Changes in Amino Acid Transport During Red Cell Maturation

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Abstract
Amino acid transport was studied in sheep red blood cells (RBC) as a function of cell maturation. Transport of amino acids was decreased strikingly in the mature mammalian RBC compared to the immature reticulocyte. Blood obtained 5-6 days after massive bleeding was fractionated on dextran gradients. In the mature erythrocyte amino acids were taken up only slowly and in the normal experimental interval (60 min) the concentration in the cell does not reach that of the medium. The reticulocyte-rich (top) fraction (50-90% reticulocytes) accumulated certain amino acids, particularly histidine, methionine and leucine. The underlying process was ATP-independent and Na+-insensitive and had properties consistent with exchange diffusion, i.e., accelerated uptake or efflux when unlabeled solute was present on the trans side. The process was apparent in intact cells and resealed ghosts. The decrease in activity of amino acid transport was a function of red maturation. Separation of cells according to their density 1, 2 and 3 wk after bleeding led to progressively lower amino acid transport activity with increasing cell density. During in vitro long-term incubation at 37.degree. C of reticulocyte-rich, unfractionated blood (5-10% reticulocytes), amino acid transport decreased while red cell integrity was maintained, as evidenced by the retention of a normal K+ gradient and the absence of hemolysis. The progressive loss was seen with resealed ghosts and with intact cells. Not all the amino acids examined participated in this exchange process. The most actively exchanged were histidine, leucine, methionine and phenylalanine. Glycine, proline, arginine and .alpha.-amino isobutyric acid did not participate in the exchange process.