Levy H M, Ryan E M
J Gen Physiol. 1967 Nov;50(10):2421-35. doi: 10.1085/jgp.50.10.2421.
The various contractile and control sites of natural actomyosin gel were studied by comparing the kinetics of ATP hydrolysis with those of gel contraction, measured as an increase in turbidity. Contraction of actomyosin gel seems to require the cooperative reaction of ATP (with Mg) at two different sites. One of these sites catalyzes the hydrolysis of ATP and most probably contributes the driving force for contraction; the binding of ATP to the other site appears to break certain links that retard movement of the gel components. At limiting concentrations of ATP, the rate of contraction seems to depend on the rate of breaking these links as well as on the rate of ATP hydrolysis. But when both sites are saturated, the rate of contraction appears to be limited only by the rate of ATP hydrolysis. In addition to these two contractile sites, there are also two different control sites. At one, the relaxing site, the binding of ATP with Mg inhibits ATP hydrolysis and gel contraction. At the other, the binding of calcium activates contraction by overcoming the inhibitory action of Mg and ATP at the relaxing site. This control system-inhibition by substrate and disinhibition by calcium-can be selectively inactivated by heat and reactivated by dithiothreitol, a disulfide-reducing agent. These observations on the isolated contractile system are discussed in relation to the contraction and relaxation of muscle.
通过比较ATP水解动力学与以浊度增加来衡量的凝胶收缩动力学，研究了天然肌动球蛋白凝胶的各种收缩和控制位点。肌动球蛋白凝胶的收缩似乎需要ATP（与镁离子结合）在两个不同位点的协同反应。其中一个位点催化ATP水解，很可能为收缩提供驱动力；ATP与另一个位点的结合似乎会破坏某些阻碍凝胶成分移动的连接。在ATP浓度受限的情况下，收缩速率似乎取决于破坏这些连接的速率以及ATP水解的速率。但当两个位点都饱和时，收缩速率似乎仅受ATP水解速率的限制。除了这两个收缩位点外，还有两个不同的控制位点。在一个位点，即松弛位点，ATP与镁离子的结合会抑制ATP水解和凝胶收缩。在另一个位点，钙的结合通过克服镁离子和ATP在松弛位点的抑制作用来激活收缩。这种控制系统——底物抑制和钙去抑制——可被热选择性失活，并被二硫苏糖醇（一种二硫键还原剂）重新激活。关于分离出的收缩系统的这些观察结果，结合肌肉的收缩和舒张进行了讨论。
