Macrophage arachidonate-mobilizing phospholipase A2: role of Ca2+ for membrane binding but not for catalytic activity.

A recently purified Ca(2+)-dependent intracellular phospholipase A2 from spleen, kidney and macrophage cell lines is activated by Ca2+ at concentrations achieved intracellularly. Using enzyme from the murine cell line J774 we here demonstrate the formation of a ternary complex of phospholipase, 45Ca2+ and phospholipid vesicle, and provide evidence for a single Ca(2+)-binding site on the enzyme involved in its vesicle binding. Although Ca2+ binds to and functions as an activator of the enzyme, this ion does not appear to be involved in its catalytic mechanism, since enzyme brought to the phospholipid vesicle by molar concentrations of NaCl or NH4+ salts exhibited Ca(2+)-independent catalytic activity.