Relation of properties of isolated myosin to those of intact muscles of the cat and sloth.

Abstract

Myosin was prepared from the extensor digitorum longus, gastrocnemius medialis and lateralis, tibialis anterior, flexor of the claws of the anterior limb, and diaphragm muscles of the cat and didactyl sloth (Choloepus hoffmanni Peters). Actin‐activated, Ca2+‐activated, and EDTA‐activated ATPase activities of the myosins from cat muscles were two to four times higher than those of the myosins from the same muscles of the sloth. The difference in the Ca2+‐activated ATPase activity was found in the pH range of 5.5 to 10.0, and in the KCl concentration range of 50 mM to 500 mM at pH 7.0.The kinetic characteristics of the two myosins indicated that both the Km and the Vmax of the ATPase activities of the sloth myosin were lower than those of cat myosin by a factor of several‐fold.Reconstituted cat actomyosin superprecipitated four to six times faster than sloth actomyosin. This difference in the superprecipitation did not vary appreciably when the concentration of ATP and the ratio of myosin to actin were varied. A correlation between the ATPase activity and the speed of superprecipitation of actomyosins of the cat and sloth was shown.The contraction time of various muscles of the cat and sloth was found to be inversely proportional to the actin‐activated ATPase activity of their respective myosins. The speed constant of shortening of the muscles of the diaphragm appeared to be proportional to the actin‐activated ATPase activity of their myosin.In contrast to the ATPase activity of myosin, which varied according to the speed of contraction, the actin‐binding ability of myosins of the cat and sloth was rather constant. The extent of superprecipitation of various reconstituted actomyosins of the cat and sloth was also the same. These properties of the isolated myosins were related to the similar tension output of the muscles of the cat and sloth.