Myosin modified in the presence or in the absence of pyrophosphate by 2,4-dinitrophenyl β-hydroxyethyl disulphide was treated with iodo[1-14C]acetamide. The residual Ca2+-stimulated adenosine triphosphatase (ATPase) activity of the modified myosin was different depending on the presence or absence of PPi during modification and the number of 2,4-dinitrophenyl β-hydroxyethyl disulphide-modified thiol groups. The radioactivity incorporated into the light components of myosin correlated with the Ca2+-stimulated ATPase activity of the modified myosin and decreased with decreasing residual Ca2+-stimulated ATPase activity of the modified myosin. When native myosin was treated with low concentrations of iodo[1-14C]acetamide the residual Ca2+-stimulated ATPase activity of carboxyamidomethylated myosin was high and the radioactivity incorporated into the light components of myosin was negligible. The thiol groups of the light components of myosin are essential to preserve the ATPase activity of the protein and are close to the pyrophosphate-binding sites.

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