The effect of pH on the transient-state kinetics of Ca2+-Mg2+-ATPase of cardiac sarcoplasmic reticulum. A comparison with skeletal sarcoplasmic reticulum.

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Abstract
The effect of pH on the Ca2+-Mg2+-dependent ATPase of sarcoplasmic reticulum (SR) was investigated with a rapid mixing quench-flow apparatus capable of measuring phosphorylation and dephosphorylation at times as rapid as 4 ms. The rates of formation and decomposition of the phosphorylated intermediate (E .apprx. P) of the Ca2+-Mg2+-ATPase were studied in the pH range between 7.6 and 6.0. At pH 6.8, the rates of formation of the phosphorylated intermediate of the Ca2+-Mg2+-ATPase of sarcoplasmic reticulum are the same (t1/2 = 10 ms) for cardiac [dog] and skeletal [rabbit] sarcoplasmic reticulum preloaded with Ca, but decrease as the pH is lowered. The effect of acid pH (6.0) is more pronounced for cardiac sarcoplasmic reticulum (t1/2 = 47 ms) than for skeletal sarcoplasmic reticulum (t1/2 = 17 ms), in agreement with studies showing that acidosis has a more pronounced effect on cardiac muscle than on skeletal muscle. In addition, a decrease in pH results in a decrease in the rate of the E .apprx. P decomposition step (the slowest step in the SR reaction sequence). The E .apprx. P decomposition half-lives were 97 and 77 ms, respectively, for cardiac and skeletal SR at pH 6.8. At pH 6.0, the half-lives were increased to 136 and 178 ms for cardiac and skeletal SR, respectively.