A shift in the optimum pH of phospholipase D produced by activating long-chain anions

Abstract

1. The activity of phospholipase D (phosphatidylcholine phosphatidohydrolase, EC 3.1.4.4) towards ultrasonically treated phosphatidylcholine or large phosphatidylcholine particles activated with ether was maximal near pH5, and there was little activity above pH6. 2. When the enzyme was activated by the addition of phosphatidic acid to large phosphatidylcholine particles the pH optimum was shifted to pH6·5 irrespective of the amount of activator added. 3. When the enzyme was activated with low concentrations of dodecyl sulphate the pH optimum was 5·5 with little activity above pH6. With higher concentrations of dodecyl sulphate the pH–activity profile was shifted upwards towards a pH optimum of 6·5–6·6, the magnitude of the shift depending on the extent of the hydrolysis. 4. The shifts in the pH–activity profiles cannot be correlated with changes in the ‘surface pH’ of the substrate particles calculated from the measurement of their ζ-potentials (electrophoretic mobilities).