Phosphatidylinositol-specific phospholipase C (PLC) plays a crucial role in initiating the surface receptor mediated signal transduction by generating two second messenger molecules, diacylglycerol and inositol 1,4,5-triphosphate. We resolved two forms of bovine brain enzyme, PLC-I and PLC-II, on a HPLC-DEAE column and purified PLC-II to homogeneity. Upon analysis of PLC-II on SDS-PAGE, a single band of MW = 145,000 was observed. When the same sample was subjected to native gradient polyacrylamide gel, four bands, one major band of MW = 200,000 and three minor bands with molecular weights corresponding to different oligomeric states of the 200K Da protein, were visible. Western blot experiments using anti-PLC-II antibody indicated that PLC-I might be derived from PLC-II by proteolytic cleavage. Multiple forms of brain PLC enzymes described in the literature might be, therefore, attributed to the oligomerization and proteolysis of PLC-II. PLC-I and PLC-II hydrolyzed both phosphatidylinositol and phosphatidylinositol 4,5-diphosphate. Both activities were stimulated by Ca(II). However, the presence of Ca(II) was not an absolute requirement for the hydrolysis of phosphatidylinositol 4,5-diphosphate while phosphatidylinositol hydrolysis at neutral pH required Ca(II). Protein kinase C phosphorylates PLC-II in a Ca(II) and phospholipid-dependent manner. The physiological meaning of this phosphorylation is not known yet.
