Unusual features of the Ca2+-ATPase activity of myosin from fast skeletal muscle of the frog: effect of actin and SH1 thiol group modification.

The K+-ATPase and actin-activated Mg2+-ATPase activity of myosin from fast skeletal muscle of the frog, Rana esculenta or Rana temporaria, are comparable to the respective activities of rabbit fast skeletal muscle. On the other hand, the Ca2+-ATPase activity of the same preparations of frog myosin is 6-7-fold lower than that of myosin from rabbit muscle. Various control experiments indicate that the small extent of Ca2+ stimulation is an intrinsic property of frog muscle myosin. Unlike myosin from rabbit muscle, the Ca2+-ATPase activity of frog myosin is strongly activated by actin; at high actin concentrations it approaches the level of the Ca2+-ATPase activity of rabbit myosin. The levels of Ca2+-ATPase activity of frog and rabbit myosins also become comparable upon modification of myosin SH1 thiol groups; this means that the modification of the SH1 groups results in a much higher activation of the Ca2+-ATPase of frog myosin than that of rabbit myosin. The results suggest a difference in the active site conformation in frog and rabbit muscle myosins. The effects of actin and SH1 group modification are discussed in terms of allosteric changes which diminish the difference in the active site conformation of the two myosins. We have also observed a difference in the reactivity of thiol groups which are not essential for the enzymatic activity in frog and rabbit myosin, indicating structural differences in regions other than the active site.