摘要
１，５二磷酸核酮糖羧化酶／加氧酶（Ｒｕｂｉｓｃｏ）是光合碳同化的关键酶，研究其降解机理对合理调控水稻生长后期光合衰退具有重要意义。前人用人为诱导植物衰老的方法，研究了Ｒｕｂｉｓｃｏ的降解机理，认为该酶降解之前，必需发生亚基间的交联聚合和向类囊体膜转移，这样在结构和空间上有利于水解酶的作用。我们用自然衰老叶片进行研究的结果表明：Ｒｕｂｉｓｃｏ在降解过程中其比活基本保持恒定，意味着未发生酶的失活，也就是说酶结构未发生根本性改变，由此也可初步判断酶未发生亚基间的交联聚合（已证明亚基交联可导致酶失活）。接着用ＳＤＳＰＡＧＥ和蛋白印迹技术证实了上述观点：Ｒｕｂｉｓｃｏ降解之前只有极少量的大亚基聚合体，随后同未聚合大亚基一起很快降解。此外，研究结果进一步表明酶分子在降解之前有少量与叶绿体膜结合，但降解过程中并未见膜结合蛋白增加。根据上述结果我们认为，亚基间交联聚合和向膜转移并非水稻叶片自然衰老时Ｒｕｂｉｓｃｏ降解的必要条件。
The mechanism of Rubisco degradation in naturally senescing rice leaves was investigated in terms of its inactivation, subunit aggregation, and translocation to chloroplast membrane. During natural senescence of rice leaves, Rubisco and other soluble proteins were degraded at similar rates, starting from the 5th day after full expansion of the leaf, but the specific activity, expressed as μmol CO 2 per milligram Rubisco per minute, remained almost constant throughout the senescence period, meaning that inactivation of the enzyme did not occur during the process. Only a small portion of Rubisco large subunits (LSUs) was found to be aggregated (about 5% of total) before its breakdown and the aggregated LSUs were degraded as rapidly as the non aggregated ones. The membrane associated Rubisco LSUs did not increase during senescence, but were lost rapidly like the stroma ones. Based on the present result,it is thought that in naturally senescing rice leaves oxidative aggregation and translocation to membrane of Rubisco LSUs may not be a necessary step before Rubisco breaks down.
