Myocardial ischemia is accompanied by accelerated phospholipid catabolism resulting in the depletion of membrane phospholipids and the accumulation of amphiphilic metabolites which alter electrophysiologic function and predispose to ventricular dysrhythmia. Alterations in myocyte ionic permeability and electrophysiologic function are most likely mediated by changes in the chemical composition of the sarcolemmal membrane. Recently we have identified plasmalogen as the major phospholipid constituent of canine myocardial sarcolemma. Since catabolism of sarcolemmal phospholipids likely contributes to ventricular dysrhythmia and ischemic membrane dysfunction and since the catabolic pathways of plasmalogens in myocardium have not been identified, we developed a convergent synthesis for the preparation of homogeneous radiolabeled choline and ethanolamine plasmalogens and have identified two novel mammalian phospholipase activities in canine myocardium which hydrolyze plasmalogen. A neutral active calcium-independent phospholipase A2 which is selective for the hydrolysis of the sn-2 fatty acid of plasmalogens and a neutral active phospholipase C specific for choline and ethanolamine glycerophospholipid have been identified. Flux through these catabolic pathways results in the production of several biologically important lipids including arachidonic acid, lysoplasmalogen, and diglycerides. The objective of the proposed research is the purification and characterization of myocardial phospholipases A2 and C and the identification of the regulatory mechanisms which modulate the enzymic activity of these phospholipases in myocardium. We will purify myocardial neutral active phospholipase C activity which hydrolyzes choline and ethanolamine glycerophospholipids, identify fhte chemical nature of an endogenous inhibitor of enzymic activity present in the cytosol and determine factors which modify the inhibitor-enzyme interaction and therefore likely modulate enzymic activity. Next we will purify myocardial plasmalogen-selective phospholipase A2 activity and characterize the mechanism of its modulation by cAMP. FInally we will identify, characterize and purify sarcolemmal enzymes which catalyze the catabolism of plamsalogens and characterize their modulation by ligant-receptor coupling. Taken together the proposed research will delineate the catabolic pathways of plasmalogens in myocardium and identify modulators of enzymic activity which regulate the production of several biologically active metabolites which likely contribute to the sequelae of myocardial ischemia.
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