PERSISTENCE OF COMPLEXED ACIDIC PHOSPHOLIPIDS IN RAPIDLY MINERALIZINGTISSUES IS DUE TO AFFINITY FOR MINERAL AND RESISTANCE TO HYDROLYTIC ATTACK - IN-VITRO DATA

Acidic phospholipids, complexed with calcium and inorganic phosphate, are components of extracellular matrix vesicles. Both the complexed ac idic phospholipids and matrix vesicles have previously been shown to s erve as hydroxyapatite (HA) nucleators in solution and when implanted in a muscle pouch. The present study supplies evidence that complexed acidic phospholipids can persist in mineralizing tissues both because of their affinity for HA and because of their resistance to hydrolysis by phospholipase A(2). Calcium-phosphatidylserine-phosphate complex ( CPLX-PS) synthesized with C-14-labeled phosphatidylserine (PS) was use d to measure CPLX-PS affinity for HA using a Langmuir adsorption isoth erm model. The affinity was shown to be higher and more specific than that of PS itself (K = 8.66 ml/mu mol; N, the number of binding sites = 20.4 mu mol/m(2) as compared with previously reported values for PS of K = 3.33 ml/mu mol, and N = 4.87 mu mol/m(2)). Incorporated into sy nthetic liposomes and incubated in a calcium phosphate solution in whi ch mineralization is induced by an ionophore, CPLX-PS showed behavior distinct from free PS. As previously reported, PS in these liposomes t otally blocked HA formation. On the other hand, CPLX-PS in similar con centrations had a varied response, having no effect, slightly inhibiti ng, or actually promoting HA formation. CPLX-PS was also shown to be a poorer substrate for phos pholipase A(2) than PS, with Km = 4.63 mM f or CPLX-PS and Km = 0.27 mM for PS; and V-max = 0.029 ml/minute for CP LX-PS and V-max = 0.066 ml/minute for PS. These data explain how compl exed acidic phospholipids may persist in the growth plate and facilita te initial mineral deposition.