The Activity of Phospholipase D on Aggregates of Phosphatidylcholine, Dodecylsulfate and Ca²⁺

Abstract

Phospholipase D from peanut seeds catalyzes the hydrolysis of phosphatidylcholine (PC) to phosphatidic acid and choline. The catalytically active enzyme has a molecular weight of 200,000. Smaller subunits were obtained upon dilution, low pH or in the presence of certain detergents. Activators and Ca²⁺ ions are required for the hydrolysis; the amphipath sodium dodecylsulfate (SDS) is the most effective in activating the reaction. The dependence of the reaction on SDS is complex. Some physical properties of the PC — SDS mixed aggregates, as a function of their concentration ratio, were studied. Proton PMR, turbidity and ultracentrifugal data in the absence or presence of Ca²⁺, indicate that “dispersible” PC — SDS mixed micelles are formed at any SDS to PC molar ratio below 4. Upon the addition of Ca²⁺ however, optimal dispersibility was demonstrated at SDS to PC molar ratio ranging from 0.25–0.65, probably because high concentrations of the insoluble calcium dodecylsulfate causes a precipitation of the mixed micelles. Kinetic experiments showed that PC in low speed precipitated aggregates is not available for hydrolysis by phospholipase D. On the other hand PC aggregates obtained by high speed centrifugation were degraded by the enzyme. These observations serve as a basis for a partial explanation of the dependence of the enzymatic activity on the presenca of SDS.