Abstract
1. 1. Some kinetic properties of pyruvate kinase from Dicrocoelium dendriticum have been studied and the results are compared with those obtained for the enzyme of higher animals. 2. 2. The substrate saturation curve shows low co-operative interaction in the kinetics with a Hill coefficient of 1·2 and an apparent K m of 1 mM. Low concentrations of fructose-1,6-diphosphate strongly activate the enzyme lowering the K m seven-fold. 3. 3. The enzyme has an essential bivalent cation requirement. The activation was found to be slightly more efficient in the presence of optimal concentrations of manganese than with magnesium. 4. 4. The influence of adenosine diphosphate (ADP) on the pyruvate kinase reaction was studied at bivalent cation concentrations within the physiological range. ADP concentrations probably prevailing in the parasitic tissue produce higher reaction velocities with magnesium, whereas low concentrations of the nucleotide favour the manganese-activated enzyme reaction. 5. 5. Adenosine triphosphate shows an inhibitory effect even when an excess of magnesium was added to counteract chelation by the nucleotide. Furthermore, the enzyme was found to be sensitive to inhibition by malate. 6. 6. The possible role of pyruvate kinase in the metabolism of the parasite is discussed.
Published Version
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