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As previously reported, an increased concentration of plasma free amino acids, particularly lysine and threonine, was found when chicks were fasted for 24 hours. This increase was suppressed where a nonprotein diet was fed. Dextrose as the chief energy source in the nonprotein diet was more effective than shortening or lard in this respect. However, the degree to which the two energy sources differed was less where birds were adapted to the utilization of fat by feeding a diet rich in fat for a considerable time before making the comparison. Insulin administration to fasted chicks was found to lower plasma amino acid levels. The possible influence of interrelationships among dextrose, fat, and insulin on plasma amino acid levels is discussed.

Inhibition of uptake of cationic amino acids by certain neutral amino acids in the presence of alkali metal ions has been studied in the Ehrlich ascites tumor cell, in the rabbit reticulocyte and in yeast. When a hydroxyl group is introduced on carbon 4 (for the reticulocyte) or carbon 5 (for the Ehrlich cell) of straight chain amino acids, a sharp shift in the selectivity among the alkali metal ions occurs in favor of Na+ or of Na+ and K+ in the two animal cells. This shift is associated with attainment of a maximal interaction between the neutral amino acid and the alkali metal, and is taken to support a bridging by the ion between the amino acid and the receptor site. Although in both potassium-treated and glucose-treated yeast neutral amino acids also become much more inhibitory to cationic amino acid uptake in the presence of alkali metal ions, no augmentation has been observed on introduction of a side chain hydroxyl group, and selectivity among the ions is weak. Hence a bridging function is doubted for that organism. The ions exert additional inhibitory effects, independent of the presence of a neutral amino acid, which appear to be competitive with H+.