We investigated which activity of myosin B (natural actomyosin), superprecipitation or ATPase, is better suited as a parameter for measuring in vitro contraction with emphasis on the response of myosin B to Sr2+. The Sr2+ concentration giving half-maximum (KSr2+) binding of Sr2+ to cardiac troponin was not different from that to skeletal troponin, whether the troponins were contained in the actomyosin system or in the isolated state. However, KSr2+ for the ATPase activity of cardiac myosin B is slightly smaller than that of skeletal myosin B. This difference in KSr2+ is more marked when superprecipitation was used as the indication for in vitro contraction. If the above results were compared with those of Kitazawa (1976) using glycerinated fibers of cardiac and skeletal muscle, the differences in KSr2+ between cardiac and skeletal muscle increased in the order: Sr-binding to troponin<ATPase activity superprecipitation<contraction of glycerinated fibers. The increase in the effect of pH on Ca2+-related properties is also in the order: Ca-binding to troponin≤ATPase activity superprecipitation<contraction of glycerinated fibers. All the results coincide with one another that the results with superprecipitation are invariably more akin to those with glycerinated fibers than to those with the ATPase activity. It is concluded that superprecipitation is a better parameter for representing in vitro contraction than ATPase activity.
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