Megerman J, Murphy R A
Biochim Biophys Acta. 1975 Dec 15;412(2):241-55. doi: 10.1016/0005-2795(75)90038-0.
The contractile proteins from arterial smooth muscle are highly soluble, and can be extracted at I = 0.05. However, they can be precipitated by a prolonged dialysis at pH 6 to give an actomyosin with a high, although variable, actin:myosin ratio. The sedimentation behavior of this actomyosin at high ionic strength was examined as a function of pH, protein concentration and composition by preparative ultracentrifugation. Comparisons with synthetic skeletal muscle actomyosins of similar composition demonstrated significant differences in the behaviors of these two systems. It was found that much smooth muscle actomyosin is not dissociated by normally relaxing conditions, and that it sediments at a slower rate than F-actin. The solubility of the supernatant protein (a myosin-enriched actomyosin) in 0.2 M K Cl (pH 7) depended on the pH during centrifugation. A lower solubility was associated only with a higher actin concentration in the supernatant, suggesting a dependence on actin repolymerization. Pure myosin was selectively precipitated from the supernatant by polyethylene glycol-6000, but only when the protein was soluble at low ionic strength. The solubility of purified myosin was similar to that of myosin from striated muscles. A relationship between the presence of depolymerized actin and the high solubility of smooth muscle contractile proteins is suggested.
动脉平滑肌的收缩蛋白高度可溶，在离子强度I = 0.05时即可提取。然而，通过在pH 6下长时间透析可使其沉淀，从而得到肌动球蛋白，其肌动蛋白与肌球蛋白的比例虽有变化但很高。通过制备性超速离心，研究了这种肌动球蛋白在高离子强度下的沉降行为与pH、蛋白质浓度和组成的关系。与组成相似的合成骨骼肌肌动球蛋白的比较表明，这两个系统的行为存在显著差异。研究发现，许多平滑肌肌动球蛋白在正常的松弛条件下不会解离，并且其沉降速度比F -肌动蛋白慢。上清液蛋白（富含肌球蛋白的肌动球蛋白）在0.2M KCl（pH 7）中的溶解度取决于离心过程中的pH值。较低的溶解度仅与上清液中较高的肌动蛋白浓度相关，这表明其依赖于肌动蛋白的再聚合。聚乙二醇 - 6000可从该上清液中选择性沉淀出纯肌球蛋白，但前提是该蛋白在低离子强度下可溶。纯化后的肌球蛋白的溶解度与横纹肌肌球蛋白的溶解度相似。文中提出了解聚的肌动蛋白的存在与平滑肌收缩蛋白的高溶解度之间的关系。
