Phosphoglucose isomerase from bananas: Partial characterization and relation to main changes in carbohydrate composition during ripening

Some characteristics of phosphoglucose isomerase (PGI, EC 5.3.1.9) from ban ana were measured during fruit ripening of three banana cultivars. In banan a, PGI was present as two dimeric isoenzymes, named PGI(1) and PGI(2), whic h had similar native molecular masses but differed in relation to heat stab ility and isoelectric point. Total PGI activity showed a distinct two-step change during fruit ripening. Before the climacteric period, PGI activity g radually decreased with the starch content, then its activity began to incr ease with sucrose accumulation. The ratio of PGI(1), and PGI(2) was constan t, indicating that both enzymes would be involved in starch degradation and sucrose synthesis. PGI activity and changes in carbohydrate composition su ggests the existence of some control to fit the requirements of the intense carbon How from starch to sucrose.