Stienen G J, van der Laarse W J, Elzinga G
Laboratory for Physiology, Free University, Amsterdam, The Netherlands.
Biophys J. 1988 Jun;53(6):849-55. doi: 10.1016/S0006-3495(88)83165-5.
MgATP binding to the actomyosin complex is followed by the dissociation of actin and myosin. The rate of this dissociation process was determined from the relationship between the maximum velocity of shortening and the MgATP concentration. It is shown here that the overall dissociation rate is rather similar in different types of muscle fibers. The relation between MgATP concentration and the maximum shortening velocity was investigated in fast and slow fibers and bundles of myofibrils of the iliofibularis muscle of Xenopus laevis at 4 degrees C from which the sarcolemma was either removed mechanically or made permeable by means of a detergent. A small segment of each fiber was used for a histochemical determination of fiber type. At 5 mM MgATP, the fast fibers had a maximum shortening velocity (Vmax) of 1.74 +/- 0.12 Lo/s (mean +/- SEM) (Lo: segment length at a sarcomere length of 2.2 microns). For the slow fibers Vmax was 0.41 +/- 0.15 Lo/s. In both cases, the relationship between Vmax and the ATP concentration followed the hyperbolic Michaelis-Menten relation. A Km of 0.56 +/- 0.06 mM (mean +/- SD) was found for the fast fibers and of 0.16 +/- 0.03 mM for the slow fibers. Assuming that Vmax is mainly determined by the crossbridge detachment rate, the apparent second order dissociation rate for the actomyosin complex in vivo would be 3.8.10(5) M-1s-1 for the fast fibers and 2.9.10(5) M-1 s-1 for the slow fibers. Maximum power output as a function of the MgATP concentration was derived from the force-velocity relationships. At 5 mM MgATP, the maximum power output in fast fibers was (73 +/- 8) mW.g-1 dry weight and (15 +/- 5) mW.g-1 in slow fibers. The Km for MgATP for the maximum power output for the fast fibers was (0.15 +/- 0.03) mM, which is about a factor of 4 lower than the Km for Vmax. The implications of these results are discussed in terms of a kinetic scheme for crossbridge action.
MgATP与肌动球蛋白复合体结合后，肌动蛋白和肌球蛋白发生解离。根据缩短的最大速度与MgATP浓度之间的关系确定了这种解离过程的速率。结果表明，在不同类型的肌纤维中，整体解离速率相当相似。在4℃下，研究了非洲爪蟾髂腓肌的快肌纤维、慢肌纤维和肌原纤维束中MgATP浓度与最大缩短速度之间的关系，这些肌纤维的肌膜要么被机械去除，要么通过去污剂使其具有通透性。每根纤维的一小段用于纤维类型的组织化学测定。在5 mM MgATP时，快肌纤维的最大缩短速度（Vmax）为1.74±0.12 Lo/s（平均值±标准误）（Lo：肌节长度为2.2微米时的节段长度）。慢肌纤维的Vmax为0.41±0.15 Lo/s。在这两种情况下，Vmax与ATP浓度之间的关系均符合双曲线米氏关系。快肌纤维的Km为0.56±0.06 mM（平均值±标准差），慢肌纤维的Km为0.16±0.03 mM。假设Vmax主要由横桥解离速率决定，那么在体内肌动球蛋白复合体的表观二级解离速率对于快肌纤维为3.8×10⁵ M⁻¹s⁻¹，对于慢肌纤维为2.9×10⁵ M⁻¹s⁻¹。最大功率输出作为MgATP浓度的函数是根据力-速度关系推导出来的。在5 mM MgATP时，快肌纤维的最大功率输出为（73±8）mW·g⁻¹干重，慢肌纤维为（15±5）mW·g⁻¹。快肌纤维最大功率输出时MgATP的Km为（0.15±0.03）mM，约比Vmax的Km低4倍。根据横桥作用的动力学方案讨论了这些结果的意义。
