Levy H M, Moy W W
Biochim Biophys Acta. 1981 Apr 14;658(2):318-26. doi: 10.1016/0005-2744(81)90302-8.
The turbidity changes induced by MgATP in suspensions of actomyosin gel particles have been studied systematically over a wide range of MgATP concentrations at different temperatures with and without the regulatory proteins. An analysis of these changes distinguishes three separate protein-protein interactions in the gel: (1) The transient cyclic interactions between actin and myosin involved in the hydrolysis of MgATP and contraction. (2) Cross-links that cause turbidity in the original suspension. (3) Cross-links that cause the high turbidity usually associated with superprecipitation. In general, there was a good correlation between the half-time for reaching maximum turbidity during superprecipitation and the rate of hydrolysis. On the other hand, the actual magnitude of the turbidity increase was progressively diminished as the concentration of substrate was raised in the millimolar range. It appears that some essential phase of the superprecipitation process is limited by the same enzymatic step that limits the rate of hydrolysis. However, whether or not this phase leads to an increase in turbidity depends on the concentration of MgATP. Apparently, high physiological levels of MgATP (1-5 mM) inhibit the formation of the specific cross-links that cause high turbidity in the superprecipitate. It is proposed that MgATP interferes with these links when it can bind to a low-affinity site on myosin that is separate from the high-affinity active sites for hydrolysis. Thus, in contracting muscle, interfilament interactions analogous to those that increase turbidity and cause isodimensional shrinkage in the gel would be prevented by the high level of MgATP in the sarcoplasm. Observations relating the shortening of isolated myofibrils to their turbidity in suspension lend support to this interpretation.
在有和没有调节蛋白的情况下，于不同温度下，对广泛的MgATP浓度范围内，肌动球蛋白凝胶颗粒悬浮液中MgATP诱导的浊度变化进行了系统研究。对这些变化的分析区分了凝胶中三种不同的蛋白质 - 蛋白质相互作用：（1）参与MgATP水解和收缩的肌动蛋白和肌球蛋白之间的瞬时循环相互作用。（2）导致原始悬浮液中浊度的交联。（3）导致通常与超沉淀相关的高浊度的交联。一般来说，超沉淀过程中达到最大浊度的半衰期与水解速率之间存在良好的相关性。另一方面，随着底物浓度在毫摩尔范围内升高，浊度增加的实际幅度逐渐减小。似乎超沉淀过程的某些关键阶段受到限制水解速率的相同酶促步骤的限制。然而，这个阶段是否导致浊度增加取决于MgATP的浓度。显然，高生理水平的MgATP（1 - 5 mM）抑制了在超沉淀物中导致高浊度的特定交联的形成。有人提出，当MgATP能够结合到肌球蛋白上与高亲和力水解活性位点分开的低亲和力位点时，它会干扰这些交联。因此，在收缩的肌肉中，肌浆中高水平的MgATP会阻止类似于在凝胶中增加浊度并导致等尺寸收缩的丝间相互作用。将分离的肌原纤维的缩短与其悬浮液中的浊度相关联的观察结果支持了这一解释。
