Respiration of barley plants IV. Protein catabolism and the formation of amides in starving leaves

Abstract

In a continuation of earlier studies the nitrogenous constituents of barley leaves have been investigated with the object of gaining more direct evidence on the nature of protein catabolism in starving leaves. Particular attention was given to the origin and identification of the amides. Tissue proteins were separated and partially analyzed by direct and indirect methods. The amino-acids of the barley-leaf protein were very similar to those of the proteins prepared from grasses. From these and other data estimates of the amide-N, glutamic acid and aspartic acid were obtained. The amide-N content of the whole protein of the leaves agreed closely with that of the separated protein. There was evidence that the release of amide-N proceeded uniformly as the proteins were broken down during starvation. Asparagine was identified as a product of catabolism by direct isolation from starved leaves; recovery of the crystalline product amounted to 60% of the total stable amide-N of the crude extract. Glutamine was not isolated, but additional evidence of its presence in the leaves was obtained. An analysis of the relation between proteolysis and formation of amides confirmed several of the suggestions made in a previous discussion. Most of the amide-N of the starved leaves was of secondary origin, and there were clear indications of a secondary synthesis of the aspartic acid combined in asparagine. Glutamine amide-N was formed secondarily during the early stages of starvation, but evidence of the origin of glutamic acid was inconclusive. The data of different experiments indicated that the breakdown of tissue proteins contributed between 20 and 40% of the total carbon lost from the leaves as respiratory carbon dioxide.