The activity of the Ca++ activated adenosinetriphosphatase (Ca-ATPase) of actomyosin systeme of rabbit and frog skeletal muscle has been studied at varying pH and temperature. The PH optima of the Ca-ATPase activity of the rabbit actomyosin was rather broad. Over the temperature range of 16-36℃ activity of the enzyme was not appreciably changed between pH 6.4-8.5; below and above which it rapidly reduced. The pH at the inflection point of the enzyme activity increased as temperature decreased, showing the ΔpH inflection/ΔT of approximately -0.018 unit/℃. Consequently, (OH-)/(H+) ratio at the inflection point was constant regardless of assay temperature. In the frog actomyosin systems the Ca-ATPase activity was not apparently altered between PH 6.4-7.0 when the incubation temperature was 15~30℃ . Outside of this range of pH, however, the enzyme activity was dramatically decreased. The pH of the inflection point changed inversely with temperature. ΔpH inflection/ΔT at the acidic side was approximately -0.018 unit/℃, whereas that at the alkaline side it was about -0.037 unit/℃. The Arrhenius Plot on the Ca-ATPase activity at constant (OH-)/(H+) ratio of 1.0 was not linear, but showed break at arround 20℃ for both rabbit and frog actomyosin Preparations. From these results it was speculated that pH dependence of Ca-ATPase activity of rabbit actomyosin systems might reflect titrations of histidine-imidazole and -SH groups, and that of the frog actomyosin represents titrations of histidine-imidazole and lysyllysine α-NH2 groups.
