Effect of sphingosine and stearylamine on the interaction of phosphatidylserine with calcium. A study using DSC, FT-IR and 45Ca(2+)-binding.
The lamellar gel to lamellar liquid-crystalline phase transition of dipalmitoylphosphatidylserine (DPPS) multilamellar membranes is abolished by the presence of Ca2+ at DPPS/Ca2+ molar ratios of 2:1 or lower. However, when equimolar sphingosine (SPH) or stearylamine (SA), which are positively charged at the pH studied in this work, were included in DPPS vesicles, the phase transition of DPPS was still observed by differential scanning calorimetry, even in the presence of very high Ca2+ concentrations such as a DPPS/Ca2+ molar ratio of 1:10. According to that, delta H was similar for samples formed by equimolar DPPS and SPH and SA, either in the presence or in the absence of Ca2+, whereas no phase transition was observed for the pure phospholipid in the presence of Ca2+ at molar ratios lower than DPPS/Ca2+ 2:1. 45Ca(2+)-binding experiments showed that for DPPS/SPH or DPPS/SA molar ratios of 2:1, only half of the Ca2+ was bound to DPPS with respect to pure DPPS, i.e., in the absence of SPH or SA. At concentrations of SPH or SA equimolar with DPPS, the Ca2+ binding was nearly abolished. The effect of SPH and SA on the the apparent pKapp of the carboxyl group of DPPS was also studied in the presence and in the absence of Ca2+ by using Fourier transform infrared spectroscopy. The dehydration of the phosphate group of DPPS induced by the binding of Ca2+ was followed through the observation of the PO2- antisymmetric stretching, and the percentage of dehydrated PO2- groups quantitatively assayed. It was again confirmed that, in the presence of equimolar concentrations of SPH or SA, Ca2+, at concentrations which are saturating for pure DPPS, was not bound at all to DPPS. It was also found that the pKapp was considerably shifted to lower values in the presence of the amino bases, decreasing from 4.6 in pure DPPS to 2.1 and 2.2 for the equimolar mixtures of DPPS with SPH and SA, respectively. These results show that SPH and SA, being positively charged molecules anchored in the membrane, are able of preventing the binding of positively charged ions such as Ca2+ through an electrostatic charge neutralization.