The specificity of substrate hydrolysis by bovine brain phospholipase A1 (PLA1) was examined. In the presence of Mg2+, using pH values of 7 to 9, the purified enzyme deacylated 1-palmitoyl-2-[1-14C]arachidonoyl-phosphatidylethanolamine yielding 2-[1-14C]arachidonoyl-lysophosphatidylethanolamine at a rate of 70 mumol/min per mg. In the absence of Mg2+, however, the reaction rate slowed at pH values above 7.25. In contrast, brain PLA1 slowly (3.8 mumol/min per mg) hydrolyzed 1-palmitoyl-2-[1-14C]arachidonoyl-phosphatidylcholine (PAPC) unless phosphatidylserine (PS) was included. Maximal PAPC hydrolyzing activity required a PAPC/PS molar ratio of 2.5:1, Mg2+, and a pH value of 8.5-9.5. Replacing PS with phosphatidylethanolamine (PE) or phosphatidic acid (PA), but not phosphatidylinositol (PI), produced a similar effect. Moreover, hydrolysis of either arachidonoyl-substituted or dipalmitoyl-substituted PC at pH 7.5 was enhanced by increasing the mol fraction of PE. Brain PLA1 also hydrolyzed 1-stearoyl-[1-14C]arachidonoyl-PI with high velocity, but only if the substrate was dispersed in PE vesicles. In contrast, the velocity of PS, 1-palmitoyl-lyso-PC or diacylglycerol hydrolysis was low and unaffected by PE. In summary, PLA1 hydrolyzed PE with high velocity and specificity, whereas a high rate of PC or PI hydrolysis was observed only if PS, PE, or PA was present. In addition, PLA1 activity was greatly influenced by pH and Mg2+, implying that the substrate conformation is important to the catalytic efficiency of PLA1. Finally, the high rate of PE, PC or PI hydrolysis suggests PLA1 significantly contributes to the turnover of these phospholipids in the brain.