Structural changes in myosin during contraction and the state of ATP in the intact frog muscle.

The reactivity of myosin to [14C]-labeled N-ethylmaleimide ([14C]NEM) or to tritium was determined in functionally different frog muscles. The incorporation of [14C]NEM into myosin decreased during isotonic or isometric contractions, as compared to resting muscle. The cysteine residues which were protected during contraction were not involved in the ATPase activity or the actin-binding ability of myosin.