Effects of phospholipase A2 and albumin on the calcium-dependent ATPase and the lipid composition of sarcoplasmic membranes.

1. The calcium-dependent ATPase activity of phospholipase-A2-digested sarcoplasmic vesicles decreases concomitantly with the contents of residual lysophospholipids and fatty acids when increasing albumin concentrations are applied. 2. Delipidated albumin preferentially removes unsaturated fatty acids and lysophosphatidylcholine. A complete removal of the phospholipids by albumin does not occur. 3. The membrane-bound lysophospholipids were analysed with respect to type of phospholipid, plasmalogen content and fatty acid chains by means of thin-layer chromatography and gas chromatography. 4. While the fatty acid composition of the lysophospholipids is independent of the degree of delipidation, the composition of the residual free fatty acids is found to change with the albumin concentration. 5. Reactivation of the Ca2+-ATPase by oleate leads to reasonable activities at room temperature as long as a minimum of about 30 lysophospholipid molec-les per ATPase is left. The course of the residual Ca2+-ATPase activity with the degree of delipidation is related to the presence of unsaturated fatty acids. 6. No specific role of either sphingomyelin or the plasmalogens has been found.