Hsu C J, Sanborn B M
Endocrinology. 1986 Feb;118(2):499-505. doi: 10.1210/endo-118-2-499.
Relaxin treatment altered the kinetic properties of rat myometrial cell myosin light chain kinase (MLCK) by increasing the K50 of the enzyme for calmodulin (CaM) from 1.1 +/- 0.1 to 38 +/- 14 nM. When MLCK was assayed in the presence of 7 nM CaM to maximize the effect of the decreased affinity for CaM between control and relaxin-treated groups, a rapid concentration-dependent effect of the hormone was observed. Relaxin decreased MLCK activity significantly within 1 min. The ED50 of the effect was 0.4 microgram/ml. In addition to its effect on Ca2+-CaM-dependent activity, relaxin also decreased Ca2+-CaM-independent MLCK activity. This decrease was not attributable to a decrease in the affinity of the enzyme for myosin. There was a temporal association between the effects of relaxin on mean cell length, elevation of cAMP levels in the presence of 0.4 microM forskolin previously shown in other studies, and the alteration of MLCK activity. All three parameters were changed significantly within 1 min after exposure to relaxin. The ED50 of relaxin for cell shape changes, cAMP elevation, and effects on MLCK activity were all approximately 0.4 microgram/ml. Relaxin may act in part by a cAMP-mediated phosphorylation of MLCK, thereby decreasing its affinity for CaM. The effect on MLCK may be linked to a decrease in the phosphorylation of myosin light chains and the promotion of uterine relaxation.
松弛素处理通过将大鼠子宫肌层细胞肌球蛋白轻链激酶（MLCK）对钙调蛋白（CaM）的K50从1.1±0.1 nM增加到38±14 nM，改变了该酶的动力学特性。当在7 nM CaM存在下测定MLCK以最大化对照组和松弛素处理组之间对CaM亲和力降低的影响时，观察到该激素具有快速的浓度依赖性效应。松弛素在1分钟内显著降低了MLCK活性。该效应的半数有效剂量（ED50）为0.4微克/毫升。除了对Ca2 + -CaM依赖性活性的影响外，松弛素还降低了Ca2 + -CaM非依赖性MLCK活性。这种降低并非归因于该酶对肌球蛋白亲和力的降低。松弛素对平均细胞长度的影响、先前在其他研究中显示的在0.4 microM福斯高林存在下cAMP水平的升高以及MLCK活性的改变之间存在时间关联。暴露于松弛素后1分钟内，所有这三个参数均发生了显著变化。松弛素对细胞形状变化、cAMP升高以及对MLCK活性影响的ED50均约为0.4微克/毫升。松弛素可能部分通过cAMP介导的MLCK磷酸化起作用，从而降低其对CaM的亲和力。对MLCK的影响可能与肌球蛋白轻链磷酸化的降低以及子宫松弛的促进有关。
