Change in the reactivity of myosin during muscle contraction.

Myosin can be labeled with 1 -fluoro-2,4-dinitrobenzene in frog muscle. The incorporation into myosin is decreased during isotonic contraction. In contrast no difference is found in the incorporation into the sarcoplasmic proteins of contracting and resting muscle. Myosin also reacts with iodoacetate in frog muscle. However, this reaction does not depend on the functional state of the muscle. The reactivity of myosin in contracting muscle follows a similar pattern in model systems. The dinitrophenyl content of myosin isolated from myofibrils dinitrophenylated in the relaxed state is higher than that of myosin from myofibrils dinitrophenylated while contracting. Incorporation of lfluoro-2,4-dinitrobenzene into pure frog myosin is greatly increased in the presence of additions which relax myofibrils as compared to the additions which contract the fibrils. With the same additions, frog actin, or myosin-free myofibrillar residues show no change in the dinitrophenylation. Furthermore, the reactivity of contracting and relaxed myofibrils remains the same when 1-fluoro-2,4-dinitrobenzene is substituted by iodoacetate. These data show that l-fluoro-2,4-dinitrobenzene is a valuable probe to differentiate the functional states of myosin in myofibrils and living muscle.