Stein L A, Chalovich J M
Department of Medicine, State University of New York, Stony Brook.
Biophys J. 1991 Aug;60(2):399-407. doi: 10.1016/S0006-3495(91)82065-3.
Regulation in striated muscles primarily involves the effect of changes in the free calcium concentration on the interaction of subfragment-1 (S-1) with the actin-tropomyosin-troponin complex (henceforth referred to as [acto]R). At low concentrations of free Ca++ the rate of ATP hydrolysis by (acto)R S-1 can be as much as 20-fold lower than that in the presence of high free Ca++, even though the binding of S-1 to (actin)R in the presence of ATP is virtually independent of the calcium concentration. This implies that the mechanism of regulation involves a kinetic transition between actin-bound states, rather than the result of changes in actin binding. In the current work, we have investigated the fluorescence transient that occurs with the binding and hydrolysis of ATP both at low and high free [Ca++]. The magnitude of this transition at low free [Ca++] is higher than at high free [Ca++]. At low free [Ca++], the rate of the fluorescence transient either stays constant or decreases slightly with increasing free actin concentrations, but at high free [Ca++] the rate increases slightly with increasing free actin concentration. The observed changes in rate are not great enough to be of regulatory importance. The results of the fluorescence transient experiments together with the binding studies performed at steady state also show that neither the binding of M.ATP or M.ADP.Pi to (actin)R is appreciably Ca++ sensitive. These data imply that an additional step (or steps) in the ATPase cycle, i.e., other than the burst transition, must be regulated by calcium.
横纹肌中的调节主要涉及游离钙浓度的变化对亚片段-1（S-1）与肌动蛋白-原肌球蛋白-肌钙蛋白复合物（以下简称[肌动蛋白]R）相互作用的影响。在游离Ca++浓度较低时，（肌动蛋白）R S-1水解ATP的速率比游离Ca++浓度高时低多达20倍，尽管在ATP存在下S-1与（肌动蛋白）R的结合实际上与钙浓度无关。这意味着调节机制涉及肌动蛋白结合状态之间的动力学转变，而不是肌动蛋白结合变化的结果。在当前的工作中，我们研究了在低游离[Ca++]和高游离[Ca++]条件下，ATP结合和水解时发生的荧光瞬变。在低游离[Ca++]时，这种转变的幅度高于高游离[Ca++]时。在低游离[Ca++]时，荧光瞬变的速率要么保持恒定，要么随着游离肌动蛋白浓度的增加而略有下降，但在高游离[Ca++]时，速率随着游离肌动蛋白浓度的增加而略有增加。观察到的速率变化不足以具有调节重要性。荧光瞬变实验的结果以及在稳态下进行的结合研究还表明，M.ATP或M.ADP.Pi与（肌动蛋白）R的结合对Ca++均不敏感。这些数据表明，ATP酶循环中的一个额外步骤（或多个步骤），即除了爆发转变之外，必须受钙调节。
