Zum Stoffwechsel der Stärke

Abstract

The determination of the activities of the enzymes which could be involved in sucrose-starch conversion showed that the first step of this transfer is performed by the sucrose-synthetase. In this reaction UDPG and almost no ADPG is formed. However, under the given conditions the concentration of UDPG is low, and with the weak activity of the starch synthetase present no starch synthesis takes place. In a further step Glu-1-P is formed from UDPG, a reaction catalysed by the UDPG-pyrophosphorylase. Glu-1-P can also be furnished from the fructose liberated in the first step. In the immature cotyledons of Vicia faba there is sufficient fructokinase, phosphoglucose isomerase and phosphoglucomutase for these transformations. Most of starch synthesis proceeds catalysed by the electrophoretically slower migrating phosphorylase which can be adsorbed on starch granules. The fact that phosphorylase could only solubilize the radioactivity incorporated from Glu-1-P-C¹⁴ (due to phosphorylase) and not the one from ADPG-C¹⁴ (due to synthetase) shows that the two pathways are different. This sucrose-starch conversion is controlled by the concentration of glucose, glu-1-P, glu-6-P and fru-1,6-diP, which inhibit the formation of UDPG from sucrose.