The role of Mg²⁺ in the contraction and adenosine triphosphatase activity of myofibrils

1. 1. The activation by Mg²⁺ of the ATP splitting of myofibrils depends on the concentration of Ca²⁺ ions present. If the concentration of Ca²⁺ is high enough to suppress the dissociation of actomyosin, Mg²⁺ activates, otherwise it inhibits. 2. 2. The course of the curve describing the activation by Mg²⁺ does not depend on the concentration of ATP present. 3. 3. In the presence of 10^-4 M Ca²⁺, the addition of increasing concentrations of Mn²⁺, Ba²⁺, Sr²⁺ or Ca²⁺ causes an activation much resembling that caused by Mg²⁺. Activation is lowest when Ca²⁺ is the only bivalent cation present. 4. 4. Mg²⁺ and Ca²⁺ are both essential for contraction. In the presence of 2·10^-5 M Mg²⁺ and 2·10^-3 M ATP there is no contraction unless Ca²⁺ in a concentration sufficient to repress dissociation (approx. 10^-6 M) is also present. In the presence of 10^-5 M Ca²⁺ a maximal rate of contraction is reached only when a minimal level of Mg²⁺ concentration (approx. 10^-6 M) is reached. There is, however, contraction in a Ca²⁺-free milieu too if the concentration of MgATP does not exceed 1·10^-5 M. 5. 5. The substrate proper of myofibrillar ATPase (ATP phosphohydrolase, EC 3.6.1.3) is shown to be free ATP. The inhibition by a total Mg²⁺ concentration exceeding that of ATP is thus caused by the decrease in available substrate, i.e. free ATP. If a constant free ATP concentration is ensured there is no inhibition even by 10^-2 M Mg²⁺.