The activity of the Ca++ activated adenosinetriphosphatase (Ca-ATPase) of actomyosin systems 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°C 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 ?pH inflection/?T of approximately. -0. 018 unit/°C. 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°C. Outside of this range of pH, however, the enzyme activity was dramatically decreased. The pH of the inflection point changed inversely with temperature. ?pH inflection/?T at the acidic side was approximately -0.018 unit/°C, whereas that at the alkaline side it was about -.0.037 unit/°C.
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?C 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, W and that -of the. frog actomyosin represents titrations of histidine-imidazole and lysyllysine -NH_2 groups.
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