Fundamental study on degeneration mechanisms of the masseter muscle

从变态牛蛙蝌蚪尾中提取肌动蛋白降解酶。该酶进行了部分纯化和表征。通过凝胶色谱法测定分子量为26，000。该酶在较宽的pH范围内（5.5 ~ 7.5）具有降解肌动蛋白分子的活性，在pH 6.0时活性最高，并能水解天然肌动蛋白、肌球蛋白H链、肌钙蛋白C和I。为了获得关于这种酶的生理作用的信息，进行了以下研究。将原变态蝌蚪的尾肌纤维用皂甙处理后，在体外与酶一起孵育，以获得均匀的酶渗透，并观察肌原纤维的超微结构随时间的变化。5 min后，I带内的肌动蛋白丝开始改变，排列不规则，呈废丝状。15 min时，大部分I带和Z盘消失，肌原纤维在I带水平断裂，或肌丝从完整肌原纤维的末端呈扇形散开。A带与相邻的A带重叠，而H带包括M线保持完整，这些超微结构变化与肌原纤维自发变态过程中的变化有许多相似之处。因此，该酶可能在无尾两栖类蝌蚪尾部肌原纤维自发变态过程中起重要作用。

The actin-degrading enzyme was extracted from the metamorphosing bullfrog tadpole tail. This enzyme was partially purified and characterized. The molecular weight was determined as 26,000 by the gel chromatography. The enzyme was active in degrading actin molecules in the broad range of pH (5.5 to 7.5) with the maximum activity at pH 6.0, and hydrolyzed native actin, myosin H chain, troponin C and I. To get information on the physiological role of this enzyme, the following investigation was carried out. The tail muscle fibers of prometamorphic tadpole were incubated in vitro with the enzyme after they were treated with saponin to obtain uniform penetration of the enzyme, and ultrastructural changes of the myofibrils in course of time were investigated. The initial changes were seen after 5 min in actin filaments within I-band to be irregular arrangement as like waste threads. Most of the I-bands and Z-discs disappeared by 15 min so that the myofibrils fragmentized at the level of the I-bands, or the myofilaments fanned out from the terminals of the intact myofibrils. The A-band then overlapped with the neighboring A-bands, but the H-bands including M-lines remained intact.These ultrastructural changes have many points of resemblance to those of the myofibrils during spontaneous metamorphosis. Therefore, this enzyme was supposed to play an important role in degeneration of the myofibrils of the anuran tadpole tail during spontaneous metamorphosis.