ATP-ADP exchange reaction by fragmented sarcoplasmic reticulum from bullfrog skeletal muscle

The ATP-ADP exchange reaction and its related partial reactions of fragmented sarcoplasmic reticulum from bullfrog skeletal muscle (frog FSR) were investigated and compared with those of rabbit FSR in order to understand the characteristics of calcium-activated ATPase (Ca²⁺-ATPase) of frog FSR. MgATP and magnesium-free ADP are substrates for the forward and backward reaction of the ATPase activity, respectively, which is consistent with the conclusion obtained with rabbit FSR. The ATP-ADP exchange rate of frog FSR increased sharply with an increase in Ca²⁺ concentration up to 3 µm, and then decreased as Ca²⁺ concentration increased from 3 µm to 100 µm, where the level of EP continued to increase. The exchange rate of frog FSR had a value similar to the overall ATPase activity at steady state. These results contrast with observations using rabbit FSR. The exchange rate of rabbit FSR, which is 10–30 times as high as the overall ATPase activity, reached a plateau at 1 µm Ca²⁺, and the decrease in the exchange rate with the increase in Ca²⁺ concentration was not observed until the concentration was greater than 30 µm, where the plateau of the ATPase activity was maintained. These results were discussed in reference to a possible ordered reaction sequence of ATP followed by calcium in the Ca²⁺-ATPase reaction. It is suggested thatk ₋₅/k ₋₆ for rabbit FSR at steady state should be larger than that for frog FSR by a factor of about 10 in the following reaction sequence. $$E.ATP.Ca EP + ADP E + P_i + ADP$$