Ca²⁺-Binding of S-100 Protein in the Presence of K⁺ and Mg²⁺

Abstract

Ca²⁺-binding of S-100 protein was studied using a Ca²⁺ electrode at pH 6.80. In the presence of 0.1 M KC1 and 10 mM MgCl₂ (ionic strength 0.13), Ca²⁺-binding to S-100 protein occurred in three steps with positive cooperativity. The numbers of bound Ca²⁺ ions in the three steps were 2, 2, and 4. The Ca²⁺-binding constants were 6.9×10³M^-1, 2.9×10³M^-1, and 3.7×10²M^-1, respectively. The Ca²⁺-binding constants of the first and second steps obtained in the presence of 33.3 mM MgCl₂ or 0.1 M KCl (ionic strength 0.10) were 1.4 times larger than those described above. This suggests that Mg²⁺ does not inhibit Ca²⁺-binding of S-100 protein. The increase of KC1 concentration from 0.1 to 0.2 M caused a decrease of the Ca²⁺-binding constants to ca. 50%.