Are calcium-dependent protein kinases involved in the regulation of glycolytic/gluconeogenetic enzymes? Studies with Ca2+/calmodulin-dependent protein kinase and protein kinase C

Local energy on demand: Are 'spontaneous' astrocytic Ca2+-microdomains the regulatory unit for astrocyte-neuron metabolic cooperation?

Astrocytes are a neural cell type critically involved in maintaining brain energy homeostasis as well as signaling. Like neurons, astrocytes are a heterogeneous cell population. Cortical astrocytes show a complex morphology with a highly branched aborization and numerous fine processes ensheathing the synapses of neighboring neurons, and typically extend one process connecting to blood vessels. Recent studies employing genetically encoded fluorescent calcium (Ca²⁺) indicators have described 'spontaneous' localized Ca²⁺-transients in the astrocyte periphery that occur asynchronously, independently of signals in other parts of the cells, and that do not involve somatic Ca²⁺ transients; however, neither it is known whether these Ca²⁺-microdomains occur at or near neuronal synapses nor have their molecular basis nor downstream effector(s) been identified. In addition to Ca²⁺ microdomains, sodium (Na⁺) transients occur in astrocyte subdomains, too, most likely as a consequence of Na⁺ co-transport with the neurotransmitter glutamate, which also regulates mitochondrial movements locally - as do cytoplasmic Ca²⁺ levels. In this review, we cover various aspects of these local signaling events and discuss how structural and biophysical properties of astrocytes might foster such compartmentation. Astrocytes metabolically interact with neurons by providing energy substrates to active neurons. As a single astrocyte branch covers hundreds to thousands of synapses, it is tempting to speculate that these metabolic interactions could occur localized to specific subdomains of astrocytes, perhaps even at the level of small groups of synapses. We discuss how astrocytic metabolism might be regulated at this scale and which signals might contribute to its regulation. We speculate that the astrocytic structures that light up transiently as Ca²⁺-microdomains might be the functional units of astrocytes linking signaling and metabolic processes to adapt astrocytic function to local energy demands. The understanding of these local regulatory and metabolic interactions will be fundamental to fully appreciate the complexity of brain energy homeostasis as well as its failure in disease and may shed new light on the controversy about neuron-glia bi-directional signaling at the tripartite synapse.

Activity-dependent modulation of intracellular ATP in cultured cortical astrocytes

Brain function is absolutely dependent on an appropriate supply of energy. A shortfall in supply-as occurs, for instance, following stroke-can lead rapidly to irreversible damage to this vital organ. While the consequences of pathophysiological energy depletion have been well documented, much less is known about the physiological energy dynamics of brain cells, although changes in the intracellular concentration of adenosine triphosphate (ATP), the major energy carrier of cells, have been postulated to contribute to cellular signaling. To address this issue more closely, we have investigated intracellular ATP in cultured primary cortical astrocytes by time-lapse microscopy using a genetically encoded fluorescent sensor for ATP. The cytosolic ATP sensor signal decreased after application of the neurotransmitter glutamate in a manner dependent on both glutamate concentration and glutamate transporter activity, but independent of glutamate receptors. The application of dopamine did not affect ATP levels within astrocytes. These results confirm that intracellular ATP levels in astrocytes do indeed respond to changes in physiological activity and pave the way for further studies addressing factors that affect regulation of ATP. © 2017 Wiley Periodicals, Inc.

In vivo phosphoproteome of human skeletal muscle revealed by phosphopeptide enrichment and HPLC-ESI-MS/MS

Protein phosphorylation plays an essential role in signal transduction pathways that regulate substrate and energy metabolism, contractile function, and muscle mass in human skeletal muscle. Abnormal phosphorylation of signaling enzymes has been identified in insulin-resistant muscle using phosphoepitope-specific antibodies, but its role in other skeletal muscle disorders remains largely unknown. This may be in part due to insufficient knowledge of relevant targets. Here, we therefore present the first large-scale in vivo phosphoproteomic study of human skeletal muscle from 3 lean, healthy volunteers. Trypsin digestion of 3-5 mg human skeletal muscle protein was followed by phosphopeptide enrichment using SCX and TiO(2). The resulting phosphopeptides were analyzed by HPLC-ESI-MS/MS. Using this unbiased approach, we identified 306 distinct in vivo phosphorylation sites in 127 proteins, including 240 phosphoserines, 53 phosphothreonines, and 13 phosphotyrosines in at least 2 out of 3 subjects. In addition, 61 ambiguous phosphorylation sites were identified in at least 2 out of 3 subjects. The majority of phosphoproteins detected are involved in sarcomeric function, excitation-contraction coupling (the Ca(2+)-cycle), glycolysis, and glycogen metabolism. Of particular interest, we identified multiple novel phosphorylation sites on several sarcomeric Z-disk proteins known to be involved in signaling and muscle disorders. These results provide numerous new targets for the investigation of human skeletal muscle phosphoproteins in health and disease and demonstrate feasibility of phosphoproteomics research of human skeletal muscle in vivo.

Use of a novel method to find substrates of protein kinase C delta identifies M2 pyruvate kinase

Protein kinase C (PKC) family members have been implicated in numerous cellular processes. However, identifying the substrates of each PKC isozyme remains a challenge. Here, we describe a method using two-dimensional (2D) isoelectric focusing gel electrophoresis to identify substrates of delta PKC (deltaPKC) in MCF-7 breast carcinoma cells. We show that M2 pyruvate kinase is a substrate of deltaPKC, and further characterize the interaction between M2 pyruvate kinase and deltaPKC in MCF-7 cells by immunoprecipitation. deltaPKC activation in vitro or in cells did not appear to alter the enzyme activity or polymerization of M2 pyruvate kinase.

High osmolarity glycerol (HOG) pathway-induced phosphorylation and activation of 6-phosphofructo-2-kinase are essential for glycerol accumulation and yeast cell proliferation under hyperosmotic stress

In response to changes in the environment, yeast cells coordinate intracellular activities to optimize survival and proliferation. The transductions of diverse extracellular stimuli are exerted through multiple mitogen-activated protein kinase (MAPK) cascades. The high osmolarity glycerol (HOG) MAPK pathway is activated by increased environmental osmolarity and results in a rise of the cellular glycerol concentration to adapt the intracellular osmotic pressure. We studied the importance of the short time regulation of glycolysis under hyperosmotic stress for the survival and proliferation of yeast cells. A stimulation of the HOG-MAPK pathway by increasing the medium osmolarity through addition of salt or glucose to cultivated yeast leads to an activation of 6-phosphofructo-2-kinase (PFK2), which is accompanied by a complex phosphorylation pattern of the enzyme. An increase in medium osmolarity with 5% NaCl activates PFK2 3-fold over the initial value. This change in the activity is the result of a 4-fold phosphorylation of the enzyme mediated by protein kinases from the HOG-MAPK pathway. In the case of hyperosmolar glucose a 5-fold PFK2 activation was achieved by a single phosphorylation with protein kinase A near the carboxyl terminus of the protein on Ser(644) and an additional 5-fold phosphorylation within the same amino-terminal fragment as in the presence of salt. The effect of hyperosmolar glucose is the result of an activation of the Ras-cAMP pathway together with the HOG-MAPK pathway. The activation of PFK2 leads to an activation of the upper part of glycolysis, which is a precondition for glycerol accumulation. Yeast cells containing PFK2 accumulate three times more glycerol than cells lacking PFK2, which are not able to grow under hypertonic stress.

Plasma membrane Ca(2+)-ATPase overexpression reduces Ca(2+) oscillations and increases insulin release induced by glucose in insulin-secreting BRIN-BD11 cells

In the mouse beta-cell, glucose generates large amplitude oscillations of the cytosolic-free Ca(2+) concentration ([Ca(2+)](i)) that are synchronous to insulin release oscillations. To examine the role played by [ Ca(2+)](i) oscillations in the process of insulin release, we examined the effect of plasma membrane Ca(2+)-ATPase (PMCA) overexpression on glucose-induced Ca(2+) oscillations and insulin release in BRIN-BD11 cells. BRIN-BD11 cells were stably transfected with PMCA2wb. Overexpression could be assessed at the mRNA and protein level, with appropriate targeting to the plasma membrane assessed by immunofluorescence and the increase in PMCA activity. In response to K(+), overexpressing cells showed a markedly reduced rise in [Ca(2+)](i). In response to glucose, control cells showed large amplitude [Ca(2+)](i) oscillations, whereas overexpressing cells showed markedly reduced increases in [Ca(2+)](i) without such large oscillations. Suppression of [Ca(2+)](i) oscillations was accompanied by an increase in glucose metabolism and insulin release that remained oscillatory despite having a lower periodicity. Hence, [Ca(2+)] (i) oscillations appear unnecessary for glucose-induced insulin release and may even be less favorable than a stable increase in [ Ca(2+)](i) for optimal hormone secretion. [Ca(2+)](i) oscillations do not directly drive insulin release oscillations but may nevertheless intervene in the fine regulation of such oscillations.

Correlated oscillations in glucose consumption, oxygen consumption, and intracellular free Ca(2+) in single islets of Langerhans

Micron-sized sensors were used to monitor glucose and oxygen levels in the extracellular space of single islets of Langerhans in real-time. At 10 mM glucose, oscillations in intraislet glucose concentration were readily detected. Changes in glucose level correspond to changes in glucose consumption by glycolysis balanced by mass transport into the islet. Oscillations had a period of 3.1 +/- 0.2 min and amplitude of 0.8 +/- 0.1 mM glucose (n = 21). Superimposed on these oscillations were faster fluctuations in glucose level during the periods of low glucose consumption. Oxygen level oscillations that were out of phase with the glucose oscillations were also detected. Oscillations in both oxygen and glucose consumption were strongly dependent upon extracellular Ca(2+) and sensitive to nifedipine. Simultaneous measurements of glucose with intracellular Ca(2+) ([Ca(2+)](i)) revealed that decreases in [Ca(2+)](i) preceded increases in glucose consumption by 7.4 +/- 2.1 s during an oscillation (n = 9). Conversely, increases in [Ca(2+)](i) preceded increases in oxygen consumption by 1.5 +/- 0.2 s (n = 4). These results suggest that during oscillations, bursts of glycolysis begin after Ca(2+) has stopped entering the cell. Glycolysis stimulates further Ca(2+) entry, which in turn stimulates increases in respiration. The data during oscillation are in contrast to the time course of events during initial exposure to glucose. Under these conditions, a burst of oxygen consumption precedes the initial rise in [Ca(2+)](i). A model to explain these results is described.

The influence of Ca2+ on the effects of glucagon on hepatic glycolysis

1. The influence of Ca2+ on the effects of glucagon on glycolysis was investigated in the isolated perfused rat liver. Livers from fed rats were perfused in an open system with Krebs/Henseleit-bicarbonate buffer (pH 7.4). Glucose release, lactate plus pyruvate production (glycolysis) and oxygen uptake were measured. The following results were obtained: 2. In livers perfused with Ca(2+)-free Krebs/Henseleit-bicarbonate buffer and after depletion of the intracellular pools, the initial and transient stimulation of glycolysis, which is normally observed shortly after the onset of glucagon infusion, was more pronounced when compared to livers perfused with normal perfusion fluid (2.5 mM Ca2+) and without previous depletion of the intracellular pools (controls); the subsequent inhibition of glycolysis was delayed in Ca(2+)-free perfused livers and was less pronounced in comparison with the controls at the end of the glucagon infusion period (20 min). 3. Perfusion with a Ca(2+)-free medium supplemented with EDTA, without previous depletion of the intracellular pools, also produced a substantial reduction in the effects of glucagon on glycolysis. 4. Ca(2+)-free perfusion did not affect the stimulative action of glucagon on glucose release (glycogenolysis) and oxygen uptake. 5. Glycolysis inhibition by cAMP also was abolished in Ca(2+)-free perfused livers, and the initial stimulation was enhanced. 6. Mn2+, a metal ion known as a competitor of Ca2+, considerably reduced the action of glucagon on glycolysis; Mn2+ did not affect the basal rates of glycolysis. 7. Sr2+, a metal ion that is often recognized as Ca2+ by several biological structures and processes, increased the inhibitory action of glucagon on glycolysis. 8. Several organic compounds, which directly or indirectly take part in Ca2+ fluxes, were also able to diminish (e.g., verapamil) or even to abolish (carbenoxolone) the inhibitory action of glucagon on glycolysis. 9. It was concluded that, under the conditions of the living cell, Ca2+ is important for glycolysis inhibition by glucagon. In principle at least, the results can be explained in terms of the known Ca2+ dependencies of several protein kinases and protein phosphatases.

Ca2+ dependence of gluconeogenesis stimulation by glucagon at different cytosolic NAD(+)-NADH redox potentials

The influence of Ca2+ on hepatic gluconeogenesis was measured in the isolated perfused rat liver at different cytosolic NAD(+)-NADH potentials. Lactate and pyruvate were the gluconeogenic substrates and the cytosolic NAD(+)-NADH potentials were changed by varying the lactate to pyruvate ratios from 0.01 to 100. The following results were obtained: a) gluconeogenesis from lactate plus pyruvate was not affected by Ca(2+)-free perfusion (no Ca2+ in the perfusion fluid combined with previous depletion of the intracellular pools); gluconeogenesis was also poorly dependent on the lactate to pyruvate ratios in the range of 0.1 to 100; only for a ratio equal to 0.01 was a significantly smaller gluconeogenic activity observed in comparison to the other ratios. b) In the presence of Ca2+, the increase in oxygen uptake caused by the infusion of lactate plus pyruvate at a ratio equal to 10 was the most pronounced one; in Ca(2+)-free perfusion the increase in oxygen uptake caused by lactate plus pyruvate infusion tended to be higher for all lactate to pyruvate ratios; the most pronounced difference was observed for lactate/pyruvate ratio equal to 1. c) In the presence of Ca2+ the effects of glucagon on gluconeogenesis showed a positive correlation with the lactate to pyruvate ratios; for a ratio equal to 0.01 no stimulation occurred, but in the 0.1 to 100 range stimulation increased progressively, producing a clear parabolic dependence between the effects of glucagon and the lactate to pyruvate ratio. d) In the absence of Ca2+ the relationship between the changes caused by glucagon in gluconeogenesis and the lactate to pyruvate ratio was substantially changed; the dependence curve was no longer parabolic but sigmoidal in shape with a plateau beginning at a lactate/pyruvate ratio equal to 1; there was inhibition at the lactate to pyruvate ratios of 0.01 and 0.1 and a constant stimulation starting with a ratio equal to 1; for the lactate to pyruvate ratios of 10 and 100, stimulation caused by glucagon was much smaller than that found when Ca2+ was present. e) The effects of glucagon on oxygen uptake in the presence of Ca2+ showed a parabolic relationship with the lactate to pyruvate ratios which was closely similar to that found in the case of gluconeogenesis; the only difference was that inhibition rather than stimulation of oxygen uptake was observed for a lactate to pyruvate ratio equal to 0.01; progressive stimulation was observed in the 0.1 to 100 range. f) In the absence of Ca2+ the effects of glucagon on oxygen uptake were different; the dependence curve was sigmoidal at the onset, with a well-defined maximum at a lactate to pyruvate ratio equal to 1; this maximum was followed by a steady decline at higher ratios; at the ratios of 0.01 and 0.1 inhibition took place; oxygen uptake stimulation caused by glucagon was generally lower in the absence of Ca2+ except when the lactate to pyruvate ratio was equal to 1. The results of the present study demonstrate that stimulation of gluconeogenesis by glucagon depends on Ca2+. However, Ca2+ is only effective in helping gluconeogenesis stimulation by glucagon at highly negative redox potentials of the cytosolic NAD(+)-NADH system. The triple interdependence of glucagon-Ca(2+)-NAD(+)-NADH redox potential reveals highly complex interrelations that can only be partially understood at the present stage of knowledge.

Limitation of glycolysis by hexokinase in rat brain synaptosomes during intense ion pumping

Incubation of rat brain synaptosomes under conditions of either increased energy utilization (addition of Na+ channel opener, veratridine, or ionophores, monensin and nigericin) or inhibition of oxidative phosphorylation (addition of rotenone), or a combination thereof, decreased [ATP], increased [ADP] and stimulated glycolysis. The rates of lactate generation were linear over a 15-min interval in the presence of rotenone alone but decreased in the other two conditions. During the first 5 min, the amount of lactate formed with veratridine, monensin or nigericin was as high or higher than with rotenone, but it was lower in the last 10 min. With a combination of one of the stimulators of ion movements and rotenone the rate of glycolysis was always markedly lower than with each compound added singly. The stimulated rates of lactate formation correlated positively with the synaptosomal content of [ATP]. After 15 min, [ATP] was 0.9-1.0 nmol/mg with rotenone, 0.5-0.9 nmol/mg with veratridine (or ionophores), and <0.3 nmol/mg with a combination of the two. Under the conditions used, calcium did not affect glycolytic activity directly. The Lineweaver-Burk plot of the rate of lactate formation against [ATP] yielded a straight line with a Km for ATP of about 0.1 mM, which is very similar to the Km for this nucleotide of brain hexokinase bound to mitochondria. In C6 cells glycolytic rate measured with a combination of an ionophore and rotenone was higher than with each of these compounds added singly while [ATP] never declined below about 9 nmol/mg prot. It is concluded that in synaptosomes, the high rate of energy utilization required for intense ion movement decreases [ATP] to a level that limits hexokinase activity kinetically. This may contribute to a reduction in the rate of glycolysis and hence energy production in brain hypoxia and ischemia.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

D-glyceraldehyde-3-phosphate dehydrogenase purified from rabbit muscle contains phosphotyrosine

Homogeneous preparations of D-glyceraldehyde-3-phosphate dehydrogenase purified from rabbit muscle were found to contain 0.2-0.7 moles of covalently bound phosphate per mole of the enzyme. With the use of anti-phosphotyrosine antibodies, evidence was obtained that the enzyme is phosphorylated at tyrosine residues.

Selective stimulation of in situ intermediary metabolism by free calcium in permeabilized rat adipocytes

The hypothesis that ionized calcium [Ca2+]i may stimulate in situ rat adipocyte intermediary metabolism distal to glucose transport was tested. A metabolically active porous adipocyte model was employed in which pathway metabolism is exclusively pore-dependent using glucose 6-phosphate (G6P) as substrate. Cellular [Ca2+]i was, furthermore, directly adjusted to between 0-2.5 microM via the membrane pores. Three metabolic fluxes were examined, (1) glycolysis-Krebs ([6-14C]G6P oxidation), (2) glycolysis to lactate ([U-14C]G6P to [14C]lactate) and (3) pentose pathway ([1-14C]G6P oxidation). Glycolysis-Krebs oxidation was was found to be selectively (33% above basal P less than 0.001) stimulated by 0.625 microM free calcium. In contrast, there was no effect of [Ca2+]i on the other, exclusively cytoplasmic, pathways. The stimulation of glycolysis-Krebs by [Ca2+]i was inhibited by a mitochondrial calcium channel blocker (Ruthenium red) and persisted over a range of ATP/ADP ratios. Separate studies demonstrated that 2-[1-14C]ketoglutarate oxidation was also calcium-stimulated in the porous adipocytes (160% over baseline at 1 microM [Ca2+]i). These studies thus demonstrate that physiologically relevant increments in porous adipocyte [Ca2+]i enhance overall in situ glycolytic-Krebs pathway oxidation by a mechanism which entails mitochondrial calcium uptake. Methodologically, this metabolically active porous adipocyte model presents a novel experimental approach to investigations regarding the effects of ionized calcium on intermediary metabolism beyond glucose transport.

Phosphorylation of heart phosphofructokinase by Ca2+/calmodulin protein kinase

Phosphofructokinase (PFK) from sheep heart was shown to be phosphorylated by Ca2+/calmodulin protein kinase (CaM-kinase) as well as by cyclic AMP-dependent protein kinase (PKA). HPLC analysis of phosphorylated PFK indicated that phosphorylation by CaM-kinase occurs at least at two sites that are distinct from those recognized by PKA. Phosphorylation by either CaM-kinase of PKA resulted in an increase in sensitivity to ATP inhibition and a small but consistent decrease in Ki for ATP. Phosphorylation by either protein kinase caused a slight increase in the Km of PFK for fructose-6-P. Protein kinase C failed to phosphorylate PFK. Combinations of PKA, CaM-kinase and protein kinase C did not alter the stoichiometry of phosphorylation and did not change the effect on enzyme activity.

Stimulation by insulin of glycolysis in cultured hepatocytes is attenuated by extracellular ATP and puromycin through purine-dependent inhibition of phosphofructokinase 2 activation

Activation of glycolysis by insulin in cultured rat hepatocytes is preceded by an activation of phosphofructokinase 2 (PFK 2) and subsequent rise of the fructose 2,6-bisphosphate [Fru(2,6)P2] level. Extracellular addition of ATP or puromycin prevented the hormonal effect on glycolysis. The mechanism through which the purines abolished glycolytic stimulation was investigated. 1. 50 microM ATP completely prevented the 3-5-fold insulin-dependent increase of glycolysis, irrespective of whether the cells initially possessed a low or a high Fru(2,6)P2 content. 50 microM puromycin prevented the stimulation of glycolysis by insulin only in cells whose initial Fru(2,6)P2 levels were low and had to be increased by insulin prior to the increase in glycolysis. It did not antagonize the action of insulin cells with initial high Fru(2,6)P2 content. 2. ATP exerted effects on its own; it decreased initially high Fru(2,6)P2 levels by 95% within 10 min and decreased the basal glycolytic rate by 60%. Half-maximal effects on the Fru(2,6)P2 level were obtained with about 25 microM ATP or 15 microM adenosine 5'[beta, gamma-methylene]triphosphate. ADP and adenosine-5-[gamma-thio]triphosphate were as effective as ATP, whereas 100 microM adenosine 5'[alpha, beta-methylene]triphosphate elicited no effect. Puromycin neither decreased high Fru(2,6)P2 levels nor inhibited basal glycolysis. 3. Extracellular ATP (100 microM) led to inhibition of the active form of PFK 2. Intracellular levels of Glc6P, citrate, ATP, ADP and AMP were increased by extracellular ATP, the phosphoenolpyruvate content was decreased, Fru6P and glycerol 3-phosphate levels stayed constant. Puromycin did not inhibit PFK 2. 4. Both puromycin and ATP prevented the insulin-dependent rise of the Fru(2,6)P2 level, they abolished the activation of PFK 2 by the hormone. Puromycin did not block the accumulation of Fru(2,6)P2 provoked by glucose addition; ATP also antagonized the glucose-dependent increase. 5. 100 microM ATP elevated the cAMP-dependent protein kinase activity ratio from 0.1 to 0.38 and increased the level of inositol trisphosphate by 16-fold within 5 min, whereas puromycin was without effect on either level. It is concluded that the two purines block the insulin effect on glycolysis by preventing the hormone increasing the Fru(2,6)P2 level. The mode of action, however, seems to be different: ATP antagonizes insulin action in that it leads to increased inhibition of PFK 2 whereas puromycin prevents the activation of PFK 2 by insulin.

Phosphorylation of 6-phosphofructokinase in rat lung

1. 6-Phosphofructokinase of both fetal and adult rat lung consists of L, M and C subunits in a ratio of 65:25:10. 2. 6-Phosphofructokinase was purified to homogeneity from adult rat lung and subjected to phosphorylation in vitro by the catalytic subunit of cyclic AMP-dependent protein kinase. 3. This resulted in phosphorylation of the L and M subunit of 6-phosphofructokinase. 4. The C subunit was not phosphorylated. 5. However, if the phosphorylation of 6-phosphofructokinase was studied in the cytosol fraction of either fetal or adult lung using endogenous protein kinase(s), only the L subunit was phosphorylated. 6. This phosphorylation was dependent on cyclic AMP. 7. No influence of calcium, calmodulin or phosphatidylserine/diolein on the phosphorylation was observed. 8. It is concluded that although both L and M subunits of rat lung 6-phosphofructokinase are potential substrates for cyclic AMP-dependent protein kinase, their phosphorylation in situ is differentially regulated.

Phosphorylation of D-glyceraldehyde-3-phosphate dehydrogenase by Ca2+/calmodulin-dependent protein kinase II

Rabbit muscle D-glyceraldehyde-3-phosphate dehydrogenase was shown to serve as a substrate for Ca2+/calmodulin-dependent protein kinase II with a Km of 0.33 microM and a Vmax of 2.63 mumol.min-1.mg-1 at pH 7.5 and 30 degrees C. In the absence of calmodulin, the Vmax was halved and Km unchanged. 0.99 mol of phosphate was incorporated per tetrameric molecule of D-glyceraldehyde-3-phosphate dehydrogenase under the experimental conditions employed.

Does rat liver 6-phosphofructo-1-kinase exhibit 'half-of-the-sites-phosphorylation'?

6-Phosphofructo-1-kinase (PFK-1) from a variety of species and organs can undergo phosphorylation by cAMP-dependent protein kinase. In most studies the stoichiometry of the phosphorylation reaction was far below the expected minimum value of 4 mol phosphate/mol PFK-1 tetramer. The present study with rat liver PFK-1 and purified catalytic subunit of cAMP-dependent protein kinase was undertaken in order to find the maximum phosphorylation stoichiometry under well-defined conditions. Irrespective of whether PFK-1 had been first treated with purified protein phosphatase 2C or not, no more than 1.66 +/- 0.22 mol phosphate/mol PFK-1 tetramer was incorporated, the highest single value being 2 mol phosphate/PFK-1 tetramer. This stoichiometry was found to be independent from the method of protein evaluation (gel dye-binding assay or amino acid analysis) and from the concentration of PFK-1 in the phosphorylation system (15.6 nM-0.53 microM). The stoichiometry was not affected by the presence of allosteric ligands, fructose-1,6-bisphosphatase or the PFK-1-inactivating protein. The possibility could be excluded that partial proteolysis was responsible for the incomplete phosphorylation. Two-dimensional polyacrylamide gel electrophoresis gave no indication of the existence of two different subunits in rat liver PFK-1. Possible reasons why rat liver PFK-1 undergoes 'half-of-the-sites' phosphorylation are discussed.

Substrate specificity of Ca2+/CaM-dependent multifunctional protein kinases: comparison of isoenzymes from brain, liver and skeletal muscle

Ca2+/CaM-dependent multifunctional protein kinase isoenzymes from brain, skeletal muscle and liver were compared by their phosphorylation of a number of protein substrates. Under the conditions of assay, the three isoenzymes demonstrated rapid phosphorylation of synapsin I and glycogen synthase. In contrast, rates of phosphorylation of pyruvate kinase and phenylalanine hydroxylase were almost two orders of magnitude slower. Differences in phosphorylation specifically of the latter two substrates was also observed among the three protein kinases. Phosphorylation by Ca2+/CaM-dependent protein kinases was contrasted with cAMP-dependent protein kinase, which phosphorylates these proteins in vitro and in vivo. The potential role of Ca2+/CaM-dependent multifunctional protein kinases in the Ca2+-dependent phosphorylation of these substrates is discussed.