dioleoylphosphatidic-acid and dicetylphosphate

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 degrees 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 metasable 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Biological Transport; Calcification, Physiologic; Calcium Phosphates; Chemical Precipitation; Cholesterol; Kinetics; Liposomes; Membrane Lipids; Models, Chemical; Organophosphates; Phosphatidic Acids; Phosphatidylcholines; Sphingomyelins

Liposome prepared from 7:2:1 molar mixtures of phosphatidylcholine, dicetyl phosphate, and cholesterol to which 1-20 mole % dioleoylphosphatidic acid (DOPA) was added were used to examine the effect of membrane-bound monoester phosphatidate anions on calcium phosphate formation in aqueous solutions at 22 degrees C, pH 7.4 and 240 mOsm. Results showed that up to 20 mole % DOPA in the liposomal envelope did not initiate mineralization in solutions made metastable with 2.25 mM CaCl2 and 1.50 mM KH2PO4. Results alos revealed that precipitation induced in metastable solutions by the seeding action of intraliposomally formed mineral was measurably reduced with as little as 1 mole % DOPA and completely suppressed with 5 mole % DOPA. In contrast, 10 mole % concentrations of diester phosphate lipids either had no effect on extraliposomal precipitation (e.g., phosphatidylglycerol and phosphatidylinositol) or, as reported previously for phosphatidylserine) only partially reduced the amount of precipitate formed. Transmission electron microscopical analysis suggests that DOPA-containing lipid bilayers adhering to the seed crystals inhibited extraliposomal mineralization by encapsulating the crystals within the liposomes and/or by blocking potential growth sites on the crystal faces. The polar head group of DOPA, being more negatively charged and sterically less encumbered than diester phosphate ligands, most probably was responsible for this adherence of the lipid bilayers to the crystal surfaces.

Topics: Animals; Anions; Calcium; Calcium Phosphates; Liposomes; Membrane Lipids; Organophosphates; Phosphatidic Acids; Phosphatidylglycerols; Phosphatidylinositols; Phosphatidylserines