Phospholipase D from savoy cabbage: purification and preliminary kinetic characterization.

Phospholipase D has been purified 680-fold from an acetone powder of savoy cabbage in an overall yield of 30%. The purification involves solubilization of the acetone powder in a Ca2+-containing buffer and subsequent ammonium sulfate fractionation. Gel filtration on Sephadex G-200 and hydrophobic affinity chromatography using a gamma-aminopropane-agarose gel complete the purification. The two chromatographic steps were conducted in buffers containing 50% ethylene glycol, which was necessary in order to maintain stability of the enzyme. Purity was established on the basis of gel electrophoresis and ultracentrifugation. A preliminary kinetic characterization of the enzyme was carried out by using lecithins with short-chain fatty acids below the critical micelle concentration. A complex series of results were obtained which demonstrated the following. (1) The enzyme is quite sensitive to ionic strength, being inhibited at high ionic strength. (2) The pH optimum depends on the concentration of Ca2+ used in the assay. At 0.5 mM Ca2+ the pH optimum is 7.25, but it is 6.0 at 50 mM Ca2+. (3) The effect of substrate concentration at a given pH and ionic strength did not show simple hyperbolic kinetics but rather regions of parabolic and hyperbolic kinetics.