title: The effect of pH on the transient-state kinetics of Ca2+-Mg2+-ATPase of cardiac sarcoplasmic reticulum. A comparison with skeletal sarcoplasmic reticulum.
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 msec. The rates of formation and decomposition of the phosphorylated intermediate (E approximately 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 msec) for cardiac and skeletal sarcoplasmic reticulum preloaded with calcium, but decrease as the pH is lowered. The effect of acid pH (6.0) is more pronounced for cardiac sarcoplasmic reticulum (t 1/2 = 47 msec) than for skeletal sarcoplasmic reticulum (t 1/2 = 17 msec), 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 approximately P decomposition step (the slowest step in the SR reaction sequence). The E approximately P decomposition half-lives were observed to be 97 and 77 msec, respectively for cardiac and skeletal SR at pH 6.8. At pH 6.0, the half-lives were increased to 136 and 178 msec for cardiac and skeletal SR, respectively.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004285:Dogs; D004533:Egtazic Acid / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism*; D010766:Phosphorylation; D010770:Phosphotransferases / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D013997:Time Factors
chemical_list: D004533:Egtazic Acid; D010770:Phosphotransferases; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium