Phosphofructokinase from mollusc muscle is activated by phosphorylation

Comparative physiological, biochemical, and molecular thermal stress response profiles for two Unionid freshwater mussel species

Freshwater mussels, aquatic keystone species, are in global decline. Long life spans, sedentary lifestyles, and unique reproductive strategies involving obligate parasitic stages make unionid freshwater mussels particularly sensitive to environmental perturbations resulting from global climate change. A greater understanding of the mechanisms by which closely related species differ in their response to thermal challenge is critical for successful conservation and management practices. As such, both an acute heat shock and a chronic warming simulation were conducted in order to evaluate responses between hypothesized thermally tolerant, Villosa lienosa, and thermally sensitive, Villosa nebulosa, freshwater mussels in response to predicted thermal warming. Multiple biological responses were quantified, including mortality, condition index, growth rates, glycogen and triglyceride content, and candidate gene expression. During acute heat shock, both species upregulated HSP90 and HSP70, though V. lienosa showed consistently greater transcript levels during upregulation. This pattern was consistent during the chronic warming simulation, with V. nebulosa showing greater induction of HSP60. Chronic warming stimulated increases in condition index for V. nebulosa, however declines in growth rates during a recovery period were observed with no concurrent tissue glycogen levels changes. This contrasts with V. lienosa, where tissue glycogen significantly increased during chronic warming, though no response was observed for condition index or growth rates. These differences might indicate disparate thermal stress response mechanisms correlated with metabolic demands and resource utilization. These biological differences could thus be a factor influencing current ranges and these two species‘ future ability to cope with persistent warming in their native habitats.

Purification and kinetic properties of 6-phosphofructo-1-kinase from gilthead sea bream muscle

The kinetic properties of 6-phosphofructo-1-kinase (PFK) from skeletal muscle (PFKM) of gilthead sea bream (Sparus aurata) were studied, after 10,900-fold purification to homogeneity. The native enzyme had an apparent molecular mass of 662 kDa and is composed of 81 kDa subunits, suggesting a homooctameric structure. At physiological pH, S. aurata PFKM exhibited sigmoidal kinetics for the substrates, fructose-6-phosphate (fru-6-P) and ATP. Fructose-2,6-bisphosphate (fru-2,6-P(2)) converted the saturation curves for fru-6-P to hyperbolic, activated PFKM synergistically with other positive effectors of the enzyme such as AMP and ADP, and counteracted ATP and citrate inhibition. The fish enzyme showed differences regarding other animal PFKs: it is active as a homooctamer, and fru-2,6-P(2) and pH affected affinity for ATP. By monitoring incorporation of (32)P from ATP, we show that fish PFKM is a substrate for the cAMP-dependent protein kinase. The mechanism involved in PFKM activation by phosphorylation contrasts with previous observations in other species: it increased V(max) and did not affect affinity for fru-6-P. Unlike the mammalian muscle enzyme, our findings support that phosphorylation of PFKM may exert a major role during starvation in fish muscle.

Phosphofructokinase from muscle of the marine giant barnacle Austromegabalanus psittacus: kinetic characterization and effect of in vitro phosphorylation

The kinetic properties of phosphofructokinase from muscle of the giant cirripede Austromegabalanus psittacus were characterized, after partial purification by ion exchange chromatography on DEAE-cellulose. This enzyme showed differences regarding PFKs from other marine invertebrates: the affinity for fructose 6-phosphate (Fru 6-P) was very low, with an S(0.5) of 22.6+/-1.4 mM (mean+/-S.D., n=3), and a high cooperativity (n(H) of 2.90+/-0.21; mean+/-S.D., n=3). The barnacle PFK showed hyperbolic saturation kinetics for ATP (apparent K(m ATP)=70 microM, at 5 mM Fru 6-P, in the presence of 2 mM ammonium sulfate). ATP concentrations higher than 1 mM inhibited the enzyme. Ammonium sulfate activated the PFK several folds, increasing the affinity of the enzyme for Fru 6-P and V(max). 5'-AMP (0.2 mM) increased the affinity for Fru 6-P (S(0.5) of 6.2 mM). Fructose 2,6-bisphosphate activated the PFK, with a maximal activation at concentrations higher than 2 microM. Citrate reverted the activation of PFK produced by 0.2 mM 5'-AMP (IC(50 citrate)=2.0 mM), producing a higher inhibition than that exerted on other invertebrate PFKs. Barnacle muscular PFK was activated in vitro after exposure to exogenous cyclic-AMP (0.1 mM) as well as by phosphatidylserine (50 microg/ml), indicating a possible control by protein kinase A and a phospholipid dependent protein kinase (PKC). The results suggest a highly regulated enzyme in vivo, by allosteric mechanisms and also by protein phosphorylation.

Mechanisms of myocardial protection by adenosine-supplemented cardioplegia: differential response of calcium-independent protein kinase C isozymes

BACKGROUND

Adenosine-supplemented cardioplegia improves myocardial function after cardioplegic arrest. However, the underlying cellular mechanism(s) responsible for adenosine's protective actions remains unclear. We tested the hypothesis that protection by adenosine-supplemented cardioplegia would be associated with selective activation of protein kinase C (PKC) isozymes delta and epsilon.

MATERIALS AND METHODS

Isolated rat hearts were perfused (37 degrees C, Krebs-Ringer bicarbonate buffer) for 30 min, after which baseline functional measurements were made. This was followed by 120 min of cold cardioplegic arrest at 4 degrees C with either St. Thomas No. 2 (ST#2), ST#2 + adenosine (100 microM, ADO) or ST#2 + ADO + 8-sulfophenyltheophylline (50 microM, SPT). Hearts were reperfused for 60 min and functional measurements made. Distribution of PKC isoforms was determined (immunoblotting) after 30 min of warm perfusion (No-CDPL) or after 30 min of perfusion followed by 15 min of cardioplegic arrest.

RESULTS

ADO prevented myocardial dysfunction after cardioplegic arrest. PKC-delta did not differ in the cytosolic fraction among groups. However, ADO prevented increases in particulate fraction PKC-delta, but elicited a significant increase in the particulate fraction PKC-epsilon, while ST#2 or SPT significantly decreased the cytosolic fraction PKC-epsilon. Both functional and cellular changes associated with ADO were receptor mediated.

CONCLUSION

This novel, dual action of adenosine-supplemented cardioplegia on PKC isoforms may be responsible for the associated functional improvements.

Growth factors stimulate the activity of key glycolytic enzymes in isolated digestive gland cells from mussels (Mytilus galloprovincialis Lam.) through tyrosine kinase mediated signal transduction

Digestive gland cells isolated from mussels (Mytilus) have previously been demonstrated to respond to mammalian EGF with a cytosolic Ca(2+) transient and stimulated DNA synthesis; both responses were mediated by activation of tyrosine kinase receptors. The present study examines the mechanisms involved in further signal progression and possible targets of phosphorylation/dephosphorylation processes. The effects of EGF, IGF-I, and insulin on the activity of two key glycolytic enzymes PFK (phosphofructokinase) and PK (pyruvate kinase) were evaluated. All the peptides tested induced a transient and dose-dependent stimulation of the activity of both PFK and PK, which involved activation of MAPKs. Quantitative immunoelectron microscopy, utilizing monoclonal anti-phosphotyrosine antibodies, revealed that EGF induced a transient increase in tyrosine phosphorylation. The results demonstrate that, in marine invertebrate cells, activation of tyrosine kinase membrane receptors by growth factors triggers signal transduction pathways involving a phosphorylative cascade similar to that of mammalian cells. Moreover, these data suggest that, in mussel cells, growth factors may play a physiological role in the in vivo regulation of glucose metabolism by modulating, through reversible phosphorylation, the activity of key glycolytic enzymes.

In vivo phosphorylation of phosphofructokinase from the bivalve mollusk Mytilus galloprovincialis

The phosphorylation state of phosphofructokinase from the mantle tissue of the facultative anaerobe mollusk Mytilus galloprovincialis was determined by a back-phosphorylation technique. The incubation of intact mantle tissue with 8-bromoadenosine 3':5'-cyclic monophosphate increased significantly the phosphate content of phosphofructokinase, which indicates that the enzyme can be phosphorylated in vivo by endogenous cAMP-dependent protein kinase. The phosphate content of mussel phosphofructokinase changes significantly during the year, in agreement with the kinetic data that show a more active enzyme form in earlier autumn. These results suggest that cAMP-dependent phosphorylation of phosphofructokinase can be partially responsible for the observed glycolytic changes associated with the annual gametogenic cycle that takes place in the mantle tissue of the mollusk. On the contrary, no differences were observed between aerobic and 24-h hypoxic mussels with regard to the phosphorylation state and the kinetic constants of phosphofructokinase. This result is inconsistent with the hypothesis that phosphorylation of phosphofructokinase is involved in the glycolytic depression that takes place during the long-term environmental hypoxia that the mollusk can undergo.

Activation by phosphorylation of phosphofructokinase from the annelid Lumbricus terrestris and comparison of phosphorylated sites in invertebrate phosphofructokinases

Purified phosphofructokinase from the earthworm Lumbricus terrestris was phosphorylated in vitro by the catalytic subunit of cAMP-dependent protein kinase from the same organism to an extent of approx. 0.5 mol/mol of subunit. Activation of the enzyme occurred in parallel to the incorporation of covalently bound phosphate and was reversed by the action of the catalytic subunit of protein phosphatase 2A. Phosphorylation decreased the co-operativity of fructose 6-phosphate saturation in the presence of inhibitory concentrations of ATP, and increased the apparent Vmax obtained with saturating concentrations of the activators 5'-AMP and fructose 2,6-bisphosphate. The phosphorylated sites of phosphofructokinase from L. terrestris and from two molluscs (Helix pomatia and Mytilus edulis) were sequenced and shown to exhibit distinct similarity to sequences located near to the N-terminus of nematode phosphofructokinases [Klein, Olson, Favreau, Wintertowed, Hatzenbuhler, Shea, Nulf and Geary (1991) Mol. Biochem. Parasitol. 48, 17-26.

The Pasteur effect in facultative anaerobic metazoa

The existence and the regulatory mechanisms of the Pasteur effect in facultative anaerobic metazoa are discussed. There are three reasons for the controversy surrounding this phenomenon. 1) The different definitions of the Pasteur effect, 2) the antagonistic effect of metabolic depression and its species specific response to hypoxia, as well as 3) the laboratory-specific differences in the experimental procedures for analyzing the Pasteur effect and its regulation. This review aims to clarify the confusion about the existence of the Pasteur effect in facultative anaerobic metazoa and to offer possible molecular mechanisms.

Conservation, evolution, and specificity in cellular control by protein phosphorylation

The glycolytic control enzyme phosphofructokinase from the parasitic nematode Ascaris lumbricoides is regulated by reversible phosphorylation. The enzyme is phosphorylated by an atypical cyclic adenosine monophosphate (cAMP)-dependent protein kinase whose substrate specificity deviates from that of the mammalian protein kinase. This variation is explained by structural peculiarities on the surface part of the catalytic groove of the protein kinase. Also, the protein phosphatases responsible for the reversal of phosphorylation appear to act specifically in glycolysis and are different from those participating in regulation of glycogenolysis.

Purification and properties of phosphofructokinase from Dictyostelium discoideum

Phosphofructokinase (PFruK) from the slime mold Dictyostelium discoideum has been purified to homogeneity over 15,000-fold with a 29% yield. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the final preparation revealed a single band of 95 kDa. The native molecular mass was determined by gel filtration to be 382 kDa, indicating that the enzyme is a homotetramer. An antibody raised in rabbits against the 95-kDa band immunoprecipitated PFruK activity while it did not react with the enzyme from yeast and mammalian cells. The apparent pI was 6.8 and the pH optimum was 7.6. The enzyme had an activation energy (Ea) of 29.1 kJ/mol. The amino acid composition was distinctive in having high Ser, Gly and Glx and low Ala, Val and Tyr compared with other eukaryotic PFruKs. Enzyme activity did not have a sigmoidal saturation curve for fructose 6-phosphate, was only mildly inhibited by MgATP at acidic pH values, was not affected by enzyme concentration and was insensitive to any of the typical allosteric effectors of PFruKs from other sources. However, the enzyme binds fructose 2,6-bisphosphate as indicated by protection against thermal denaturation. Treatment with cAMP-dependent protein kinase led to phosphorylation of the enzyme without change in activity. The metabolic significance of these properties and their relationship to structure/function are discussed.