Marston S B, Trevett R M, Walters M
Biochem J. 1980 Feb 1;185(2):355-65. doi: 10.1042/bj1850355.
Myosin and actin competition tests indicated the presence of both thin-filament and myosin-linked Ca2+-regulatory systems in pig aorta and turkey gizzard smooth-muscle actomyosin. A thin-filament preparation was obtained from pig aortas. The thin filaments had no significant ATPase activity [1.1 +/- 2.6 nmol/mg per min (mean +/- S.D.)], but they activated skeletal-muscle myosin ATPase up to 25-fold [500 nmol/mg of myosin per min (mean +/- S.D.)] in the presence of 10(-4) M free Ca2+. At 10(-8) M-Ca2+ the thin filaments activated myosin ATPase activity only one-third as much. Thin-filament activation of myosin ATPase activity increased markedly in the range 10(-6)-10(-5) M-Ca2+ and was half maximal at 2.7 x 10(-6) M (pCa2+ 5.6). The skeletal myosin-aorta-thin-filament mixture gave a biphasic ATPase-rate-versus-ATP-concentration curve at 10(-8) M-Ca2+ similar to the curve obtained with skeletal-muscle thin filaments. Thin filaments bound up to 9.5 mumol of Ca2+/g in the presence of MgATP2-. In the range 0.06-27 microM-Ca2+ binding was hyperbolic with an estimated binding constant of (0.56 +/- 0.07) x 10(6) M-1 (mean +/- S.D.) and maximum binding of 8.0 +/- 0.8 mumol/g (mean +/- S.D.). Significantly less Ca2+ bound in the absence of ATP. The thin filaments contained actin, tropomyosin and several other unidentified proteins. 6 M-Urea/polyacrylamide-gel electrophoresis at pH 8.3 showed proteins that behaved like troponin I and troponin C. This was confirmed by forming interspecific complexes between radioactive skeletal-muscle troponin I and troponin C and the aorta thin-filament proteins. The thin filaments contained at least 1.4 mumol of a troponin C-like protein/g and at least 1.1 mumol of a troponin I-like protein/g.
肌球蛋白与肌动蛋白竞争试验表明，猪主动脉和火鸡砂囊平滑肌肌动球蛋白中存在细肌丝和与肌球蛋白相关的钙离子调节系统。从猪主动脉中获得了一种细肌丝制剂。这些细肌丝没有显著的ATP酶活性[1.1±2.6纳摩尔/毫克每分钟（平均值±标准差）]，但在存在10⁻⁴摩尔/升游离钙离子的情况下，它们能将骨骼肌肌球蛋白ATP酶的活性激活高达25倍[500纳摩尔/毫克肌球蛋白每分钟（平均值±标准差）]。在10⁻⁸摩尔/升钙离子浓度下，细肌丝对肌球蛋白ATP酶活性的激活程度仅为前者的三分之一。细肌丝对肌球蛋白ATP酶活性的激活在10⁻⁶至10⁻⁵摩尔/升钙离子浓度范围内显著增加，在2.7×10⁻⁶摩尔/升（pCa²⁺ 5.6）时达到最大值的一半。在10⁻⁸摩尔/升钙离子浓度下，骨骼肌肌球蛋白 - 主动脉细肌丝混合物给出了一条双相的ATP酶速率与ATP浓度曲线，类似于用骨骼肌细肌丝得到的曲线。在存在MgATP²⁻的情况下，细肌丝结合的钙离子高达9.5微摩尔/克。在0.06至27微摩尔/升钙离子浓度范围内，钙离子结合呈双曲线关系，估计结合常数为（0.56±0.07）×10⁶摩尔⁻¹（平均值±标准差），最大结合量为8.0±0.8微摩尔/克（平均值±标准差）。在没有ATP的情况下，结合的钙离子明显减少。细肌丝含有肌动蛋白、原肌球蛋白和其他几种未鉴定的蛋白质。在pH 8.3条件下进行的6M尿素/聚丙烯酰胺凝胶电泳显示出一些行为类似于肌钙蛋白I和肌钙蛋白C的蛋白质。通过形成放射性骨骼肌肌钙蛋白I和肌钙蛋白C与主动脉细肌丝蛋白之间的种间复合物，这一点得到了证实。细肌丝每克至少含有1.4微摩尔类似肌钙蛋白C的蛋白质和至少1.1微摩尔类似肌钙蛋白I的蛋白质。
