Isoenzymes of sugar phosphate metabolism in endosperm of germinating castor beans

Abstract

Two isoenzymes each of phosphoglucomutase, hexose phosphate isomerase, aldolase, fructose diphosphatase, phosphofructokinase, and 6-phosphogluconate dehydrogenase have been separated by DEAE-cellulose column chromatography of extracts from endosperm of germinating castor beans (Ricinus communis cv. Hale). One of each of the enzymes is localized in the cytosol and the other is confined to plastids. Developmental studies of these isoenzymes were carried out to clarify their roles in the endosperm. In extracts from ungerminated seeds the activities of marker enzymes of mitochondria (fumarase), plastids (ribulose bisphosphate carboxylase), and glyoxysomes (catalase) were low, but phosphoglucomutase, hexose phosphate isomerase, aldolase, and 6-phosphogluconate dehydrogenase were present in relatively high activity. The total amounts of these enzymes increased 3- to 4-fold during the first 5 days of growth. The activities of isoenzymes in the plastids rose in parallel with that of ribulose bisphosphate carboxylase to reach a maximum at day 4, and like the carboxylase they declined sharply thereafter. The activities of the cytosolic isoenzymes peaked at day 5. These changes are consistent with the roles previously proposed for the sequences present in plastid and cytosol.