Adenosinetriphosphatase (ATPase) activity of actomyosin of the obliquely striated mantle muscle of squid was studied to confirm and supplement the previous work. ATPase activity was assayed at ionic strength of 0.15 or 0.6, Tris-maleate buffer 30 mM (pH 7.0-7.5), protein 0.1mg/m
l, ATP 3.2 mM, CaCl
2, MgCl
2 or EDTA 3.2 mM, and 25°C; for 10 min. Effects of Ca
2+ and Mg
2+ were tested in the function of temperature. Both Ca
2+ and Mg
2+ revealed marked activation at eithe ionic strength of 0.15 or 0.6, but EDTA none. At ionic strength 0.15, Ca
2+-ATPase showed a steep peak at 30°C, while Mg
2+-ATPase demonstrated a gradual peak with the top at 35°C. At ionic strenght of 0.6, the macimum of the former appeared at 25°C and that of the latter at 40°C. Mg
2+ ion seemed to stabilize the ATPase. When the activety was assayed at various ratios of actomyosin and ATP, the full activity levels were attained below the ratio of 0.1-0.2 mg/m
l protein per 3 mM ATP. Addition of Mg
2+ to Ca
2+-activated ATPase, reduced the activity to the level activated with Mg
2+ alone, while addition of Ca
2+ to Mg
2+-activated ATPase, did not alter the activity. Effects of Mg
2+ appeared to overcome that of Ca
2+. During 60 min incubation of enzymic reaction mixture at 25°C prior to adding ATP, the activity was reduced appreciably. This seemed to satand with the so far reported lability of the squid actomyosin which was illustrated in changes of several physiconchemical properties.
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