Blood:bone disequilibrium. VI. Studies of the solubility characteristics of brushite: apatite mixtures and their stabilization by noncollagenous proteins of bone

Abstract

Recent evidence of the occurrence of brushite in newly formed bone mineral prompted a study of the solubility properties of brushite:apatite mixtures under physiological conditions and the influence on them of pH, lactate, pyruvate, and, particularly, noncollagenous bone proteins (NCBPs). Brushite alone was surprisingly stable in solution at pH 7.4, 37 degrees C. In the presence of increasing amounts of apatite, hydrolysis of brushite to an insoluble phase occurred. A decrease of 0.1 pH unit or the addition of 1.5 mM pyruvate or 10 mM lactate increased the ion activity product (Ca2+ x HPO4(2-) 3 or more times. However, within the loose envelope of bone such conditions so different from those in the circulation might be only local or temporary. NCBPs, on the other hand, stabilized brushite in solution alone as well as in the presence of apatite for days. They probably act by adsorbing strongly to the crystal surface and preventing nucleation by apatite. This brushite-apatite-bone protein system exhibits solubility characteristics that can resolve the old problems presented by the participation of the skeleton in extracellular calcium homeostasis on the one hand, and by the apparent insolubility of the apatite mineral of bone on the other.