The interaction of α-tocopherol with phosphatidylserine vesicles and calcium

Abstract

The interaction of α-tocopherol with dimyristoylphosphatidylserine (DMPS) has been studied in the presence and in the absence of Ca 2+ by using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and 45Ca 2+-binding. In the absence of Ca 2+, DSC showed that α-tocopherol decreases the temperature of the lamellar gel to lamellar liquid crystalline phase transition as well as it decreases ΔH of this transition. Two different peaks were detected at 10 mol% of α-tocopherol and probably one of the peaks correspond to pure or nearly pure DMPS and the other to DMPS incorporating most of the α-tocopherol. The phase transition was totally abolished at 30 mol% of α-tocopherol. In the presence of Ca 2+ this L β to L α phase transition of DMPS was even more perturbed by α-tocopherol, so that it was totally abolished by only 7 mol% of α-tocopherol, at Ca 2+ concentrations which were clearly non-saturating, like those giving DMPS/Ca 2+ molar ratio of 4:1 and 10:1. Furthermore, the transition of the DMPS/Ca 2+ complex observed at 91.6°C was perturbed by the presence of α-tocopherol, indicating a change in the structure of the crystalline complex. The FT-IR analysis of the effect of α-tocopherol on DPMS phase transition confirmed the decrease in the phase transition temperature of the phospholipid, and also that α-tocopherol increases the number of gauche isomers in the gel state but has no effect in the liquid crystalline state. The binding of 45Ca 2+ was also affected by the presence of α-tocopherol, so that the number of binding sites was decreased, and this may be interesting for situations in which phosphatidylserine and Ca 2+ are simultaneously implicated in biological functions, such as membrane fusion and enzyme activation.