Amino acid transport by human erythrocyte membranes.

The human erythrocyte plasma membrane is permeable to several free amino acids usually present in the bloodstream. Seven distinct routes of entry have been described which represent both secondary active transport and facilitated diffusion (passive transport). Additionally, certain amino acids can enter the cell by simple diffusion, at least to a limited extent. The function of most of these transport systems is unclear, although it has been suggested that the cell can take up certain amino acids and carry them to various parts of the body. In the case of glutamine, cysteine, and glycine, however, it is believed that the biosynthesis of the tripeptide glutathione is the primary reason for their uptake into the cell. Much of the amino acid transport probably has no function in mature red cells, but might be a remnant of the immature cell's needs. This review discusses the various amino acid transport systems known to be present in the red cell plasma membrane.