The exothermic reaction of calcium with unilamellar phosphatidylserine vesicles: Titration microcalorimetry

Phospholipid membranes have been used extensively as model systems for understanding the mechanism of membrane fusion in biological systems (reviews [I ,2]). Phosphatidylserine (PS) membranes have unusual properties with respect to their interaction with divalent ions. For example, planar bilayer membranes (black lipid films) are unstable if Ca*+ is added to only one side of the membrane [3,4]. Small unilamellar PS vesicles have a similar instability in the presence of Ca*+ in that they fuse [5], leak their contents [3], and eventually form cochlear cylinders [6]. PS also exhibits a specificity for the binding of Ca*+, and the gel to liquid crystalline transition temperature of the phospholipid is shifted to above 100°C when Ca*’ is added [7,8]. The binding of divalent ions to PS appears to be correlated with their ability to fuse sonicated vesicles made of this phospholipid [8-l I]. The addition of Ca*+ to these vesicles results in the release of heat as measured in a batch calorimeter [8]. To identify the source of the heat released and its possible involvement in the fusion reaction requires a detailed knowledge of the relationship of the extent of the exothermic reaction and the amount of Ca*+ bound to the vesicles. For this purpose, we have measured the heat of reaction in a titrating microcalorimeter and have determined the Ca*+ bound by means of a