ADP-glucose pyrophosphorylase (AGPase) is a key enzyme of starch synthesis in seeds, tubers and fruits. UDP-glucose pyrophosphorylase (UGPase) is an important enzyme of sucrose metabolism in the cytosol while alkaline phosphatase (ALP) is a marker enzyme of the amyloplast that keeps the production of ADPG by removing PPi. Unripe banana accumulates starch in the pulp during development, while ripe fruits are characterized by the accumulation of soluble sugars. The aim of the study was to compare starch granule structure, carbohydrate levels, subcellular location and activities of three enzymes: AGPase, UGPase and ALP. Protein extracts from the cytosolic and amyloplastidial fractions were obtained from the pulp of banana fruit at three developmental stages (11, 16 and 21 weeks after flowering) and analyzed by electrophoresis and immunodetection. Protein profiles were similar during ripening, showing a main electrophoretic band at 50–55 kDa. Higher protein content was found in the cytosolic than in the amyloplastidial fraction. Starch granules and ALP activity were enriched in the amyloplast, whereas AGPase showed a subcellular distribution similar to UGPase. Immunoblot analysis also confirmed the presence of AGPase in both cytosol and amyloplast. AGPase activity was higher in the cytosol than in the amyloplast. Both AGPase activity and western blot band intensity were highest at 16 weeks. UGPase activity was highest at 21 weeks. We conclude that cytosolic production of ADP-glucose is not an exclusive feature of cereal endosperms due to plant breeding, but it also occurs in fruits of non-domesticated plants such as tropical banana (Musa acuminata). This work increases our understanding about pyrophosphorylase activities in the pulp of banana fruit.

中文翻译：

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ADP-葡萄糖焦磷酸化酶位于热带香蕉果实（Musa acuminata）果肉质体和细胞质中

ADP-葡萄糖焦磷酸化酶（AGPase）是种子、块茎和果实中淀粉合成的关键酶。UDP-葡萄糖焦磷酸化酶 (UGPase) 是细胞溶质中蔗糖代谢的重要酶，而碱性磷酸酶 (ALP) 是淀粉体的标记酶，通过去除 PPi 保持 ADPG 的产生。未成熟的香蕉在发育过程中会在果肉中积累淀粉，而成熟的香蕉则以积累可溶性糖为特征。该研究的目的是比较三种酶的淀粉颗粒结构、碳水化合物水平、亚细胞定位和活性：AGPase、UGPase 和 ALP。从香蕉果实的三个发育阶段（开花后 11、16 和 21 周）的果肉中获得细胞质和淀粉体部分的蛋白质提取物，并通过电泳和免疫检测进行分析。成熟过程中的蛋白质谱相似，在 50-55 kDa 处显示主要电泳条带。在细胞溶质中发现比在淀粉质体部分中更高的蛋白质含量。淀粉粒和 ALP 活性在淀粉体中富集，而 AGPase 显示出类似于 UGPase 的亚细胞分布。免疫印迹分析还证实了细胞质和淀粉体中都存在 AGPase。胞质溶胶中的 AGPase 活性高于淀粉体。AGPase 活性和蛋白质印迹条带强度均在 16 周时最高。UGPase 活性在 21 周时最高。我们得出的结论是，由于植物育种，ADP-葡萄糖的胞质产生不是谷物胚乳的独有特征，但它也发生在非驯化植物的果实中，例如热带香蕉（Musa acuminata）。