The asymmetric distribution of phospholipids in bovine endothelial-cell membranes was probed with 2,4,6-trinitrobenzenesulphonate and purified phospholipase A2. The data suggest that phosphotidylethanolamine is primarily located in the inner lipid bilayer, as reported for other cell types. Stearic acid is taken up by the endothelial cells and is randomly distributed among the membrane phospholipids. In contrast, the polyunsaturated fatty acids (arachidonic, eicosatrienoic and eicosapentaenoic acids) have initial incorporation into the phosphatidylcholine fraction. These fatty acids then undergo a time-dependent transfer from phosphatidylcholine to phosphatidylethanolamine. Thus we propose that endothelial cells possess a mechanism for the selective internalization of polyunsaturated fatty acids.
Plasminogen activators were isolated from porcine heart, lung and kidney and from human heart and lung. Highly purified porcine heart activator has been shown to have plasminogen activating properties different from human urinary urokinase (UK). The tissue and urinary activators were subjected to isoelectric focusing in granulated dextran gel (Ultradex) over the pH gradient range 3.5 to 11.0, and activator activity was measured by bovine fibrin plate assay. Electrofocusing of Abbott UK revealed isoelectric forms at pI 8.6, 8.8, 9.2, 9.4 and above 9.8, while Serono UK gave forms at 8.2 and 8.6. In contrast, activators from porcine tissues commonly focused at about 7.3, although occasional preparations showed non-symmetrical activity profiles, and focusing of these preparations in a pH gradient 5 to 9 revealed major PI forms at 7.2 and 7.7. Activators from human heart and lung were more closely related to activators from porcine tissues than to UK. Since UK preparations probably contain proteolytically degraded molecules, tissue activators with lower isoelectric points may represent the native molecular forms. Further evidence also indicates that a more catalytically efficient form of tissue activator may be composed of an acceleratory protein and an amidase complexed to a plasminogen activator molecule. (Supported by USPHS NIH Grant HL-15817)
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