Hydrolysis of membrane phospholipids by phospholipases of rat liver lysosomes

Abstract

The hydrolysis of 32P- or myo[2-3H]inositol-labeled rat liver microsomal phospholipids by rat liver lysosomal enzymes was studied. The relative rates of hydrolysis of phospholipids at pH 4.5 are: sphingomyelin > phosphatidylethanolamine > phosphatidylcholine > phosphatidylinositol. The predominant products of phosphatidylcholine and phosphatidylethanolamine hydrolysis are their corresponding lyso-compounds, indicating a slow rate of total deacylation. Ca2+ inhibits the hydrolysis of all phospholipids, though only appreciably at high (> 5 mM) concentration. The hydrolysis of sphingomyelin is considerably less sensitive to Ca2+ than that of glycerophospholipids. Analysis of the water-soluble products of phosphatidylinositol hydrolysis (by using myo[3H]inositol-labeled microsomal fraction as a substrate) produced evidence that more than 95% of the product is phosphinositol, which was derived by direct cleavage from phosphatidylinositol, rather than by hydrolysis of glycerophosphoinositol. This production of phosphinositol, allied with negligible lysophosphatidylinositol formation and a detectable accumulation of diacylglycerol, indicates that lysosomes hydrolyse membrane phosphatidylinositol almost exclusively in a phospholipase C-like manner. Comparisons are drawn between the hydrolysis by lysosomal enzymes of membrane substrates and that of pure phospholipid substrates, and also the possible role of phosphatidylinositol-specific lysosomal phospholipase C in cellular phosphatidylinositol catabolism is discussed.