A membrane-bound activity catalysing phosphatidylinositol breakdown to 1,2-diacylglycerol, d-myoinositol 1:2-cyclic phosphate and d-myoinositol 1-phosphate. Properties and subcellular distribution in rat cerebral cortex

1. Breakdown of phosphatidylinositol was studied in homogenates and subcellular fractions of rat cerebral cortex by using both membrane-bound and externally added [ 32 P]phosphatidylinositol as substrate. 2. In the presence of deoxycholate breakdown followed first-order kinetics at low substrate concentrations ([unk]1mm) and zero-order kinetics at higher concentrations (6–9mm). 3. Maximum breakdown by cerebral-cortex homogenates was approximately 0.5μmol/h per mg of protein and occurred at pH7.0 in the presence of 8mm-phosphatidylinositol, 2mm-CaCl 2 and 2mg of deoxycholate/ml. Activity was abolished by 1mm-ethanedioxybis(ethylamine)tetra-acetate. 4. The products of phosphatidylinositol breakdown were 1,2-diacylglycerol and a mixture of d- myo inositol 1:2-cyclic phosphate (55%) and d- myo inositol 1-phosphate (45%). The two phosphate esters appeared to be produced together and in constant proportions. 5. Some 51% of the activity was particle-bound, with the highest activities in small nerve endings, microsomal material and two synaptic membrane fractions (fractions Mic 20 , Mic 100 , M 1 1.0 and M 1 0.9 respectively), all of which were also rich in acetylcholinesterase and which have been shown to be rich in other surface-membrane enzymes. Much of the particle-bound activity therefore appears to be present in cerebral-cortex plasma membranes. 6. The results are discussed in relation to previously described soluble activities that catalyse the same reaction, and to a possible role of the membrane-bound enzyme in enhanced phosphatidylinositol turnover in externally stimulated cells.