The calcification of fibrin in vitro

In the present work we employed fibrin in order to assess its capability to induce biological mineralization. Fibrin is a very important factor in the blood-clotting system. Structurally, fibrin is an ordered organic matrix w hich has a periodic structure that repeats every 230 Angstrom. Hydroxyapati te, HAP and octacalcium phosphate (OCP) are the most interesting calcium ph osphate salts. Hydroxyapatite is thermodynamically the most stable calcium phosphate which is mostly used as a model compound for the study of biologi cal-calcification processes. On the other hand, octacalcium phosphate has b een proposed as a precursor of hydroxyapatite whose formation is favoured k inetically in solutions supersaturated to both salts. The kinetics of cryst allization of HAP and OCP on fibrin were studied using the constant composi tion technique. The onset of HAP crystallization started immediately after introducing the substrate in the supersaturated solution. Unlike HAP crysta llization induction periods were observed before the appearance of OCP prec ipitate in a solution supersaturated with respect to both HAP and OCP. Usin g nucleation rate equations derived from the classical homogeneous nucleati on theory, interfacial energies and the size of the critical nucleus for bo th HAP and OCP were calculated. Phosphate was taken up extensively by the b iological molecule studied. The dependence of adsorption upon ionic strengt h and pH of the medium suggests an appreciable contribution of electrostati c forces. Controversially calcium ions did not exhibit any detectable adsor ption from solutions containing calcium dichloride at concentrations rangin g from 1 x 10(-4) to 5 x 10(-3) M in 0.15 M NaCl supporting electrolyte, 37 degrees C, pH = 7.4. From the results above it follows that formation of H AP on fibrin may be initiated via adsorption of inorganic phosphate on the biological substrate. (C) 2000 Elsevier Science B.V. All rights reserved.