The Uptake of Amino-Acids and their Incorporation into the Proteins of Excised Barley Embryos

Methods are described by which barley embryos may be excised and grown under sterile conditions on a medium containing sucrose, minerals, and a complete mixture of amino-acids. Growth under these conditions was comparable with that of intact seedlings and the uptake of sugar and amino-acid could be studied without disturbing the metabolic steady state. Purified preparations of the embryo proteins have been made and the constituent amino-acids separated. 14C labelling in these amino-acids was determined by a new gas-scintillation method. In an isotopic competition experiment embryos were grown in 14C-sucrose with nitrate or an amino-acid mixture as nitrogen source. The presence of exogenous amino-acids suppressed the incorporation of carbon from carbohydrate into amino-acid residues of the embryo protein. The degree of suppression varied, being undetectable for glutamic acid but almost complete for lysine and leucine; it appeared to be related to the length of the synthetic pathway from carbohydrate to the amino-acid. The evidence suggested that amino-acids are protein precursors, and this conclusion was confirmed in further experiments in which 14C-aspartic acid, -glutamine, -proline, -leucine, or -lysine were supplied singly in a complete mixture of amino-acids. The 14C was found predominantly in the amino-acid residue of protein corresponding to the 14C-amino-acid supplied, with smaller amounts in other amino-acids of the same or related families. Aspartic acid and glutamine yielded appreciable quantities of respiratory carbon dioxide, although the contribution was small compared to that of sucrose. Little carbon was lost as carbon dioxide from leucine or lysine. The results are discussed in relation to the role of amino-acids in protein synthesis, and to the existence of feedback control in the amino-acid metabolism of higher plants.