Interaction of phosphatidylserine with mast cells

Phosphatidylserine (PtdSer) potentiates histamine secretion from mast cells exposed to concanavalin A and Ca2+. In order to identify the form of PtdSer that is responsible for its effect on mast cell secretion, PtdSer containing a tritium-labeled serine moiety (3H-PtdSer) was synthesized from egg yolk phosphatidylcholine. The critical micelle concentration (CMC) of 3H-PtdSer and the binding isotherm for 3H-PtdSer interaction with mast cells were determined. The midpoints of the binding isotherm and the dose-response curve for potentiation of secretion coincide and are 2 orders of magnitude greater than the CMC. The shape of the binding curve is explicable either in terms of simple binding of preformed PtdSer micelles or of cooperative binding of monomeric PtdSer in which the number of molecules cooperatively associating with a mast cell binding site is equal to the number of monomers in a PtdSer micelle. In either case, at equilibrium, PtdSer micelles are bound to the mast cells. The number of PtdSer molecules bound to a single mast cell at equilibrium was estimated to be 3.7 X 10(9).