Studies on the exchange of G-actin-bound calcium with bivalent cations

1. The possibility of the replacement of G-actin-bound calcium by various bivalent cations has been investigated. After the reaction with all cations studied, with the exception of Cu²⁺, action remains active, i.e., contains bound ATP and polymerizes in 0.1 M KCl.

2. The amount of G-actin-bound calcium, as well as the sum of bivalent cation after replacement, not removable by short-time Dowex-50 treatment, accounts to about 1 mole per 50000 g of G-actin.

3. The rate of exchange is of the same order for bivalent cations studied, including calcium.

4. G-actin-bound Ca²⁺ is fully replaced, besides free Ca²⁺, by free Mn²⁺ and Cd²⁺. The replacement with Mg²⁺, Co²⁺, Ni²⁺ and Zn²⁺ is not complete, and there is practically no reaction with Ba²⁺ and Sr²⁺.

5. Assuming the affinity constant of Ca²⁺ as 1, the following affinity constants for other bivalent cations were obtained: Mn²⁺, 0.90; Cd²⁺, 1.07; Mg²⁺, 0.27; Zn²⁺, 0.22; Co²⁺, 0.18; Ni²⁺, 0.08.

6. The results obtained show that there exists a close correlation between the ionic radius of a particular bivalent cation, and its ability to replace bound Ca²⁺.