Amino acid metabolism in plant leaf

¹⁴C-labelled sodium bicarbonate and ¹⁵N-labelled ammonium sulfate were simultaneously vacuum-infiltrated into detached sunflower leaves, and the incorporation of ¹⁴C and ¹⁵N into free amino acids was chased during 60-min period in the light and in the dark.

In the light, the ue specific activity of aspartic acid, alanine, serine and glycine rapidly increased for 5 min and thereafter decreased. On the other hand, that of gultamic acid continued to increase slowly during the entire 60-min period. In the dark, aspartic acid most actively incorporated ¹⁴C. The difference of changes in ¹⁴C specific activity between glutamic acid and other amino acids was also observed in the dark as in the light. These results suggest that the carbon skeleton of glutamic acid is synthesized from aspartic acid, alanine, serine and glycine.

¹⁵N content of glutamine was the highest of all amino acids investigated in the light, and it was followed by glutamic acid. alanine, aspartic acid, serine and glycine, in this order. In the dark, ¹⁵N content of glutamic acid fell remarkably and was lower than that of alanine up to 5 min. From these ¹⁵N tracer experiments, it is suggested that the incorporation of ammonium into glutamic acid is strictly dependent on light and that alanine incorporates ammonium by the direct amination besides the transamination from glutamic acid.