State of Calcium in Protein-containing Fluids. ∗

Abstract

We have investigated the nature of the relationship between calcium and protein in blood serum and other protein-containing fluids, using the frog-heart method, previously described, 1 for direct estimation of calcium ion concentrations, and have accumulated evidence concerning the hypothesis 2 that there is a substance, like citrate, and other than protein, which binds calcium in serum and other fluids of the body. When purified electrolyte-free serum proteins (beef) are dissolved in solutions of approximately the electrolyte concentration of the serum, calcium combines with the protein in a manner analogous to that previously described for citrate. 3 The combination represents an equilibrium between protein, total calcium, and calcium ions, and this equilibrium may as a first approximation be described by the mass law equation

When normal human serum is diluted with a solution containing approximately the electrolyte concentration of the serum, but without calcium, and when calcium is added to the various dilutions, the same relationship is found. From observations on both serum and purified serum proteins at pH 7.35 (approximately) a value of 2.22 has been derived for pK_CaProt when grams of protein are reduced to base combining capacity by use of the factors given by Van Slyke, Hastings, Hiller, and Sendroy. 4 Agreement of the values for pK_CaProt obtained from protein-containing fluids and from purified serum proteins indicates that it is unnecessary to postulate any appreciable amount of other calcium-binding substances in human serum.

Table I includes all observations on undiluted human material in which total calcium, total protein, and calcium ions have been determined upon the same specimens. In all cases the factor used for reducing grams of protein to base combining capacity at pH 7.35 is 0.243, which assumes a normal distribution of albumin and globulin. Further work may indicate the desirability of applying separate factors to these proteins.