Fundamental study on degeneration mechanisms of the masseter muscle

从牛蛙蝌蚪尾中提取肌动蛋白降解酶。对该酶进行了部分纯化和表征。凝胶色谱法测定分子量为26000。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时活性最大，可水解天然肌动蛋白、肌凝蛋白H链、肌钙蛋白C和i。为了了解该酶的生理作用，我们进行了以下研究。用皂苷处理前变质蝌蚪尾肌纤维，使其均匀渗透后，体外培养该酶，观察其肌原纤维随时间的超微结构变化。5 min后，i带内的肌动蛋白丝开始呈废丝状不规则排列。大部分i带和z盘在15 min后消失，肌原纤维在i带水平碎裂，或肌丝从完整的肌原纤维末端呈扇形散开。然后，a带与相邻的a带重叠，但包括m线在内的h带保持完整。这些超微结构的变化与自发性变态过程中肌原纤维的变化有许多相似之处。因此，该酶可能在无尾蝌蚪自发变态过程中肌原纤维的退化中起重要作用。

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.