Migration of Amino Acids across the Membrane of the Human Erythrocyte

Before the work of Costantino, the erythrocyte was considered to be impermeable to ammo acids (1, 2).Costantino observed, however, a higher level in the red blood cell than in the serum of groups titratable in the presence of formaldehyde(3).He was also able to demonstrate by this method uptake by red cells in vitro of glycine, asparagine, and the mixture of amino acids contained in a casein hydrolysate (4).He therefore proposed that these cells are permeable to amino acids.Abderhalden and Kiirten (5) found that the amount of amino acid taken up in 45 minutes did not bear a linear relationship to the concentration in the serum.These authors therefore proposed that the process was one of adsorption on the cell surface, and used the Freundlich adsorption isotherm to describe their results.Danielson ( 6), on the other hand, concluded that the cell was freely permeable to amino acids, since a fraction of the cell or-amino nitrogen as estimated by the method of Folin was easily washed from the cell.He suggested that the remainder of the a-amino nitrogen in the cell probably consists largely of glutathione, to which the membrane is impermeable.Ussing (7) compared the rates of uptake of several amino acids and found that aspartic and glutamic acids enter the red blood cell slowly, if at all, whereas leucine is taken up quite readily.He was also able to confirm, by specific methods, Danielson's assumption that glutathione accounted for the excess nonprotein a-amino nitrogen in the cell over that in the plasma.I'ssing concluded, on the basis of the temperature dependence of uptake, that the process was not one of internal binding, as had been suggested by Sbarsky (8).By specific methods of analysis, Christensen et al. found that glycine and alanine enter the human red cell rather slowly (9), and that this process is insensitive to oxygen deficiency and respiratory inhibitors (10).Slightly higher concentrations were observed in the cells than in the plasma, although this was not the case with rabbit erythrocytes (11).The results of Ussing, and the more recent ones of Rieser ( 12), reveal that the entry rate into red cells is greatly affected by amino acid structure.We have therefore studied the time course of uptake of a series of natural and synthetic amino acids of the neutral group in order to define the nature of these structural effects.Also, in view of the recent finding by Oxender and Christensen of more than one mediated uptake process for neutral amino acids in the Ehrlich ascites cell (13.-15), we have attempted