Detergent Effects on the Phosphatidylinositol‐Specific Phospholipase C in Rat Brain Synaptosomes

Abstract

In the presence of Ca2+ (2.5 mM) and using [14C]arachidonyl phosphatidylinositol (PI) membrane as substrate, phosphatidylinositol-specific phospholipase C (PI-PLC) (EC 3.1.4.3) in rat brain synaptosomes was activated by deoxycholate but not taurocholate. Ca2+ stimulated enzymic hydrolysis by both detergents, but the stimulatory effect of taurocholate was < deoxycholate. Peak stimulation for deoxycholate was observed at 1 mg/ml, whereas taurocholate was 4 mg/ml. When 1 mM EDTA was added to the taurocholate (4 mg/ml) and Ca2+ (3.5 mM) system, synaptosomal PI-PLC activity was greatly stimulated, to almost the same level as the deoxycholate + Ca2+ system. This system required the presence of all 3 factors, and EGTA [ethylene glycol bis tetraacetic acid] could not effectively replace EDTA in the stimulatory action. The detergent-induced hydrolysis of synaptosomal PI by the deoxycholate + Ca2+ and the taurocholate + Ca2+ + EDTA systems was strongly inhibited by divalentmetal ions such as Zn2+, Cu2+, Pb2+ and Fe2+, whereas Mg2+ and Ca2+ were ineffective. Nevertheless, only the deoxycholate + Ca2+ system was responsive to enzyme inhibition by membrane-perturbing agents such as lysophospholipids and free fatty acids. The specific requirement for EDTA in the taurocholate system may be due to the release of a pool of inhibitory divalent metal ions from the membranes.