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Liver PK had hyperbolic saturation curves with both substrates and was not sensitive to alanine, but was inhibited by ATP (I 50 = 6 mM). Both white and red muscle PK showed hyperbolic saturation curves with PEP and ADP, and were not activated by FDP but were inhibited by ATP (I 50 values were 2.3 and 5.6 mM, respectively). Varying Mg 2+ concentrations in muscle and liver PK always showed sigmoidal kinetics, whereas with Mn 2+ only the liver PK saturation curve was sigmoidal. Ca 2+ was an inhibitor of both liver and muscle PK. Cu 2+ and Zn 2+ had an inhibitory effect on liver and muscle PK. The I 50 values were of the same order but slightly lower for liver.

1. 1. In the posterior adductor muscle of Scapharca inaequivalvis both aspartate and glycogen were substrates for anaerobic metabolism. ATP and arginine phosphate only contributed to a small extent. Alanine and succinate were major end products, whereas opines were minor end products. Lactate production was not observed. 2. 2. The ATP turnover rate was reduced by a factor of 6 in the later anoxic period. Glycolysis and aspartate contributed about 90% either during the initial step or during the prolonged anoxia. 3. 3. Recovery of 24 hr was not enough for a complete restoration of energy reserves, whereas anaerobic end products were eliminated over this period. There was no strombine or lactate accumulation.

1. 1. Pyruvate kinase from the posterior adductor muscle of bivalve molluscs Venus gallina and Scapharca inaequivalvis can be converted into a more active form by treatment with a cyclic AMP-dependent protein kinase. 2. 2. The enzyme from Scapharca is inhibited through incubation with a non-specific phosphatase. 3. 3. Purified pyruvate kinase from Venus treated with protein kinase is affected by alanine in a way similar to the untreated enzyme. 4. 4. Phosphorylation is thought to be an additional mechanism of regulation of pyruvate kinase in anaerobiosis and under gluconeogenetic conditions.

1. 1. In Scapharca inaequivalvis pyruvate kinase activity showed the following decreasing order: foot > muscle > viscera > gills > mantle. 2. 2. Pyruvate kinase from the posterior adductor muscle was purified through ion exchange chromatography, gel filtration and affinity chromatography. 3. 3. The enzyme, in the presence of PEP, showed a hyperbolic saturation curve. 4. 4. The influence of divalent cations at two different pHs was determined: Mg2+ and Mn2+ had an activator effect (pH-dependent); Ca2+ and Cu2+ had always an inhibitory effect, whereas Cd2+ and Co2+, at physiological pH, inhibited and at lower pH activated multiphasically the enzyme; also Zn2+ presented an activator/inhibitory effect depending on its concentration. 5. 5. These results give more information about the characteristics of Scapharca and Venus pyruvate kinase and lead us to think that this enzyme could be regulated in a different manner in these two species.

1. 1. Pyruvate kinase from the posterior adductor muscle of the Adriatic bivalve mollusc Venus gallina has been partially purified through ion-exchange chromatography and gel filtration. In both cases one peak of activity was revealed. 2. 2. The molecular weight has been computed by gel filtration, the subunits' composition has been determined by SDS electrophoresis. 3. 3. The enzyme kinetics are sensitive to several ions, Mg2+ and Mn2+ had an activatory effect which was pH dependent, whereas Ca2+, Co2+, Zn2+ and Cd2+ had an inhibitory effect.