Abstract
Summary Mg-activated ATPase of natural actomyosin prepared from guinea pig skeletal muscle was determined after the preincubation with trypsin. The ATPase activity of the white vastus (composed predominately of fast-twitch white fibers) was greatly activated by a few minutes of preincubation, as was that of the red vastus which is composed predominately of fast-twitch red fibers. However, the ATPase of the soleus and the semimembranosus proprius (composed exclusively of slow-twitch intermediate fibers) was never activated by the trypsin treatment. The activation was not observed in desensitized actomyosin. The data suggest differences in the thin filament components of different types of skeletal muscle.
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