Calcium ions reverse a latency period induced by bovine serum albumin in the time course of phospholipase A2 action on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes.

The time course of hydrolysis of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes in the gel phase catalyzed by porcine pancreatic phospholipase A2 was studied at 1 mM NaCl and variable CaCl2 concentrations, in the presence of delipidated bovine serum albumin. It has been found that the duration of the latency induced by bovine serum albumin shows an inverse dependence with CaCl2 concentration. As we showed previously, the induction of a lag phase by bovine serum albumin is related to its ability to sequester the fatty acid newly released by hydrolysis. Based on this and on our observation that there is an inverse dependence between the length of the latency period and the interfacial calcium ion concentration, it is interpreted that, while a direct effect of bovine serum albumin is the diminution of the liposome negative surface charge density by sequestration of the fatty acid released during hydrolysis, an indirect effect could be the decrease in the surface Ca2+ concentration. This, in turn, should diminish the enzyme binding to the lipid-water interface. The appearance of a latency phase seems to be the final consequence of these events.