Activity and Positional Specificity of a Rat Plasma Phospholipase on Phosphatidylethanolamine

Abstract

Lysophosphatidylethanolamine (LPE), rich in arachidonic acid, has been proposed by others as a physiologically important inhibitor of the renal enzyme renin that circulates in blood. Intravenously infused phosphatidylethanolamine (PE) has proposedly inhibited the acute pressor effect of injected renin in rats, suggesting a rapid conversion of PE to arachidonate-rich LPE by blood phospholipase activity. Therefore a definition of the activity was attempted. Incubation at 37 °C, pH 7.7–7.9, of a highly purified, 32P-labeled rat liver PE added to fresh rat plasma (heparin) in diethyl ether (0.2 ml/ml plasma) or sodium deoxycholate solution (5 mg/ml plasma) indicated that one phospholipase was dominant, and its activity greater when deoxycholate, rather than ether, was present. Lysophospholipase activity was negligible in either case. Two milliliters of plasma (with deoxycholate) deacylated 47% of 1.5 mg PE (approximately 2 μmol) during 24 min of incubation. Fresh serum had higher activity than corresponding plasma, suggesting activation associated with blood coagulation. C-2 specificity was demonstrated by selective removal of the label from 2-[1-C14]arachidonoyl rat liver PE, without concurrent cholesterol esterification, and by virtually identical compositions (gas–liquid chromatography) of fatty acids removed from PE by plasma or by C-2 specific snake venom phospholipase (Crotalus atrox). Our data appear to rule out C-1 specific post-heparin phospholipase and also C-2 specific lecithin:cholesterol acyltransferase (LCAT), and we present a plasma phospholipase A2 that is highly active against PE and creates an arachidonate-poor LPE, presumably not a renin inhibitor.