The hydrolysis of phosphatidylinositol by lysosomal enzymes of rat liver and brain

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Abstract
Lysosomes from rat liver contain 2 enzymic systems for hydrolysing phosphatidylinositol: a deacylation via lysophosphatidylinositol producing glycerophosphoinositol and non-esterified fatty acid, and a phospholipase C-like cleavage into inositol 1-phosphate and diacylglycerol. The separate enzyme systems involved can be distinguished by gel filtration, differential temperature-stability and the inhibitory action of detergents. The enzyme systems both have pH optima at 4.8 and their attack on a pure phosphatidylinositol substrate is inhibited by many bivalent metals including Ca2+ and Mg2+, and cationic drugs [e.g. tetracaine, mepacrine and chlorpromazine]. Whereas, the deacylation system will attack other glycerophospholipids, the phospholipase C shows a marked specificity towards phosphatidylinositol, although it will also slowly attack phosphatidylcholine with the liberation of phosphocholine. Gel filtration and temperature-stability distinguish the phospholipase C from lysosomal phosphatidic acid phosphatase (EC.3.1.3.4) but not from sphingomyelinase (EC 3.1.4.12). Evidence is presented that an EDTA-insensitive phospholipase C degrading phosphatidylinositol is present in rat brain.