Fatty acid stimulation of membrane phosphatidylinositol hydrolysis by brain phosphatidylinositol phosphodiesterase

The hydrolysis of membrane-bound phosphatidylinositol in rat liver microsomal fraction by the soluble phosphatidylinositol phosphodiesterase from rat brain was markedly stimulated by oleic acid or arachidonic acid. The stimulation did not require added calcium, although it was abolished by EDTA. Lysophosphatidylcholine also totally suppressed the stimulation. A possible role for the fatty acid content of a membrane in controlling phosphatidylinositol turnover is suggested.

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

1. Lysosomes from rat liver contain two enzymic systems for hydrolysing phosphatidyl-inositol: a deacylation via lysophosphatidylinositol producing glycerophosphoinositol and non-esterified fatty acid, and a phospholipase C-like cleavage into inositol 1-phosphate and diaclygycerol. 2. The separate enzyme systems involved can be distinguished by gel filtration, differential temperature-stability and the inhibitory action of detergents. 3. 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. 4. Whereas the deacylation system will attack other glycerophospholipids, the phospholipase C shows a marked specificity towards phosphatidylinositol, although it will also slowly attach phosphatidylcholine with the liberation of phosphocholine. 5. Gel filtration and temperature-stability distinguish the phospholipase C from lysosomal phosphatidic acid phosphatase, but not from sphingomyelinase. 6. Evidence is presented that an EDTA-insensitive phospholipase C degrading phosphatidylinositol is present in rat brain.

Phosphatidylinositol-degrading enzymes in liver lysosomes

The major pathway by which liver lysosomal enzymes degrade phosphatidylinositol is through an EDTA-insensitive formation of phosphorylinositol. This is in distinct contrast with the Ca2+-dependent production of phosphorylinositol from phosphatidylinositol, which is located in the cytosol. Lysosomal enzymes can also totally deacylate phosphatidylinositol, producing glycerophosphorylinositol.

The effect of ethylene on (1-14C)glycerol incorporation into phospholipids of etiolated pea stems

The effect of ethylene (10 p.p.m.) on the rate of incorporation of [1-(14)C]glycerol into phospholipids of etiolated pea stems was studied. After 2-3h treatment with ethylene, incorporation was decreased by 50%. It remained at this value for as long as ethylene was supplied (8h). Handling the plants also caused a temporary decrease in incorporation, which we attribute to the production of endogenous ;wound' ethylene. The percentage decrease in incorporation was the same in four major phospholipid fractions, i.e. phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylinositol.