Optimal conditions for Ca-acidic phospholipid-PO4 formation.

Calcium-acidic phospholipid-phosphate complexes (Ca-PL-PO4) cause hydroxyapatite (HA) deposition in vitro. The acidic phospholipids--phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylglycerol (PG), and diphosphatidylglycerol (DPG)--all form Ca-PL-PO4 complexes: Nonacidic phospholipids--sphingomyelin (SPL), phosphatidylcholine (PC) and phosphatidylethanolamine (PE)--do not form Ca-PL-PO4 and do not induce hydroxyapatite (HA) deposition in low ionic strength metastable calcium phosphate solution or in synthetic lymph. The extent to which each of the acidic phospholipids reacts with calcium and inorganic phosphate, and the chemical composition of the Ca-PL-PO4 complexes is a function of pH, solution composition, the nature of the phospholipid, and the method of isolation. The addition of inorganic phosphate prior to or in combination with calcium appears to be an absolute requirement for Ca-PL-PO4 formation.