SIZE-ACTIVITY RELATIONSHIPS IN PHOSPHORYLASE OF POTATO-TUBERS AFTER STORAGE

Alpha-1,4-Glucan phosphorylase (EC 2.4.1.1) forms from fresh and stored preparations of potato (Solanum tuberosum L.) tuber amyloplasts were studied by electrophoretic and chromatographic techniques. Upon non-denaturing PAGE, fresh preparations showed two primed phosphorylase bands, of which only the slow-moving band exhibited unprimed activity. This enzyme form should be regarded as a type 11 or plastidial phosphorylase as judged by its low affinity for amylopectin, elution by high salt concentration from anion exchangers and subunit size of about 110 kDa. On storage of the amyloplast preparation, this band gradually disappeared and a novel, fast-moving band became evident. This latter phosphorylase form still had some unprimed activity in preparations stored for 4 to 7 days and underwent further modification upon prolonged storage, resulting in loss of the unprimed activity. This novel enzyme form, which may arise via proteolytic cleavage of the slow-moving one, should be considered as a type I or cytosolic phosphorylase based on its high affinity for amylopectin, lower subunit size (molecular mass of 96 kDa) and elution by lower salt concentration from anion exchangers. The relevance of these results as regards starch synthesis in potato tuber amyloplasts is discussed.