Paradoxical effects of K+ and D-600 on parathyroid hormone secretion and cytoplasmic Ca2+ in normal bovine and pathological human parathyroid cells

The effects of K+ and the Ca2+ channel blocker D-600 on parathyroid hormone (PTH) release and cytoplasmic Ca2+ activity (Ca2+i) were measured at different Ca2+ concentrations in dispersed parathyroid cells from normal cattle and from patients with hyperparathyroidism. When the extracellular Ca2+ concentration was raised within the 0.5-3.0 mM range Ca2+i increased and PTH secretion was inhibited. There was also a stimulatory effect of Ca2+ on secretion as indicated by a parallel decrease of Ca2+i and PTH release when extracellular Ca2+ was reduced to less than 25 nM. Addition of 30-50 mM K+ stimulated PTH release and lowered Ca2+i. The effect of K+ was less pronounced in the human cells with a decreased suppressability of PTH release. The Ca2+ channel blocker D-600 had no effect on Ca2+i and PTH release in the absence of extracellular Ca2+. However, at 0.5-1.0 mM Ca2+, D-600 increased Ca2+i and inhibited PTH release, whereas the opposite effects were obtained at 3.0 mM Ca2+. The transition from inhibition to stimulation occurred at a higher Ca2+ concentration in the human cells and the right-shift in the dose-effect relationship for Ca2+-inhibited PTH release tended to be normalized by D-600. It is suggested that K+ stimulates PTH release by increasing the intracellular sequestration of Ca2+ and that the reduced response in the parathyroid human cells is due to the fact that Ca2+i already is lowered. D-600 appears to have both Ca2+ agonistic and antagonistic actions in facilitating and inhibiting Ca2+ influx into the parathyroid cells at low and high concentrations of extracellular Ca2+, respectively. D-600 and related drugs are considered potentially important for the treatment of hyperparathyroidism.     

Dinitrofluorobenzene-induced rigor in ileal longitudinal smooth muscle of the guinea-pig.

1. The administration of 2,4-dinitrofluorobenzene (DFB) (0.1-1 mM) to the ileal longitudinal muscle produced contractions within seconds of its administration. 2. A component of the first 2 min duration of the phasic phase of 1 or 0.5 mM DFB contraction and the first phase of 0.35 or 0.1 mM DFB contraction was inhibited by Ca2+ antagonists, 1 x 10(-6) M D-600. 3. The DFB contraction resistant to D-600 began to develop when the tissue ATP concentration rapidly reduced. 4. The DFB contraction in ileum consists of two components; an initial fast contraction which is sensitive to Ca2+ antagonists, and a late contraction referred to as a rigor which is resistant to it.     

Blockade of Ca2+ channels inhibits K+ contractures but not twitches in skeletal muscle

The effects of the voltage-sensitive, calcium channel blocking agents, D-600 and verapamil, on twitches and K+-induced contractures were studied using frog's toe muscles. K+-contracture tension was reduced by concentrations as low as 10(-8) M and the contractures were blocked by 10(-6) M. There was no significant difference in the effects of the two drugs. Twitches were potentiated by 5 X 10(-5) M D-600 and blocked only at 3 X 10(-4) M. The latter concentration also produced contractures in the toe muscles. As shown by other workers, the higher concentration also blocks action potential production and this is probably the way in which it blocks the twitch. Raising the bathing solution Ca2+ concentration from 1.08 to 10 or 20 mM, produced only a small, inconsistent, noncompetitive antagonism of the D-600 block of K+ contractures.     

Maitotoxin-Induced Release of γ-[3H]Aminobutyric Acid from Cultures of Striatal Neurons

The potent marine toxin, maitotoxin, induced the release of gamma-[3H]aminobutyric acid (GABA) from reaggregate cultures of striatal neurons in a dose-dependent manner. Maitotoxin-induced release occurred following a lag period of several minutes and was persistent. Release induced by 70 mM K+ on the other hand was immediate and transient in nature. Co2+ (3 mM) and Cd2+ (1 mM) inhibited maitotoxin-induced release of GABA as did removal of extracellular Ca2+. However, the organic calcium antagonists nisoldipine, nitrendipine, and D-600 at concentrations of 10(-6) M did not block maitotoxin-induced or 70 mM K+-induced release. High concentrations of D-600 (10(-4) M) partially blocked both maitotoxin- and 70 mM K+-induced release. The dihydropyridine calcium agonist BAY K8644 (10(-6) M) did not enhance maitotoxin-induced or 70 mM K+-induced release. Replacement of Na+ in the incubation medium with choline led to an increased basal output of GABA and an apparent inhibition of the effect of maitotoxin. These data are discussed with reference to the hypothesis that maitotoxin can directly activate voltage-sensitive calcium channels.     

Role of calcium channel in postvagal potentiation of contraction as evident from the effects of Mn2+, La3+, D-600, and deoxycholate on isolated guinea pig atria-vagus nerve preparation

The role of the calcium channel in the first large contraction (postvagal potentiation, PVP) of the atria at the end of the inhibitory phase of its response (IPR) to vagal stimulation has been investigated by studying the effects of agents acting on the calcium channel (e.g., Ca2+, Mn2+, La3+, and D-600) or sarcoplasmic reticulum (SR) (e.g., deoxycholate (DOC)). IPR was potentiated by high [Ca2+]o (3-16 mM) and also by the calcium channel blockers, Mn2+ (1 microM-0.5 mM), La3+ (0.1 microM-0.5 mM), D-600 (1.0-10 microM), and DOC (1 microM-0.5 mM). PVP was also potentiated by enhanced [Ca2+]o, but the PVP ratio, which employs a correction for the simultaneous changes in the force of spontaneous contraction was inhibited. This indicated greater potentiation of contractility during spontaneous activity by Ca2+ than during PVP. Mn2+, La3+, and D-600 and even DOC in the above concentrations inhibited PVP but increased the PVP ratio. High concentrations of DOC (greater than 1 mM), which disrupt SR, strongly inhibited PVP. It is concluded that the calcium channel plays a more prominent role in spontaneous contractions than in PVP in guinea pig atria. PVP is suggested to be generated by excessive triggered release of Ca2+ from SR leading to a marked increase in [Ca2+]i. The calcium channel and the calcium trapped in the glycocalyx also play significant roles in PVP.     

Possible role of carbonic anhydrase in rat pancreatic islets: enzymatic, secretory, metabolic, ionic, and electrical aspects

The presence of carbonic anhydrase (type V) was recently documented in rat and mouse pancreatic islet beta-cells by immunostaining and Western blotting. In the present study, the activity of carbonic anhydrase was measured in rat islet homogenates and shown to be about four times lower than in rat parotid cells. The pattern for the inhibitory action of acetazolamide on carbonic anhydrase activity also differed in islet and parotid cell homogenates, suggesting the presence of different isoenzymes. NaN3 inhibited carbonic anhydrase activity in islet homogenates and both D-[U-14C]glucose oxidation and glucose-stimulated insulin secretion. Acetazolamide (0.3-10.0 mM) also decreased glucose-induced insulin output but failed to affect adversely D-[U-14C]glucose oxidation, although it inhibited the conversion of D-[5-3H]glucose to [3H]OH and that of D-[U-14C]glucose to acidic metabolites. Hydrochlorothiazide (3.0-10.0 mM), which also caused a concentration-related inhibition of the secretory response, like acetazolamide (5.0-10.0 mM), decreased H(14)CO3- production from D-[U-14C]glucose (16.7 mM). Acetazolamide (5.0 mM) did not affect the activity of volume-sensitive anion channels in beta-cells but lowered intracellular pH and adversely affected both the bioelectrical response to d-glucose and its effect on the cytosolic concentration of Ca2+ in these cells. The lowering of cellular pH by acetazolamide, which could well be due to inhibition of carbonic anhydrase, might in turn account for inhibition of glycolysis. The perturbation of stimulus-secretion coupling in the beta-cells exposed to acetazolamide may thus involve impaired circulation in the pyruvate-malate shuttle, altered mitochondrial Ca2+ accumulation, and perturbation of Cl- fluxes, resulting in both decreased bioelectrical activity and insulin release.     

Hexose metabolism in pancreatic islets. Regulation of aerobic glycolysis and pyruvate decarboxylation.

1. D-Glucose (0.5-16.7 mM) preferentially stimulates aerobic glycolysis and D-[3,4-14C]glucose oxidation, relative to D-[5-3H]glucose utilization in rat pancreatic islets, the concentration dependency of such a preferential effect displaying a sigmoidal pattern. 2. Inorganic and organic calcium antagonists, as well as Ca2+ deprivation, only cause a minor decrease in the ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose utilization in islets exposed to a high concentration of the hexose (16.7 mM). 3. Non-glucidic nutrient secretagogues such as 2-aminobicyclo[2,2,1]heptane-2-carboxylate (BCH), 2-ketoisocaproate and 3-phenylpyruvate fail to stimulate aerobic glycolysis and D-[3,4-14C]glucose oxidation in islets exposed to 6.0 mM D-glucose. Nevertheless, BCH augments [1-14C]pyruvate and [2-14C]pyruvate oxidation. 4. The glucose-induced increment in the paired ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose utilization is impaired in the presence of either cycloheximide or ouabain. 5. These findings suggest that the preferential effect of D-glucose upon aerobic glycolysis and pyruvate decarboxylation is not attributable solely to a Ca(2+)-induced activation of FAD-linked glycerophosphate dehydrogenase and/or pyruvate dehydrogenase, but may also involve an ATP-modulated regulatory process.     

The ionic control of 1,25-dihydroxyvitamin D-3 synthesis in isolated chick renal mitochondria. The role of potassium.

In previous studies it was found that change in the concentrations of Ca2+, H+, and HPO2-4 in the incubation medium altered the rates of synthesis of 1,25-dihydroxyvitamin D-3 (1,25(OH)2D-3) by isolated renal mitochondria obtained from D-deficient chicks. The present studies demonstrate that raising the medium concentration of K+ from 1 to 50 mM leads to a 6-fold increase in rate of 1,25(OH)2D-3 synthesis by isolated chick mitochondria; that the magnitnitude of this K+-dependent stimulation is enhaced by optimal concentrations of calcium (pCa = 5) and phosphate (pPi = 3) (3 mM) but not by pH (from 6.8 to 7.4); that the effect is not produced by similar changes in media Na+ concentration; and that the stimulatory effect of K+ is not blocked by ruthenium red, and inhibitor of calcium transport and of the calcium-dependent stimulation of mitochondrial 1,25(OH) 2D-3 synthesis. It was also found that valinomycin, a K+-specific ionophore, enhanced the sensitivity of the mitochondrial 1 alpha-hydroxylase activity to K+. In the presence of valinomycin, an increase of pK+ to 3 was sufficient to cause a significant stimulation of 1,25(OH)2D-3 synthesis. It was concluded that changes in the ion content of the mitochondrial matrix space regulated the activity of the 1 alpha-hydroxylase.     

The blocking effect of magnesium on the secretion of adrenal catecholamines induced by the omission of sodium from the extracellular medium.

The blocking action of Mg++ on catecholamine release induced by the substitution of extracellular Na+ by an osmotic equivalent amount of sucrose was studied in isolated, perfused bovine adrenal glands. Perfusing glands with 10 mM Mg++ produced at 51.1% inhibition on catecholamine release evoked by Na+ omission. Increasing the concentration of Mg++ to 20 mM this inhibitory effect was enhanced to 90.3%. D-600 (0.3 mM) promoted a marked blockade of acetylcholine-induced release of catechol hormones that was partially and significantly reverted increasing the concentration of Ca++ in the perfusion medium. D-600 (0.3 mM) failed to inhibit the catecholamine-releasing effect of Na+ deprivation. In adrenal glands previously perfused with D-600 (0.3 mM) and then exposed to a Locke solution containing D-600 (0.3 mM) + Mg++ (10 or 20 mM) the inhibition of the secretory responses evoked by the omission of Na+ was of the same magnitude as that obtained when the glands were perfused with Mg++ (10 or 20 mM) in the absence of D-600. These results are compatible with the view that the blocking effect of Mg++ may involve an intracellular site of action and that the access of Mg++ into the chromaffin cell may not be mediated through the Ca++ channels.     

Lack of coupling of D-2 receptors to adenylate cyclase in GH-3 cells exposed to epidermal growth factor. Possible role of a differential expression of Gi protein subtypes.

Exposure of GH-3 cells to epidermal growth factor for 4 consecutive days induced the expression of both D-2(415) and D-2(444) dopamine-receptor isoforms. Epidermal growth factor also promoted a remarkable increase in the content of Gi3 protein, which is responsible for receptor-induced activation of potassium channels in GH-3 cells. D-2 receptors in this model apparently activate a specific transducing pathway, leading to opening of potassium channels and inhibition of prolactin release by cAMP-independent mechanisms. This is shown by: 1) the selective D-2 agonist quinpirole, while inactive on vasoactive intestinal peptide-induced prolactin release, strongly inhibited the hormone secretion induced by neurotensin; 2) quinpirole, up to 100 microM, did not inhibit cAMP production evoked by vasoactive intestinal peptide both in intact cells and in broken cell membrane preparations; and 3) quinpirole and other D-2 agonists strongly potentiated Rb+ efflux when measured in a nominally calcium-free reaction solution containing 100 mM potassium (voltage-dependent component), but did not modify Rb+ efflux if measured in a reaction solution containing 1 mM calcium and 5 mM potassium (calcium-activated, cAMP-dependent component).     

Identification and characterization of coenzyme B12-dependent glycerol dehydratase- and diol dehydratase-encoding genes from metagenomic DNA libraries derived from enrichment cultures

To isolate genes encoding coenzyme B(12)-dependent glycerol and diol dehydratases, metagenomic libraries from three different environmental samples were constructed after allowing growth of the dehydratase-containing microorganisms present for 48 h with glycerol under anaerobic conditions. The libraries were searched for the targeted genes by an activity screen, which was based on complementation of a constructed dehydratase-negative Escherichia coli strain. In this way, two positive E. coli clones out of 560,000 tested clones were obtained. In addition, screening was performed by colony hybridization with dehydratase-specific DNA fragments as probes. The screening of 158,000 E. coli clones by this method yielded five positive clones. Two of the plasmids (pAK6 and pAK8) recovered from the seven positive clones contained genes identical to those encoding the glycerol dehydratase of Citrobacter freundii and were not studied further. The remaining five plasmids (pAK2 to -5 and pAK7) contained two complete and three incomplete dehydratase-encoding gene regions, which were similar to the corresponding regions of enteric bacteria. Three (pAK2, -3, and -7) coded for glycerol dehydratases and two (pAK4 and -5) coded for diol dehydratases. We were able to perform high-level production and purification of three of these dehydratases. The glycerol dehydratases purified from E. coli Bl21/pAK2.1 and E. coli Bl21/pAK7.1 and the complemented hybrid diol dehydratase purified from E. coli Bl21/pAK5.1 were subject to suicide inactivation by glycerol and were cross-reactivated by the reactivation factor (DhaFG) for the glycerol dehydratase of C. freundii. The activities of the three environmentally derived dehydratases and that of glycerol dehydratase of C. freundii with glycerol or 1,2-propanediol as the substrate were inhibited in the presence of the glycerol fermentation product 1,3-propanediol. Taking the catalytic efficiency, stability against inactivation by glycerol, and inhibition by 1,3-propanediol into account, the hybrid diol dehydratase produced by E. coli Bl21/pAK5.1 exhibited the best properties of all tested enzymes for application in the biotechnological production of 1,3-propanediol.     

Adenosine modulation of D-[3H]aspartate release in cultured retina cells exposed to oxidative stress

In this study we evaluated the role of adenosine receptor activation on the K+-evoked D-[3H]aspartate release in cultured chick retina cells exposed to oxidant conditions. Oxidative stress, induced by ascorbate (3.5 mM)/Fe2+ (100 microM), increased by about fourfold the release of D-[3H]aspartate, evoked by KCl 35 mM in the presence and in the absence of Ca2+. The agonist of A1 adenosine receptors, N6-cyclopentyladenosine (CPA; 10 nM), inhibited the K+-evoked D-[3H]aspartate release in control in oxidized cells. The antagonist of A1 adenosine receptor, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 50 nM), potentiated the release of D-[3H]aspartate in oxidized cells, and reverted the effect observed in the presence of CPA 10 nM. However, in oxidized cells, when DPCPX was tested together with CPA 100 nM the total release of D-[3H]aspartate increased from 5.1 +/- 0.4% to 11.4 +/- 1.0%, this increase being reverted by 3,7-dimethyl-1-propargylxanthine (DMPX; 100 nM), an antagonist of A2A adenosine receptors. In cells of both experimental conditions, the K+-evoked release of D-[3H]aspartate was potentiated by the selective agonist of A2A adenosine receptors, 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosin e (CGS 21680; 10 nM), whereas the antagonist of these receptors, DMPX (100 nM), inhibited the release of D-[3H]aspartate in oxidized cells, but not in control cells. Adenosine deaminase (ADA; 1 U/ml), which is able to remove adenosine from the synaptic space, reduced the K+-evoked D-[3H]aspartate release, from 5.1 +/- 0.4% to 3.1 +/- 0.3% in oxidized cells, and had no significant effect in control cells. The extracellular accumulation of endogenous adenosine, upon K+-depolarization, was higher in oxidized cells than in control cells, and was reduced by the inhibitors of adenosine transporter (NBTI) and of ecto-5'-nucleotidase (AOPCP). This suggests that adenosine accumulation resulted from the outflow of adenosine mediated by the transporter, and from extracellular degradation of adenine nucleotide. Our data show that both inhibitory A1 and excitatory A2A adenosine receptors are present in cultured retina cells, and that the K+-evoked D-[3H]aspartate release is modulated by the balance between inhibitory and excitatory responses. Under oxidative stress conditions, the extracellular accumulation of endogenous adenosine seems to reach levels enough to potentiate the release of D-[3H]aspartate by the tonic activation of A2A adenosine receptors.     

Purification of an inducible L-valine dehydrogenase of Streptomyces coelicolor A3(2)

Valine dehydrogenase (VDH) from Streptomyces coelicolor A3(2) was purified from cell-free extracts to apparent homogeneity. The enzyme had an Mr 41,000 in denaturing conditions and an Mr 70,000 by gel filtration chromatography, indicating that it is composed of two identical subunits. It oxidized L-valine and L-alpha-aminobutyric acid efficiently, L-isoleucine and L-leucine less efficiently, and did not act on D-valine. It required NAD+ as cofactor and could not use NADP+. Maximum dehydrogenase activity with valine was at pH 10.5 and the maximum reductive amination activity with 2-oxoisovaleric acid and NH4Cl was at pH 9. The enzyme exhibited substrate inhibition in the forward direction and a kinetic pattern with NAD+ that was consistent with a sequential ordered mechanism with non-competitive inhibition by valine. The following Michaelis constants were calculated from these data: L-valine, 10.0 mM; NAD+, 0.17 mM; 2-oxoisovalerate, 0.6 mM; and NADH, 0.093 mM. In minimal medium, VDH activity was repressed in the presence of glucose and NH4+, or glycerol and NH4+ or asparagine, and was induced by D- and L-valine. The time required for full induction was about 24 h and the level of induction was 2- to 23-fold.     

Study of mechanisms mediating contraction to leukotriene C4, D4 and other bronchoconstrictors on guinea pig trachea

Contractions of guinea pig trachea in the absence and presence of indomethacin to LTD4 greater than LTC4 greater than K+ greater than histamine greater than acetylcholine were reduced following a 45 minute exposure of the tissues to calcium-free Krebs' solution (Ca2+-free Krebs' solution), were further reduced by a transient exposure to EGTA (1.25 mM) in Ca2+-free Krebs' solution and were virtually abolished when tested in the presence of EGTA (0.125 mM) in Ca2+-free Krebs' solution. In normal Krebs' solution (2.5 mM Ca2+) the Ca2+ entry blockers nifedipine (N) much greater than D-600 greater than verapamil (V) greater than diltiazem (D) almost completely abolished the contractions to K+ but blocked only a component of the maximum response to the other agonists. After exposure to Ca2+-free Krebs' solution for 45 minutes, any residual contractions to LTC4 & LTD4, were reversed by low concentrations of N (0.3 microM) or D-600 (2.1 microM). Leukotrienes appear to mobilize a superficial and a bound store of Ca2+ which gains entry through at least two types of Ca2+ channels (or mechanisms), one of which is blocked by N and D600. K+-induced contractions appear to be dependent on superficial and tightly bound Ca2+ but entry is solely through channels which are blocked by the Ca2+ entry blockers studied. Contraction to histamine and acetylcholine persisted following exposure of the tissues to Ca2+ free Krebs' solution but contractile activity was virtually abolished in Ca2+ free Krebs' solution containing EGTA. Residual contractions to histamine and part of the residual contractions to acetylcholine in Ca2+-free Krebs' solution were blocked by low dose N (0.3 microM) or D600 (2.1 microM). These findings suggest a major role for extracellular Ca2+ during spasmogen-induced contraction in this tissue.     

Calcium accumulation by chick intestinal mitochondria. Regulation by vitamin D-3 and 1,25-dihydroxyvitamin D-3

We have the evaluated the effect of vitamin D-3 and its metabolite 1,25-dihydroxyvitamin D-3 on Ca2+ accumulation by chick intestinal mitochondria. Ca2+ accumulation appears to occur in two phases: an early, transient accumulation into an Na+-labile pool followed by an ATP-dependent accumulation into an Na+-resistant pool. Ca2+ accumulation is extensive at free Ca2+ concentrations greater than 3 . 10(-6) M in the presence of ATP. Ruthenium red and dinitrophenol block Ca2+ accumulation, but atractyloside does not. Oligomycin blocks ATP-supported accumulation completely with a partial inhibition of ATP and malate-supported accumulation. Little difference could be found in mitochondrial preparations from vitamin D-deficient chicks compared to those from vitamin D-3 (or 1,25(OH)2D-3)-supplemented chicks with respect to respiratory control, oxygen consumption, efficiency of oxidative phosphorylation, affinity for Ca2+, or the rate and extent of ATP-supported Ca2+ accumulation. Intestinal cytosol stimulated Ca2+ accumulation, but this was not specific with respect to vitamin D status or tissue of origin, nor was it duplicated by chick intestinal Ca2+-binding protein. 30 ng/ml 1,25(OH)2D-3 stimulated Ca2+ accumulation directly, regardless of the presence of intestinal cytosol. Other vitamin D metabolites were less potent: 25-hydroxyvitamin D-3 greater than 24,25-dihydroxyvitamin D-3 = vitamin D-3. Since increasing the free Ca2+ concentration from 3 . 10(-6) to 1 . 10(-5) M increased Ca2+ accumulation approx. 50-fold, whereas direct stimulation by 1,25(OH)2D-3 in vitro increased Ca2+ accumulation less than 2-fold, we conclude that 1,25(OH)2D-3 influences mitochondrial accumulation of Ca2+ in vivo primarily by altering cytosol concentrations of free Ca2+.     

Impaired enzyme-to-enzyme channelling between hexokinase isoenzyme(s) and phosphoglucoisomerase in rat pancreatic islets incubated at a low concentration of D-glucose

It was recently proposed that in rat pancreatic islets exposed to 8.3 mM D-glucose, alpha-D-glucose-6-phosphate undergoes enzyme-to-enzyme channelling between hexokinase isoenzyme(s) and phosphoglucoisomerase. To explore the identity of the hexokinase isoenzyme(s) involved in such a tunnelling process, the generation of 3HOH from the alpha- and beta-anomers of either D-[2-3H]glucose or D-[5-3H]glucose was now measured over 60 min incubation at 4 degrees C in pancreatic islets exposed only to 2.8 mM D-glucose, in order to decrease the relative contribution of glucokinase to the phosphorylation of the hexose. Under these experimental conditions, the ratio for 3HOH production from D-[2-3H]glucose/D-[5-3H]glucose at anomeric equilibrium (39.7 +/- 11.6%) and the beta/alpha ratios for the generation of 3HOH from either the D-[2-3H]glucose anomers (70.9 +/- 12.6%) or the D-[5-3H]glucose anomers (59.6 +/- 12.4%) indicated that a much greater fraction of alpha-D-glucose-6-phosphate escapes from the process of enzyme-to-enzyme channelling in the islets exposed to 2.8 mM, rather than 8.3 mM D-glucose. These findings suggest, therefore, that the postulated process of enzyme-to-enzyme channelling involves mainly glucokinase.     

Metabolism of alpha-D-[1,2-13C]glucose pentaacetate and alpha-D-glucose penta[2-13C]acetate in rat hepatocytes

Hepatocytes from fed rats were incubated for 120 min in the presence of alpha-D-[1,2-13C]glucose pentaacetate (1.7 mM), both D-[1,2-13C]glucose (1.7 mM) and acetate (8.5 mM), alpha-D-glucose penta[2-13C]acetate (1.7 mM), or D-[1,2-13C]glucose (8.3 mM). The amounts of 13C-enriched L-lactate and D-glucose and those of acetate and beta-hydroxybutyrate recovered in the incubation medium were comparable under the first two experimental conditions. The vast majority of D-glucose isotopomers consisted of alpha- and beta-D[1,2-13C]glucose. The less abundant single-labeled isotopomers of D-glucose were equally labeled on each C atom. The output of 13C-labeled L-lactate, mainly L-[2-13C]lactate and L-[3-13C]lactate, was 1 order of magnitude lower than that found in hepatocytes exposed to 8.3 mM D-[1,2-13C]glucose, in which case the total production of the single-labeled species of D-glucose was also increased and that of the C3- or C4-labeled hexose was lower than that of the other 13C-labeled isotopomers. In cells exposed to alpha-D-glucose penta[2-13C]acetate, the large majority of 13C atoms was recovered as [2-13C]acetate and, to a much lesser extent, beta-hydroxybutyrate labeled in position 2 and/or 4. Nevertheless, L-[2-13C]lactate, L-[3-13C]lactate, and single-labeled D-glucose isotopomers were also produced in amounts higher or comparable to those found in cells exposed to alpha-D-[1,2-13C]glucose pentaacetate. However, a modest preferential labelling of the C6-C5-C4 moiety of D-glucose, relative to its C1-C2-C3 moiety, and a lesser isotopic enrichment of the C3 (or C4), relative to that of C1 (or C6) and C2 (or C5), were now observed. These findings indicate that, despite extensive hydrolysis of alpha-D-glucose pentaacetate (1.7 mM) in the hepatocytes, the catabolism of its D-glucose moiety is not more efficient than that of unesterified D-glucose, tested at the same molar concentration (1.7 mM) in the presence of the same molar concentration of unesterified acetate (8.5 mM), and much lower than that found at a physiological concentration of the hexose (8.3 mM). The present results also argue against any significant back-and-forth interconversion of D-glucose 6-phosphate and triose phosphates, under conditions in which sizeable amounts of D-glucose are formed de novo from 13C-enriched Krebs cycle intermediates generated from either D-[1,2-13C]glucose or [2-13C]acetate.     

Amino acid metabolism in the thermophilic phototroph,Chloroflexus aurantiacus: properties and metabolic role of two l-threonine (l-serine) dehydratases

Two l-threonine (l-serine) dehydratases (EC 4.2.1.16) of the thermophilic phototrophic bacterium Chloroflexus aurantiacus Ok-70-fl were purified to electrophoretic homogeneity by procedures involving anion exchange and hydrophobic interaction chromatography. Only one of the two enzymes was sensitive to inhibition by l-isoleucine (K _i=2 M) and activation by l-valine. The isoleucine-insensitive dehydratase was active with l-threonine (K _m=20 mM) as well as with l-serine (K _m=10 mM) whereas the other enzyme, which displayed much higher affinity to l-threonine (K _m=1.3 mM), was inactivated when acting on l-serine. Both dehydratases contained pyridoxal-5-phosphate as cofactor. When assayed by gel filtration techniques at 20 to 25° C, the molecular weights of both enzymes were found to be 106,000±6,000. In sodium dodecylsulfate-polyacrylamide gel electrophoresis, the two dehydratases yielded only one type of subunit with a molecular weight of 55,000±3,000. The isoleucine-insensitive enzyme was subject to a glucose-mediated catabolite repression.Abbreviations A absorbance - ile isoleucine - PLP pyridoxal-5-phosphate - SDS sodium dodecyl sulfate - TDH threonine dehydratase - U unit     

Isolation and characterization of three new classes of transformation-deficient mutants of Streptococcus pneumoniae that are defective in DNA transport and genetic recombination

A bifunctional enzyme, L-(+)-tartrate dehydrogenase-D-(+)-malate dehydrogenase (decarboxylating) (EC 1.1.1.93 and EC 1.1.1. . . , respectively), was discovered in cells of Rhodopseudomonas sphaeroides Y, which accounts for the ability of this organism to grow on L-(+)-malate. The enzyme was purified 110-fold to homogeneity with a yield of 51%. During the course of purification, including ion-exchange chromatography and preparative gel electrophoresis, both enzyme activities appeared to be in association. The ratio of their activities remained almost constant [1:10, L-(+)-tartrate dehydrogenase/D-(+)-malate dehydrogenase (decarboxylating)] throughout all steps of purification. Analysis by polyacrylamide gel electrophoresis revealed the presence of a single protein band, the position of which was coincident with both L-(+)-tartrate dehydrogenase and D-(+)-malate dehydrogenase (decarboxylating) activities. The apparent molecular weight of the enzyme was determined to be 158,000 by gel filtration and 162,000 by ultracentrifugation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis yielded a single polypeptide chain with an estimated molecular weight of 38,500, indicating that the enzyme consisted of four subunits of identical size. The isoelectric point of the enzyme was between pH 5.0 and 5.2. The enzyme catalyzed the NAD-linked oxidation of L-(+)-tartrate as well as the oxidative decarboxylation of D-(+)-malate. For both reactions, the optimal pH was in a range from 8.4 to 9.0. The activation energy of the reaction (delta Ho) was 71.8 kJ/mol for L-(+)-tartrate and 54.6 kJ/mol for D-(+)-malate. NAD was required as a cosubstrate, and optimal activity depended on the presence of both Mn2+ and NH4+ ions. The reactions followed Michaelis-Menten kinetics, and the apparent Km values of the individual reactants were determined to be: L-(+)-tartrate, 2.3 X 10(-3) M; NAD, 2.8 X 10(-4) M; and Mn2+, 1.6 X 10(-5) M with respect to L-(+)-tartrate; and D-(+)-malate, 1.7 X 10(-4) M; NAD, 1.3 X 10(-4); and Mn2+, 1.6 X 10(-5) M with respect to D-(+)-malate. Of a variety of compounds tested, only meso-tartrate, oxaloacetate, and dihydroxyfumarate were effective inhibitors. meso-Tartrate and oxaloacetate caused competitive inhibition, whereas dihydroxyfumarate caused noncompetitive inhibition. The Ki values determined for the inhibitors were, in the above sequence, 1.0, 0.014, and 0.06 mM with respect to L-(+)-tartrate and 0.28, 0.012, and 0.027 mM with respect to D-(+)-malate.     

Ca entry at rest and during prolonged depolarization in dialyzed squid axons

Ca influx has been studied in squid axons under internal dialysis control. In axons dialyzed with "normal" physiological conditions (Nai = 40-50 mM, Cai2+ = 0.06-0.1 microM, ATP = 2 mM, Ki = 310 mM), 70% of the resting Ca influx is sensitive to external TTX (K0.5 congruent to 5 nM), 20% of it can be accounted by the reversal of the Na-Ca exchange, and the remaining fraction (10%) is insensitive to TTX, D-600, and Nai. The Ca antagonic drug D-600 (50-100 microM) has an inhibitory effect on the resting Ca influx. This compound was found to affect both the TTX sensitive and the Nai-dependent Ca influx components. In the presence of Nai and ATP, Cai2+ activates the carrier mediated Ca entry (Nai-dependent Ca influx). Most of the activation occurs in the submicromolar range of Cai2+ concentrations (K0.5 congruent to 0.6 microM). In the absence of Nai and/or ATP, no activation of Ca influx by Cai2+ was found up to about 5 microM Cai2+. Prolonged depolarization with high Ko causes an increase in Ca influx sustained for long time (minutes). Depolarizing the axons by removing Ki causes the same effect. This depolarization-induced Ca entry was only observed in axons containing Nai. In the absence of Nai, Ca influx decreases with increasing Ko. The activation of the carrier mediated Ca entry (electrogenic Na/Ca exchange) by membrane depolarization was found to be markedly dependent on the magnitude of Ca2+ i. Increasing the magnitude of Ca2+ i from 0.1 to 0.6 microM causes a ten fold increase in the extra Ca influx induced by a K-depolarization.