The distribution of amino acids between plasma and erythrocytes

Amino acid concentrations were determined in whole blood, plasma and washed erythrocytes from a group of healthy subjects. A comparison between the erythrocyte concentrations calculated from whole blood and plasma concentrations and those measured in washed erythrocytes showed that several amino acids (especially methionine, valine, isoleucine, leucine, tyrosine and phenylalanine) can easily be washed out of the erythrocytes. Statistical analysis of the data indicates that at least three different amino acid transport systems are operative in erythrocytes: (1) a system for anionic amino acids (aspartate and glutamate), which concentrates these amino acids intracellularly, reaching high erythrocyte/plasma (E/P) concentration ratios; (2) a concentrating system of A type, transporting serine, glycine and alanine and maintaining E/P ratios less than two; these amino acids show positive corelations between plasma and erythrocyte concentrations and are retained in the erythrocytes when they are washed; (3) a system of L type, with reactivity to the branched chain amino acids, methionine, phenylalanine, lysine and glutamine, which is not concentrating. The erythrocyte concentrations of these amino acids are independent of those in plasma and they can easily be washed out of the cells. Threonine and tyrosine seem to be transported by both the A and L system.