Effect of Diethyl Ether on the Adenosine Triphosphatase Activity and the Calcium Uptake of Fragmented Sarcoplasmic Reticulum of Rabbit Skeletal Muscle

Treatment of fragmented preparations of sarcoplasmic reticulum with diethyl ether decreases the calcium uptake capacity of reticulum, while the ATP-binding capacity as well as the ATP-splitting activity of reticulum were increased. Diethyl ether-treated reticulum preparations retained both the "basal" and the "extra" splitting activities of Hasselbach and Makinose, although the latter activity was uncoupled from the calcium uptake activity. Taking advantage of the fact that calcium-dependent ATP splitting, namely, the extra splitting of ether-treated reticulum, is not confined in a short period of calcium uptake process, the following kinetic characteristics at 25° were obtained. In the presence of an optimal concentration of calcium, the half-maximal activation of the extra splitting by magnesium was obtained with 0.1 m m added MgCl 2 . Overoptimal concentrations of calcium exerted a competitive inhibition. The plot of the extra splitting activity as a function of ATP concentration showed two discrete steps, which suggest two different calcium-dependent activities; their Michaelis-Menten constants are 10 µ m and >1 m m , and their maximal activities are 1.7 and >0.6 µmoles of phosphate liberated per min per mg of reticulum protein. The ATP splitting by ether-treated preparations of reticulum in the presence of 1,2-bis-(2-dicarboxy methylaminoethoxy)ethane with no added calcium, namely, the basal splitting, occurred at ATP concentrations about the same as those for the second step in the extra splitting, but its activity was too small to account for the activity in the second step. On the other hand, the calcium uptake by nontreated preparations of reticulum also showed two steps as the ATP concentration increased; the first step occurred at ATP concentrations about one-tenth of those for the ATP-splitting activity mentioned above, but the second step occurred at about the same concentrations as those for the ATP splitting.