Table 1.

Actin-activated ATPase activity of the myosin fragments

Myosin fragments	Vmax, S−1
	Phosphorylated	Dephosphorylated
Skeletal HMM	4.67  ±  0.40	4.50  ±  0.41
Smooth HMM	0.86  ±  0.09	0.06  ±  0.01
Chimeric myosin	4.32  ±  0.03	0.48  ±  0.01

Actin-activated ATPase activity was measured at 25°C in 0.01 mg/ml myosin fragment, 0.03 mM ATP, 60 mM KCl, 30 mM Tris·HCl, pH 7.5, 8 mM MgCl₂ with various concentrations of F-actin. To measure the activity of phosphorylated myosin, 0.2 mM CaCl₂, 15 μg/ml myosin light chain kinase, and 10 μg/ml calmodulin were added, whereas 1 mM EGTA was added for the dephosphorylated one. Results are means ± SD of three independent preparations. A computed nonlinear least-squares curve-fitting program was used to estimate the maximum actin-activated ATPase activity (V_max) and the apparent dissociation constant for actin (K_a) based on the equation V = V_max/(1 + K_a/[actin]).