A novel phospholipase A2 associated with nuclear matrix: stimulation of the activity and modulation of the Ca2+ dependency by polyphosphoinositides

Neutral phospholipase A2 activity, which hydrolyzed phosphatidylcholine and phosphatidylethanolamine with the same efficiency, was identified in the nuclear matrix prepared from purified nuclei of rat ascites hepatoma cells (AH 7974). The enzyme activity was optimal at pH 7.0 and required Ca2+ absolutely. Concentrations of Ca2+ for a maximal and a half-maximal activation were 1.10(-2) and 1.10(-3) M, respectively, and little activity was detected at Ca2+ concentrations lower than 1.10(-5) M. Addition of acidic phospholipids markedly stimulated the enzyme activity, and further, lowered the minimum Ca2+ concentration required for activation. In particular, the polyphosphoionositides phosphatidylinositol 4-monophosphate and 4,5-diphosphate were most effective. These two polyphosphoinositides lowered the Ca2+ concentration required for half-maximal activation to 10(-5) M and dramatically stimulated the activity at that Ca2+ concentration (greater than 30-fold). The neutral phospholipase A2 activity such as characterized in the present study was very low in the other subcellular fractions including mitochondria, microsome, plasma membrane and cytosol.