Multiple phospholipase A₂ activities in canine vascular smooth muscle

Three phospholipase A₂ activities from canine vascular smooth muscle were identified and characterized including: (1) a cytosolic calcium-independent phospholipase A₂ which is activated by nucleotide di- and triphosphates; (2) a cytosolic calcium-dependent phospholipase A₂ which is activated by physiologic increments in calcium ion concentration; and (3) a microsomal calcium-independent phospholipase A₂ which was highly selective for plasmenylcholine substrate. Vascular smooth muscle cytosolic calcium-independent phospholipase A₂ was activated 338% ± 11 (X + S.E.; n = 15) by physiologic concentrations of ATP. Similar amounts of activation were also present utilizing other nucleotide di- and triphosphates (e.g., ADP, CTP, GDP and GTP) as well as non-hydrolyzable nucleotide triphosphate analogs (e.g., ATP-γ-S, AMP-PNP and GTP-γ-S). Vascular smooth muscle cytosolic calcium-dependent phospholipase A₂ was purified 455-fold by sequential DEAE-Sephacel, PhenylSepharose, Mono Q, hydroxyapatite and Superose 12 chromatographies. The partially purified calcium-dependent phospholipase A₂ was activated by physiologic increments in calcium ion concentration (e.g., 1 μM) and possessed an apparent native molecular weight of 95 kDa, an acidic isoelectric point (pI = 4.8) and a neutral pH optimum (pH 7.0). Vascular smooth muscle microsomal phospholipase A₂ activity was predominantly calcium-independent and was over six-fold selective for hydrolysis of plasmenylcholine substrate. Taken together, these results demonstrate the existence of three separate and distinct phospholipase A ₂ activities in vascular smooth muscle and identify ATP and calcium ion as independent modulators of discrete phospholipase A₂ activities in vascular smooth muscle cells.