Characterization of phospholipase a 2 activity in rat aorta smooth muscle cells

Abstract

Phospholipase A activity was measured in homogenates and acid extracts of smooth muscle cells from rat aorta and mesenteric artery using [1- 14C]oleate-labeled autoclaved Escherichia coli and 1-[1- 14C]stearyl-2-acyl-3- sn-glycerophosphorylethanolamineas substrates. The results demonstrate the presence of neutral-active phospholipase(s) A that exclusively catalyze the release of fatty acid from the 2-position of phospholipids. Optimal activity was at pH 7.5, and there was an absolute requirement for low concentrations of Ca 2+. Mg 2+ did not substitute for Ca 2+, and EGTA inhibited the activity. Phospholipase A 2 activity was predominantly membrane-associated and was solubilized by homogenization in 0.18 N H 2SO 4. Sulfuric acid extracts of rat aortic smooth muscle cells were four times more active than extracts of mesenteric artery (710 vs. 170 nmol/h per mg protein). By comparison, acid extracts of rat lung, heart, and liver were less active (60–75 nmol/h per mg). Indomethacin, sodium meclofenamate, mepacrine and chlorpromazine, but not dexamethasone or aspirin, inhibited acid-solubilized phospholipase(s) A 2 between 10 −6 and 10 −3 M in a dose-dependent manner. Preincubation with p-bromophenacyl bromide or diethylpyrocarbonate inhibited phospholipase(s) A 2, suggesting the presence of a histidine residue at the active site. An extract from the leaves of feverfew plant ( Tanacetum parthenium) was also a potent inhibitor of aortic smooth muscle phospholipase(s) A 2.