The Catabolism of Phosphatidylinositol by an EDTA-Insensitive Phospholipase A1 and Calcium-Dependent Phosphatidylinositol Phosphodiesterase in Rat Brain.

1. A rat brain supernatant and microsomal fraction contained a phosholipase A₁ enzyme which hydrolysed phosphatidylinositol at pH 8 in the absence of calcium. Triolein and phosphatidylcholine were not attacked under the same incubation conditions. 2. No evidence could be obtained for a phospholipase. A₂ in the microsomal preparation, and in the presence of Ca²⁺ the release of fatty acid observed was due to phosphatidylinositol phosphodiesterase followed by diacylglycerol lipase action. 3. Brain phosphatidylinositol phosphodiesterase showed extensive activity in the alkaline range ( 7 - 8.5) as well as at pH 5 - 5.5. The activity at higher pH values required higher calcium concentrations and disappeared on purification of the soluble enzyme by ammonium sulphate fractionation. 4. In general the ratio between inositol 1,2-(cyclic)phosphate and inositol 1-phosphate produced by phosphodiesterase action decreased with increasing pH. [ABSTRACT FROM AUTHOR]