There are not many doubts about the synthesizing role of ADPG-starch glucosyl transferase or about the participation of amylases in the degradation of starch, but the function of phosphorylase remains unclear. It is not easy to explain how starch metabolism can be regulated in tissues in which all these enzymes are simultaneously present. Ghosh and Preiss have suggested that the biosynthesis of the a-1,4 glucosyl linkage in starch is regulated at the level of ADPG. In some tissues the activity of transferase is much lower than the activity of the phosphorylases,2-fi and should synthesis proceed by the phosphorylase-catalyzed pathway, such a regulation would no longer be effective.On the other hand, the production of starch by phosphorylase may be inhibited by the presence of amylases." Badenhuizen and colleagues discussed the possibility that the lack of starch in the mesophyll cells of Cynodon dactylon Pers. could be due to such an inhibition by amylase of phosphorylase activity. Also, maize leaves have two distinct types of photosynthetic cells: the bundle sheath cells, which accumulate starch in the light, and the mesophyll cells, which under normal conditions are nearly devoid of starchax In our previous investigations we were able to show that in bundle sheath cells the activity of phosphorylase is high enough to account for the starch synthesized, while the activity of transferase is poor. Both enzymes are also

No abstract

No abstract

The activities of phosphorylase and synthetase in the bundle sheath cells of maize leaves were investigated in plants that as a result of different light-dark treatments contained various amounts of starch. The material coming from the dark (67 h) had almost no starch and scarcely any synthetase activity if starch granules were not added to the assay mixture as a primer. In the presence of this primer a poor synthetase activity could be detected. After 2 h in the light the leaves produced a small amount of starch and the synthetase activity increased. When starch granules were included in the test the synthetase activity was increased 3.5-fold. This value was 1.5-fold higher than the corresponding one in the dark. Plants that were in the light for 28 h contained fair amounts of starch and the synthetase activity was independent of the addition of primer. The values were 3 fold higher than those found in plants in the dark. A further increase in the synthetase activity and decrease in starch content were brought about by a dark period of 2 h following the illumination of 28 h.The total activity of phosphorylase remained high and almost constant in all these materials and did not require the addition of primer to attain a maximal value. In the material coming from the dark, the product produced by the activity of phosphorylase in absence of added primer could serve as a good primer for the synthetase. From the comparison of the amounts of starch produced in vivo under different conditions and the enzymic activities found we conclude that the first steps of starch synthesis are carried out by phosphorylase and that further on both enzymes participate in the process.

No abstract