Studies on the biosynthesis of starch. I. Isolation and properties of the soluble adenosine diphosphate glucose: starch glucosyltransferase of Solanum tuberosum

[The role of phosphorylase in starch metabolism in plastids]

Two phosphorylases could be detected on gel-electropherograms of leaf-extracts of Spinacia oleracea and of immature cotyledons of Vicia faba. These two phosphorylases could be separated by means of ammonium sulfate fractionation. Both the slower migrating phosphorylases from spinach and from beans, but not the fast one from beans, could be adsorbed on amyloplasts. This process takes place only when the amyloplasts are suspended in a salt medium. The slow phosphorylases can also be adsorbed on chloroplasts. The specific activity of the amyloplast-adsorbable phosphorylase in spinach leaves is about ten times higher in the cytoplasmatic fraction than in chloroplasts, a fact which suggests that this phosphorylase is localised in the cytoplasma. The addition of ADP or ATP to the reaction mixture had no influence on the synthesizing activity of the slow phosphorylases when they were tested with soluble amylopectin as a primer or while they were adsorbed on amyloplasts. The presence of ADPG and UDPG was inhibitory.The results reported above suggest that phosphorylase could catalyse the synthesis of starch in the plastids when photophosphorylation or oxidative phosphorylation occurs. This starch synthesis could be controlled by the concentration of ADPG. When, on the other hand, the ATP/Pi ratio is low, phosphorylase would be involved in starch breakdown. This reverse reaction is also regulated by the concentration of glucosylnucleotides.