Activity and expression of banana starch phosphorylases during fruit development and ripening

Two main forms of starch phosphorylase (EC 2.4.1.1) were identified and purified from banana (Musa acuminata Colla. cv. Nanicão) fruit. One of them, designated phosphorylase I, had a native molecular weight of 155 kDa and subunit of 90 kDa, a high affinity towards branched glucans and an isoelectric point around 5.0. The other, phosphorylase II, eluted at a higher salt concentration from the anion exchanger, had a low affinity towards branched glucans, a native molecular weight of 290 kDa and subunit of 112 kDa. Kinetic studies showed that both forms had typical hyperbolic curves for orthophosphate (Pi) and glucose-1-phosphate, and that they could not react with substrates with a blocked reducing end or alpha-1,6 glucosidic bonds. Antibodies prepared against the purified type-II form and cross-reacting with the type-I form showed that there was an increase in protein content during development and ripening of the fruit. The changes in protein level were parallel to those of phosphorylase activity, in both the phosphorolytic and synthetic directions. Considering the kinetics, indicating that starch phosphorylases are not under allosteric control, it can be argued that protein synthesis makes a contribution to regulating phosphorylase activity in banana fruit and that hormones, like gibberellic acid and indole-3-acetic acid, may play a regulating role. For the first time, starch phosphorylases isoforms were detected as starch-granule-associated proteins by immunostaining of SDS-PAGE gels.