REGULATION OF NEUTRAL CHOLESTEROL ESTERASE-ACTIVITY TOY PHOSPHOLIPIDSCONTAINING NEGATIVE CHARGES IN SUBSTRATE LIPOSOME

The effect of phospholipids on cholesteryl ester hydrolysis by neutral cholesterol esterase in alveolar macrophages was studied. Among the p hospholipids used as emulsifiers, those with a negative charge, such a s phosphatidylserine, phosphatidic acid, phosphatidylinositol, and car diolipin, gave a higher level of hydrolysis by neutral cholesterol est erase than other less negatively charged phospholipids, such as phosph atidylcholine, lysophosphatidylcholine, phosphatidylethanolamine , and sphingomyelin. Phospholipase D treatment of liposomes emulsified with phosphatidylcholine produced phosphatidic acid and enhanced cholester yl ester hydrolysis. Phospholipase A(2) treatment produced lysophospha tidylcholine and decreased the hydrolysis. The hydrolysis of cholester yl ester in lipid droplets obtained from cholesterol-laden macrophages elicited by thioglycollate in the rat peritoneal cavity was low compa red to artificial liposomes emulsified with phosphatidylcholine. The r eason for this was speculated to be that lipid droplets were low in to tal phospholipids and poor in phospholipids with strong negative charg es but rich in phosphatidylethanolamine and sphingomyelin. These resul ts suggest that the polar heads of phospholipids may play an important role in cholesteryl ester hydrolysis by neutral cholesterolesterase.