Effect of different phospholipid-cholesterol membrane compositions on liposome-mediated formation of calcium phosphates

Summary The present report compares the effects of different membrane phospholipid (PL)-cholesterol compositions on the kinetics of liposome-mediated formation of calcium phosphates from metastable solutions (2.25 mM CaCl₂; 1.5 mM KH₂PO₄) at 22°C, pH 7.4 and 240 mOsm. In most experiments, the liposomes were composed of 7:2:X mixtures of phosphatidylcholine (PC), neutral or acidic phospholipids, and cholesterol (Chol, X=0, 10, 35, or 50 mol%). The neutral phospholipids (NPL) examined, in addition to PC, were phosphatidylethanolamine (PE) and sphingomyelin (Sph), and the acidic phospholipids (APL) examined were dicetylphosphate (DCP), dioleolylphosphatidylglycerol (DOPG), dioleolylphosphatidic acid (DOPA), phosphatidylserine (PS) and phosphatidylinositol (PI). The 7:2:X liposomes did not initiate mineralization in metastable external solutions per se or, with the exception of DOPA, show extensive Ca-PL binding. However, solution Ca²⁺ losses due to precipitation occurred when the liposomes were encapsulated with 50 mM KH₂PO₄ and made permeable to external Ca²⁺ with X-537A. The extent of these Ca²⁺ losses was sensitive to both the phospholipid and Chol makeup of the membrane. Moderate-to-extensive intraliposomal precipitation occurred in all 7PC:2APL and 7PC:2NPL liposomes containing 0 or 10 mol% Chol. In contrast, at 50 mol% Chol, mineralization inside all liposomes was negligible. The only significant discriminating effect on internal mineralization among the different phospholipids was observed at 35 mol% Chol, where mineral accumulations ranged from negligible to moderate. At 0 or 10 mol% Chol, extraliposomal precipitation was extensive in all but DOPA- and PS-containing liposomes. However, onece intraliposomal yields declined at the higher Chol levels, external mineralization was either delayed or totally blocked in all liposome preparations. Other experiments showed that Sph substituted for PC in 7NPL:2DCP:1Chol liposomes totally blocked both intra- and extraliposomal precipitaiton. PE substituted in this manner, however, blocked only extraliposomal precipitation. The results of this study suggest that interference of the membrane transport processes controlling intraliposomal precipitation 15] by high (50 mol%) Chol levels is not significantly compromised by the specific APL or NPL incorporated in the membrane. Similarly, the data suggest that Chol does not directly affect the specfic interactions of the different membrane APLs with the mineral phase. On the other hand, the substitution of other NPLs for PC can affect the role of APLs such as DCP in liposome-mediated mineralization.