Tissue specific isozymes of pyruvate kinase in the channelled whelkBusycotypus canaliculatum: enzyme modification in response to environmental anoxia

Freeze-induced suppression of pyruvate kinase in liver of the wood frog (Rana sylvatica)

The wood frog (Rana sylvatica) undergoes physiological and metabolic changes to withstand subzero temperatures and whole body freezing during the winter months. Along with metabolic rate depression, high concentrations of glucose are produced as a cryoprotectant by liver and distributed to all other tissues. Pyruvate kinase (PK; EC:2.7.1.40), the final enzyme of glycolysis, plays an important role in the modulation of glucose metabolism and, therefore, overall metabolic regulation. The present study investigated the functional and kinetic properties of purified PK from liver of control (5 °C acclimated) and frozen (-2.5 °C for 24 h) wood frogs. Liver PK was purified to homogeneity by a two-step chromatographic process, followed by analysis of enzyme properties. A significant decrease in the affinity of PK for its substrates, phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP) at 22 °C and 5 °C was noted in liver from frozen frogs, as compared with controls. Immunoblotting also revealed freeze-responsive changes in posttranslational modifications with a significant increase in serine and threonine phosphorylation by 1.46-fold and 1.73- fold for PK from frozen frogs as compared with controls. Furthermore, a test of thermal stability showed that PK from liver of frozen wood frogs showed greater stability as compared with PK from control animals. Taken together, these results suggest that PK is negatively regulated, and glycolysis is suppressed, during freezing. This response acts as an important survival strategy for maintaining continuously elevated levels of cryoprotectant in frogs while they remain in a hypometabolic frozen state.

Effects of hypoxia on the energy status and nitrogen metabolism of African lungfish during aestivation in a mucus cocoon

We examined the energy status, nitrogen metabolism and hepatic glutamate dehydrogenase activity in the African lungfish Protopterus annectens during aestivation in normoxia (air) or hypoxia (2% O(2) in N(2)), with tissues sampled on day 3 (aerial exposure with preparation for aestivation), day 6 (entering into aestivation) or day 12 (undergoing aestivation). There was no accumulation of ammonia in tissues of fish exposed to normoxia or hypoxia throughout the 12-day period. Ammonia toxicity was avoided by increased urea synthesis and/or decreased endogenous N production (as ammonia), but the dependency on these two mechanisms differed between the normoxic and the hypoxic fish. The rate of urea synthesis increased 2.4-fold, with only a 12% decrease in the rate of N production in the normoxic fish. By contrast, the rate of N production in the hypoxic fish decreased by 58%, with no increase in the rate of urea synthesis. Using in vivo (31)P NMR spectroscopy, it was demonstrated that hypoxia led to significantly lower ATP concentration on day 12 and significantly lower creatine phosphate concentration on days 1, 6, 9 and 12 in the anterior region of the fish as compared with normoxia. Additionally, the hypoxic fish had lower creatine phosphate concentration in the middle region than the normoxic fish on day 9. Hence, lowering the dependency on increased urea synthesis to detoxify ammonia, which is energy intensive by reducing N production, would conserve cellular energy during aestivation in hypoxia. Indeed, there were significant increases in glutamate concentrations in tissues of fish aestivating in hypoxia, which indicates decreases in its degradation and/or transamination. Furthermore, there were significant increases in the hepatic glutamate dehydrogenase (GDH) amination activity, the amination/deamination ratio and the dependency of the amination activity on ADP activation in fish on days 6 and 12 in hypoxia, but similar changes occurred only in the normoxic fish on day 12. Therefore, our results indicate for the first time that P. annectens exhibited different adaptive responses during aestivation in normoxia and in hypoxia. They also indicate that reduction in nitrogen metabolism, and probably metabolic rate, did not occur simply in association with aestivation (in normoxia) but responded more effectively to a combined effect of aestivation and hypoxia.

Analysis of signal transduction pathways during anoxia exposure in a marine snail: A role for p38 MAP kinase and downstream signaling cascades

The responses of members of the three main MAPK families (ERK, JNK/SAPK, p38 MAPK), as well as selected peripheral pathways, were examined in hepatopancreas of the marine periwinkle, Littorina littorea, to determine if anoxia exposure influenced the total protein content or the phosphorylation status of any key components. The content of active phospho-p38 MAPK was 2-fold higher in hepatopancreas from anoxic snails relative to controls. A 1.7-fold increase in the amount of phospho-Hsp27 and a 1.3-fold increase in phospho-CREB correlated well with the changes in p38 MAPK phosphorylation. Activation of these factors via p38 MAPK may be vital to the reorganization of metabolic responses to anoxia in hepatopancreas. No changes in components of the JNK/SAPK and ERK pathways occurred and transcription factors involved in lipid metabolism did not appear to be affected by anoxia. The present analysis of a variety of signaling pathways has implicated the p38 MAPK pathway as a key anoxia-responsive signal transduction pathway in L. littorea.

Glycolytic controls in estivation and anoxia: a comparison of metabolic arrest in land and marine molluscs

Facultative metabolic rate depression is the common adaptive strategy underlying various animal mechanisms for surviving harsh environmental conditions. This strategy is common among molluscs, enabling animals to survive over days or even months in the absence of oxygen or under extremely dry conditions. The large reductions in metabolic rate during estivation and anoxia can translate into considerable energy savings when dormant animals are compared to active animals. A complex metabolic coordination is required during the transition into the dormant state to maintain cellular homeostasis and involves both energy-consuming and energy-producing pathways. With regard to energy-producing pathways, several different mechanisms have been identified that participate in controlling flux. One such mechanism, enzyme phosphorylation, can have a wide-ranging effect. For example, phosphorylated enzymes exhibit altered substrate, activator, and inhibitor affinities. This effect may be magnified by changes in the concentrations of allosteric effectors, such as fructose 2,6-bisphosphate, that occur during hypometabolic states. Changes in fructose 2,6-bisphosphate are related to changes in enzyme phosphorylation through changes in the relative activity of phosphofructokinase-2. Alterations in glycolytic enzyme binding can also be brought about through changes in enzyme phosphorylation. The present review focuses on identifying hypometabolism-related changes in enzyme phosphorylation as well as characterizing the mechanisms involved in mediating these phosphorylation events.

The effects of aestivation on the catalytic and regulatory properties of pyruvate kinase from Helix aspersa

1. Pyruvate kinase was partially purified from the foot, mantle, and digestive gland of active and aestivating snails. 2. At pH 7.0 the apparent Km values for phosphoenolpyruvate (PEP) were 0.064 mmol/l for the enzyme from foot and 0.071 mmol/l for the enzyme from mantle; those for ADP were 0.35 mmol/l for the foot enzyme and 0.33 mmol/l for the mantle enzyme. 3. Both enzymes were inhibited by alanine, and this could be reversed by fructose 1,6-bisphosphate (FBP), although FBP alone was a weak activator. 4. Decreasing the pH to 6.5 markedly increased the inhibition by alanine and reduced the response to FBP. 5. The enzymes from these tissues of aestivating snails showed a small decrease in their affinity for PEP and a small increase in the effectiveness of alanine as an inhibitor. 6. These changes are indicative of a down-regulation of this enzyme which is consistent with the observations in other species during metabolic depression. 7. In contrast the enzyme from the digestive gland of active animals showed sigmoidal saturation kinetics for PEP with a S0.5 of 1.2 mmol/l, but had a markedly higher affinity for PEP, S0.5 = 0.20 mmol/l during aestivation. This may be indicative of other metabolic changes occurring in the digestive gland.

The role of protein kinases in anoxia tolerance in facultative anaerobes: purification and characterization of a protein kinase that phosphorylates pyruvate kinase

A protein kinase which phosphorylates pyruvate kinase (PK) in vitro was purified and characterized from the foot muscle of the anoxia-tolerant gastropod mollusc Busycon canaliculatum. Purification involved four steps: poly(ethylene glycol) fractionation, affinity chromatography on Blue agarose, ion-exchange chromatography on phosphocellulose and preparative isoelectric focusing (pI = 5.5). The activity was monitored by following changes in pyruvate kinase I50 values for L-alanine which have previously been linked to changes in the degree of enzyme phosphorylation. The correlation between enzyme phosphorylation and changes in the L-alanine inhibition constant was also directly demonstrated in the present paper by radioactively labelling PK with [tau-32P]ATP. The final purified protein kinase solution gave a single band on SDS-gel electrophoresis with a molecular weight of 37,000 +/- 2000. Kinetic analysis of the purified protein kinase (PK-kinase) showed a pH optimum of 7.0, an absolute requirement for magnesium ions (Km = 1.29 mM), a relatively high affinity for MgATP (Km = 57 microM), and inhibition by increasing salt concentrations (I50 = 55 mM KCl). The protein kinase activity was not affected by either spermine, heparin, cAMP, cGMP or concentrations of CaCl2 less than 10 mM. The enzyme did not phosphorylate either phosphofructokinase or glycogen phosphorylase, two enzymes that are also phosphorylated during anoxia in whelks. The purified enzyme is different from the catalytic subunit of cAMP-dependent protein kinase as shown by the inability of cAMP to stimulate the protein kinase at all stages of the preparation; cAMP did not activate either crude enzyme, the 7% poly(ethylene glycol) supernatant, or any of the column eluant peak fractions when measured by changes in pyruvate kinase kinetic parameters.

Phosphofructokinase from a vertebrate facultative anaerobe: effects of temperature and anoxia on the kinetic parameters of the purified enzyme from turtle white muscle

The effects of low temperature and anoxia were determined on phosphofructokinase (PFK) purified from white skeletal muscle of the freshwater turtle, Pseudemys scripta. These effects were assayed by comparing PFK kinetic constants measured at a high (20 degrees C) and low (6 degrees C) temperature using enzyme obtained from animals held under normoxic and anoxic conditions. When assayed at 20 degrees C, PFK from anoxic animals had a lower Ka for phosphate, a lower Ka for AMP and showed no inhibition with increasing concentrations of ATP (up to 10 mM) when compared to enzyme from normoxic animals. At 6 degrees C, anoxic enzyme had a higher Km for fructose 6-phosphate and a higher I50 value for citrate with respect to normoxic enzyme. Decreasing temperature also had a differential effect on PFK kinetic parameters depending on the source of the enzyme. When normoxic enzymes were compared at 20 and 6 degrees C, the enzyme measured at 6 degrees C showed a lower Km for ATP and a lower Ka for AMP. Comparison of anoxic enzymes at these two temperatures showed that anoxic PFK at 6 degrees C had a higher Ka for phosphate, a higher Ka for AMP, and a larger Hill coefficient. A comparison of maximal velocities at varying temperature showed that normoxic enzyme (Q10 = 2.22) was more temperature sensitive than the anoxic enzyme (Q10 = 1.80). It is possible to interconvert the normoxic and anoxic forms of PFK by incubating normoxic enzyme with the active subunit of protein kinase, suggesting that the kinetic changes observed during anoxia resulted from enzyme phosphorylation. These data are discussed with respect to the mechanisms underlying white muscle function during diving and hibernation in red-eared turtles.

Pyruvate kinase isoenzymes in marine invertebrates: a comparative study by the use of monoclonal antibodies

1. Six monoclonal antibodies specific to the pyruvate kinase from the foot muscle of the common limpet P. caerulea were produced. 2. They also exhibited specificity against the mouse liver where the L-type isoenzyme of pyruvate kinase is present. They did not react with the mouse skeletal muscle, heart or red blood cells isoenzymes of pyruvate kinase (PK). One of these, the monoclonal antibody B did not react with any PK isoenzymes of the mouse tissues. 3. The presence of the isoenzymic type of PK which was recognized by the monoclonals, (type L), was traced in five phyla of marine invertebrates by the application of the monoclonal antibodies A, B and C. 4. In two phyla the majority of the animals were found to possess an L-type PK isoenzyme in their muscles while in quite a few of them a different isoenzymic type was present in the other tissues. The results of this study are compared with the existing literature, and the use of monoclonal antibodies in the study of enzymic systems is considered in the discussion.

Pyruvate kinase isozymes from the green alga, Selenastrum minutum. II. Kinetic and regulatory properties

The kinetic and regulatory properties of two pyruvate kinase isozymes, PKp and PKc (apparent chloroplastic and cytosolic isozymes, respectively) from the green alga Selenastrum minutum were studied. The two isozymes differed greatly in several kinetic properties. Although both isozymes showed hyperbolic substrate saturation kinetics, the apparent Michaelis constants for PEP and ADP were about twofold and fourfold lower, respectively, for PKc as compared with PKp. ADP was the preferred nucleotide substrate for both isozymes. However, PKc utilized alternate nucleotides far more effectively than did PKp. PKc and PKp also differed strongly in the effect of activators and inhibitors on the enzymes. Although both isozymes were activated by dihydroxyacetone phosphate (DHAP) with a similar activation constant of about 30 microM, this activator (0.5 mM) caused an approximate 30% increase in the Vmax of PKc, but had no effect on the Vmax of PKp. PKp, but not PKc, was inhibited by ribose 5-phosphate, ribulose 1,5-bisphosphate, 2-phosphoglycerate, phosphoglycolate, and malate. Both isozymes were inhibited by MgATP, Mg2citrate, Mg2oxalate, and Pi. PKc was far more sensitive to inhibition by Pi, as compared with PKp. Pi was a competitive inhibitor of PKc with respect to phosphoenolpyruvate (PEP) (Ki = 1.3 mM). Glutamate was a potent inhibitor of PKc, but had no effect on PKp. In contrast with Pi, glutamate was a mixed-type inhibitor of PKc with respect to PEP (Ki = 0.7 mM). DHAP facilitated the binding of PEP by both isozymes and reversed or relieved the inhibition of PKc by Pi and/or glutamate. The regulatory properties of PKp indicate that it is likely less active in the light and more active in the dark. The in vivo activity of PKc is probably regulated by the relative cytosolic levels of DHAP, Pi, and glutamate; this provides a rationale for the activation of algal cytosolic pyruvate kinase which occurs during periods of enhanced ammonia assimilation.

In vivo and in vitro studies on the pathway of modification of mussel pyruvate kinase

1. On aerial exposure, pyruvate kinase is inactivated in various organs of M. edulis; the decrease of activity is slower in muscle than in non-muscular tissue. 2. Anoxic in vitro incubation of gills results in a rapid inactivation of pyruvate kinase. No change occurs in an aerated medium. 3. Enzyme inactivation is mimicked in part by oxic incubation in an acidified medium containing 5,5-dimethyloxazolidine-2,4-dione, and by the action of calcium ionophore A23187. 4. Incubation of supernatant of gill homogenate results in a slow inactivation of pyruvate kinase that is inhibited by trifluoperazine or EGTA and stimulated by exogenous calmodulin. 5. Addition of ATP plus cAMP stimulates pyruvate kinase inactivation in supernatant of homogenized muscle but not so in a high molecular weight fraction thereof.

Purification and properties of aerobic and anoxic forms of pyruvate kinase from the hepatopancreas of the channelled whelk, Busycotypus canaliculatum

Aerobic and anoxic variants of pyruvate kinase (termed PK-aer and PK-anx) from the hepatopancreas of the gastropod mollusc, Busycotypus canaliculatum, were purified to apparent homogeneity with final specific activities of 14 and 2.3 units/mg protein, respectively. Both enzymes were homotetramers of the same molecular weight. The enzymes also showed equivalent affinities for ADP (0.22 mM) and very similar affinities for Mg2+, Mn2+, K+, and NH4+. PK-aer and PK-anx differed strongly, however, in maximal enzyme velocity (Vmax 9-fold higher for PK-aer), in affinity for P-enolpyruvate (PEP0.5 = 0.38 mM for PK-aer and 1.1 mM for PK-anx), and in the effects of activators and inhibitors on the enzymes. PK-aer was much more strongly stimulated by fructose-1,6-P2 and aspartate as activators (a 19- and 32-fold activation of enzyme velocity at subsaturating PEP levels versus only 4.1- and 2.6-fold activation for PK-anx, respectively). K alpha for fructose-1,6-P2 was 3-fold lower (0.16 microM) for PK-aer than for PK-anx (0.48 microM), but K alpha for aspartate was the same for both enzymes (1.5 mM). Activators decreased the PEP0.5 (to 0.05 mM for PK-aer and 0.07 mM for PK-anx), relieved inhibitions by alanine, Mg ATP, ADP, and Pi, and, when added together, showed a strong synergistic activation of PK-aer (but not PK-anx). The kinetic differences between PK-aer and PK-anx are similar to those of the dephosphorylated versus phosphorylated forms of PK from other sources, including those of red muscle PK of B. canaliculatum, and indicate that the change in enzyme form brought about during anaerobiosis may be due to enzyme phosphorylation. The powerful activation of hepatopancreas PK by aspartate is a novel regulatory control of the enzyme. Aspartate is one of the substrates of anaerobic energy production in marine molluscs and its effects on the enzyme may be important in a tissue where inactivation of PK can occur for one of two reasons: anaerobiosis or gluconeogenesis.