Isoenzymes of Sugar Phosphate Metabolism in Endosperm of Germinating Castor Beans

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Abstract
Two isoenzymes each of phosphoglucomutase, hexose phosphate isomerase, aldolase, fructose diphosphatase, phosphofructokinase and 6-phosphogluconate dehydrogenase were separated by DEAE-cellulose column chromatographv of extracts from endosperm of germinating castor beans (R. 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. The activities of the cytosolic isoenzymes peaked at day 5. These changes are consistent with the roles previously proposed for the squences present in plastid and cytosol.