Reaction mechanism of the Ca(2)+-dependent ATPase of sarcoplasmic reticulum from skeletal muscle. XI. Re-evaluation of the transition of ATPase activity during the initial phase.
We previously reported (J. Biochem. 70,95--123 (1971) that the time course of Pi liberation in the reaction of Ca2+, Mg2+--dependent ATPase [EC 3.6.1.3.] of fragmented sarcoplasmic reticulum (SR) consists of a lag phase, a burst phase, and a steady phase. We also showed that the rate constant, kd, of decomposition of the phosphorylated intermediate (E approximately P) decreases during the initial phase, and suggested that the burst phase is due to transition of the kd value. Recently, Froehlich and Taylor (J. Biol. Chem. 250, 2013--2021 (1975)) claimed that the Pi burst is caused by the formation of an acid-labile intermediate containing phosphate (E.P) formed by rapid hydrolysis of E approximately P. In the present study, the transition of the kd value during the initial phase was measured precisely, and the results showed that the burst phase is due to a transition in the kd value, not to the existence of E-P. The main results obtained were as follows: 1. After the SR had been phosphorylated with [gamma-32P]ATP in the presence of Mg2+ and Ca2+ ions, further phosphorylated was stopped by the addition of EGTA. The concentration of E approximately 32P then decreased exponentially with time. 2. The first-order rate constants, kd, of decomposition of E aproximately 32P after adding EGTA decreased with increase in the interval, t, between the start of E approximately 32P formation and the time of adding EGTA...
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D009132:Muscles / Q000166:cytology*; D010710:Phosphates / Q000032:analysis; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013997:Time Factors
D010710:Phosphates; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D002118:Calcium