DIACYLGLYCEROL, PHOSPHATIDYLSERINE AND CA2- A PHASE-BEHAVIOR STUDY( )

The interaction of 1,2-dipalmitoylglycerol (1,2-DPG) with dipalmitoylp hosphatidylserine (DPPS) has been studied in the presence and in the a bsence of Ca2+ by using differential scanning calorimetry (DSC) and P- 31-nuclear magnetic resonance (P-31-NMR). In the absence of Ca2+, DSC showed that 1,2-DPG increased the phase transition of DPPS, effect alr eady noticed at very low 1,2-DPG concentrations, whereas lipid immisci bilities were detected at concentrations of 1,2-DPG higher than about 30 mol%. P-31-NMR indicated that lamellar phases were always present a t concentrations of 1,2-DPG lower than about 35 mol%, but at higher co ncentrations non-lamellar phases may be present in the fluid phase. As observed by DSC, the apparent pK(a) of the carboxyl group of DPPS was increased slightly in the presence of 1,2-DPG. In the presence of Ca2 +, the effect of 1,2-DPG was to further increase the temperature of th e onset of the phase transition, indicating an stabilization of the mo st rigid phase in the DPPS/1,2-DPG/Ca2+ samples. Even concentrations o f 1,2-DPG as low as 1 mol% of the total lipid already produced a notic eable effect. Moreover, lipid immiscibilities were apparent at concent rations of 1,2-DPG higher than 20 mol%. Furthermore, the transition of the DPPS/Ca2+ complex observed by DSC at 155-degrees-C was perturbed by the presence of 1,2-DPG, indicating a change in the structure of th e crystalline complex. Interestingly, the effect of non-saturating Ca2 + concentrations on the DPPS phase transition was enhanced by the pres ence of 1,2-DPG. The effect reported here may be significant for a num ber of situations where Ca2+, phosphatidylserine and diacylglycerols a re involved, such as fusion of membranes, where diacylglycerol may fac ilitate Ca2+-induced fusion, or the activation of enzymes such as prot ein kinase C and phospholipases.