Uptake of Glycylglycine by the Scutellum of Germinating Barley Grain

Abstract

The scutella separated from germinating barley grains (Hordeum vulgare L. cv. Himalaya) took up the dipeptide [¹⁴C]glycylglycine (Gly-Gly) rapidly from incubation media. The pH optimum of the process was about 4.5, and the rate of uptake conformed to Michaelis-Menten kinetics with an apparent K_m of 2.3 mm and V_max of 41 μmole gram⁻¹ hour⁻¹. The uptake was strongly inhibited by dinitrophenol and cyanide and by lack of O₂. After incubation of the scutella with Gly-Gly, no intact Gly-Gly was detectable in the scutella but the level of free glycine increased. The poorly hydrolyzed “model” dipeptide glycylsarcosine, which is actively taken up and accumulated by the scutella, was a competitive inhibitor of the uptake of Gly-Gly and completely inhibited the uptake at infinitely high inhibitor concentration. This suggests that Gly-Gly is taken up by the same mechanism as glycylsarcosine as an intact dipeptide (without hydrolysis in the membrane) and is hydrolyzed to free glycine by the abundant peptidases of the scutella. The uptake of Gly-Gly was not affected by glycine or leucine, but was strongly inhibited by all of the 10 dipeptides tested for inhibition. The three dipeptides tested for uptake, Ala-Gly, Pro-Gly, and Gly-Pro, were all taken up by the scutella. Thus, the uptake mechanism for the dipeptides seems to be rather nonspecific with respect to the side chains of the amino acids. The high rates of the uptake suggest that this process has an essential role in the mobilization of reserve proteins in the germinating grain.