Phosphorylation of Gizzard Myosin Light Chain and the ATPase Reaction and Superprecipitation of Skeletal Acto-Gizzard Myosin as Functions of ATP Concentration

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Abstract
Three different reactions are known to occur in a combined system of skeletal actin, gizzard myosin, and gizzard native tropomyosin. They were studied as functions of ATP concentration. (a) At around 1 μM ATP in the presence of an ATP-regenerating system, two of the three reactions, superprecipitation and ATPase reaction, occurred independently of calcium and were not accompanied by the third reaction, phosphorylation of myosin light chains. (b) Relatively high concentrations of ATP were required for calcium-dependent phosphorylation and for calcium-activated ATPase reaction. It is suggested that the apparent Km value for myosin light-chain kinase and that for acto-phosphorylated myosin-ATPase are approximately 10−4.0M and 10−5.5M, respectively. (c) The calcium-dependent phosphorylation and the calcium-activated ATPase reaction were closely coupled, but they were only indirectly coupled with superprecipitation. (d) In some of the responses to change in ATP concentration, (a)–(c), skeletal acto-gizzard myosin was similar to skeletal acto-skeletal myosin. It was also similar in that sulfhydryl groups of myosin are involved in the calcium regulation of actomyosin-ATPase and its superprecipitation in both cases.