The competition between adenosine triphosphate and inorganic pyrophosphate for myosin and its suppression by substoichiometric actin concentrations.

Inorganic pyrophosphate (PPi) inhibits not only Mg2+-ATPase activity of myosin subfragment 1 (S-1) but abolishes also the ATP-induced increment of tryptophan fluorescence of subfragment 1. At the concentrations used (25-50 micron ATP, 12 mm PPi) these effects of PPi were abolished by substoichiometric actin concentrations (approx. 0.1 microM actin vs. approx. 1 microM S-1), where ATPase activity was barely stimulated by actin.