Factors controlling the metabolism of phosphatidate by phosphohydrolase and phospholipase A-type activities: Effects of magnesium, calcium and amphiphilic cationic drugs

Abstract

1. 1. The simultaneous deacylation and dephosphorylation of 1,2-diacyl- sn-[ 3H]glycerol 3-phosphate by the microsomal and soluble fraction of rat liver was studied. The substrate was either in the form of an emulsion or bound to microsomal membranes. 2. 2. Mg 2+ stimulated the deacylation and dephosphorylation of phosphatidate emulsions by both fractions, although the stimulation of both microsomal activities was less than that in the soluble fraction. The preparations of membrane-bound phosphatidate contained Mg 2+. Further addition of Mg 2+ inhibited dephosphorylation, whereas low concentrations of EDTA stimulated. Additional Mg 2+ had little effect on the deacylation of membrane-bound phosphatidate and EDTA inhibited it. 3. 3. Ca 2+ inhibited the phosphohydrolase reactions in both fractions, but had little effect on the deacylation of phosphatidate emulsions or membrane-bound phosphatidate. 4. 4. In the absence of Mg 2+, lower concentrations of amphiphilic cations (chlorpromazine and benfluorex) stimulated the deacylation and dephosphorylation of phosphatidate emulsions by the soluble fraction. They also stimulated deacylation by the microsomal fraction, but inhibited dephosphorylation. In the presence of 5 mM MgCl 2, these drugs inhibited the dephosphorylation and deacylation of phosphatidate emulsions, the deacylation reaction being slightly less sensitive. Chlorpromazine (0.4 and 0.8 mM) also inhibited the dephosphorylation of membrane-bound Mg 2+-phosphatidate by microsomal and microsomal plus soluble fractions. The deacylation was stimulated by 0.4 mM chlorpromazine and by 1 and 2 mM norfenfluramine. Chlorpromazine (0.8 mM) inhibited the deacylation by microsomal plus soluble fractions, but not by microsomal fractions alone. 5. 5. The possible importance of the deacylation of phosphatidate in the physiological and pharmacological control of glycerolipid synthesis is discussed.