Phospholipases of the myocardium

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Abstract
The myocardium contains diverse cellular components and heterogeneous phospholipid-containing membranes. The major phospholipids are phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositnol, sphingomyelin, and cardiolipin. The phospholipases capable of hydrolyzing these membrane lipids include phospholipase A, lysophospholipase, and phosphatidylnositol-specific phospholipase C. Early studies revealed that myocardial phospholipase A with an acid pH is localized to lysosomes; those with more alkaline and neutral activities are present in cytosol, microsomes, mitochondria and sarcolemma. Recently, we have identified phosphatidylinositol-specific phospholipase C activity in bovine myocardium with molecular weights ranging from 40,000 to 271,000. Interestingly, forms I, II and III had pH optima ranging from 4.5 to 5,5; form III also had significant activity at pH 7.0. All activities were stimulated by calcium, suggesting that they are different from calcium-independent phospholipases C found in liver and brain. The pathophysiological significance of these four cytosolic forms of phospholipase C remains to be determined. Thus, under injury-promoting conditions, phospholipase C appears capable of hydrolyzing membrane-associated phosphatidylinositol and the polyphosphoinositides, whereas phospholipases A and lysosphospholiphases appear to prefer non-inositol containing phospholipids. Finally, very recent studies suggest “free radical-triggered lipolysis” by phospholipases as a possible mechnism for production of lysophospholipids in myocardial membranes. Fig.1 Percentage (± SEM) of total free fatty acids detected in biopsy samples from control and ischaemic isolated perfused canine myocardium. After lipid extraction, one dimension TLC and esterification with boron trifluoride-methanol, the fatty acid sample (in hexane) was injected into the gas chromatograph for detection by hydrogen flame ionization. Student’s t-test for paired samples was used.