Effect of α-actinin on actin structure Actin ATPase activity

α-Actinin increases the ATPase activity of actin by up to 84%, depending on pH, divalent cations present and the added Mg²⁺: ATP ratio. Dithiothreitol decreases actin ATPase activity approx. 20% but does not reduce the ability of α-actinin to increase actin ATPase activity. Increasing amounts of added α-actinin up to 1 mol α-actinin to 49 mol actin cause an increasing increment in actin ATPase activity, but adding α-actinin beyond 1 mol α-actinin to 49 mol actin elicits only small additional increments in activity. Actin ATPase activity ranges from approx 100 nmol P_i/mg actin per h (4.3 mol P_i/mol actin per h) at high levels (10 mM) of ATP in the presence of lower amounts (1 mM) of added Mg²⁺ to approx. 12.5 nmol P_i/mg actin per h (0.52 mol P_i/mol actin per h) at high pH (8.5) or at low levels (0.5–1.0 mM) of ATP in the presence of higher amounts (10 mM) of added Mg²⁺. ATP uncomplexed with Mg²⁺ inhibits the ability of α-actinin to increase F-actin ATPase activity. Activities with different divalent cations showed that the actin ATPase in these studies, which was 1/100 as great as Mg²⁺-modified actomyosin ATPase activity, was not due to trace amounts of myosin contaminating the actin preparations. The results are consistent with the concept that α-actinin can alter the structure of actin monomers.