibits enzyme activity. It is competitive inhibition. Histamine and other amines which were associated with enzyme inhibition form spectrophotometricaly demonstrable complex with PLP. The possible mechanism of the inhibitory action of PLP and complex with histamine and other amines on rat intestinal histaminase activity are discussed.