Identification of neutral active phospholipase C which hydrolyzes choline glycerophospholipids and plasmalogen selective phospholipase A₂ in canine myocardium

Two novel phospholipase activities have been identified in the cytosolic fraction of canine myocardium. Neutral active phospholipase C activity was partially purified by anion exchange, hydroxylapatite, chromatofocusing, and gel filtration chromatographies. The partially purified enzyme had similar maximum velocities (237 versus 241 nmol/mg · h) and apparent Michaelis constants (20 versus 14 μM) utilizing either plasmenylcholine or phosphatidylcholine as substrate. Myocardial phospholipase C had a pH optimum between 7 and 8, required divalent cations for maximal activity, and did not hydrolyze phosphatidylinositol or sphingomyelin. Myocardial cytosol contained a potent inhibitor of phospholipase C which masked enzymic activity until it was removed during the purification procedure. A plasmalogen selective phospholipase A₂ activity was also identified in the cytosolic fraction of canine myocardium. The protein catalyzing this activity was partially purified by DEAE-Sephacel-hydroxylapatite tandem chromatography and exhibited a maximum velocity of 5 nmol/mg·h for plasmenylcholine but only 1 nmol/mg·h for phosphatidylcholine, had a pH optimum between 6 an 7 for both substrates and did not require calcium ion for activity. These results constitute the first demonstration of a neutral active phospholipase C specific for choline and ethanolamine glycerophospholipids and a plasmalogen selective phospholipase A₂ in mammalian tissue.