Determination of calcium phosphate inhibitor activity. Critical assessment of the methodology

A method is described which determines the amount of calcium phosphate inhibitor activity in solutions with ionic compositions similar to that of undiluted plasma or serum. Constant supersaturations were obtained for the inhibitor analysis by equilibrating an aliquot of sample with dicalcium phosphate dihydrate (DCPD) and it was shown that the ion activity product of the equilibrated solutions was similar to the solubility product of DCPD. The amount of inhibitor activity in solution is assessed by determining the amount of hydroxylapatite seed needed for a predetermined rate of crystal growth. It was shown that the Langmuir adsorption isotherm is an appropriate model for describing the inhibitor data and provides a convenient linear-regression means for computing inhibitor activities. When tested at its normal physiological concentrations in plasma, albumin was the most important of the known calcium phosphate inhibitors.