Cooke R, Pate E
Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0524.
Adv Exp Med Biol. 1988;226:255-65.
We have investigated the effect of magnesium pyrophosphate (MgPPi) on the orientation and stiffness of myosin heads in glycerinated muscle fibers. MgPPi is known to bind to the nucleotide site of myosin and to weaken the bond between myosin and actin. The spectra of paramagnetic probes attached to the reactive sulfhydryl on the myosin head were used to monitor the orientation of the head. Addition of 3 mM MgPPi to rigor fibers resulted in a shift of approximately 50% of the myosin heads from a state in which the probes are highly oriented to a state in which the probes are disordered. Experiments in which MgPPi dissociated labeled subfragment-1 from unlabeled fibers support the conclusion that the disordered probes are on myosin heads that are not attached to actin. Addition of MgPPi causes only a small decrease in fiber stiffness, measured by step changes in fiber length. We conclude that up to 50% of the myosin heads can be dissociated from actin with little change in muscle stiffness. Thus stiffness is not a linear measure of the fraction of myosin heads attached to actin. We present a model in which one head of the pair of a myosin molecule binds more weakly to actin and can be dissociated preferentially by the binding of MgPPi.
我们研究了焦磷酸镁（MgPPi）对甘油化肌纤维中肌球蛋白头部取向和刚度的影响。已知MgPPi可结合到肌球蛋白的核苷酸位点，并削弱肌球蛋白与肌动蛋白之间的结合。附着在肌球蛋白头部反应性巯基上的顺磁探针光谱用于监测头部的取向。向僵直纤维中添加3 mM MgPPi导致约50%的肌球蛋白头部从探针高度取向的状态转变为探针无序的状态。MgPPi使标记的亚片段-1从未标记纤维上解离的实验支持了这样的结论，即无序的探针位于未附着于肌动蛋白的肌球蛋白头部。通过纤维长度的阶跃变化测量，添加MgPPi仅使纤维刚度略有降低。我们得出结论，高达50%的肌球蛋白头部可从肌动蛋白上解离，而肌肉刚度变化很小。因此，刚度不是附着于肌动蛋白的肌球蛋白头部比例的线性度量。我们提出了一个模型，其中肌球蛋白分子对中的一个头部与肌动蛋白的结合较弱，并且可通过MgPPi的结合而优先解离。
