Effect of Proteins on the Diffusion of Amino Acids Through Membranes.

Abstract

The concentration of amino acids in nucleated erythrocytes is known to be several times that of the surrounding plasma. The concentration in mammalian liver and muscle, as indicated by determinations on tungstic acid extracts, is 6 to 8 times that of the blood plasma.

As part of an inquiry into the factors that cause this inequality in distribution we have studied the diffusion of amino acids through tubular membranes of cellophane. The progress of dialysis was measured by amino nitrogen determinations on the inner and outer liquids by the manometric method of Van Slyke. The inner liquid consisted of a 1.5% solution of gold label gelatin. The amino acid was contained in the outer liquid in an initial concentration of 0.001 N. Glycine was generally employed. After 15 to 30 hours at a temperature of 20° the experiments were terminated. The outer fluid was analyzed directly, while the inner fluid was first rendered free of protein by treatment with phosphotungstic acid.

Over a wide range of H ion concentration, (pH 2.5-9.5) the equilibrium concentration of amino acid in the outer fluid was found to be over twice as great as that in the inner protein-containing solution. This inequality persisted, undiminished in magnitude, even in the isoelectric zone of the protein. The equilibrium, moreover, could be approached from the other end, that is by dissolving the amino acid in the inner fluid containing the dispersed protein. In neutral solutions aspartic acid and glutamic acid behaved like glycine in the establishment of a high concentration ratio when dialyzed against 1.5% gelatin. Dialyzed solutions of crystalline egg albumin (1.5%) and agar-agar (0.38%) affected the diffusion of glycine at pH 5 in similar fashion.