Glycine transport in human erythrocytes

1. Glycine transport in human erythrocytes was resolved into five separate components of uptake. The first and major component of uptake was transport by a high-affinity (apparent Km 25 microM) Na+- and Cl- -dependent system. This system was specific for glycine, sarcosine and proline; Br- but not I- was able to substitute for Cl-. Uptake by this route was inhibited less than 20% by the loop diuretics, bumetanide and furosemide (10(-4) M), suggesting that it was distinct from the Cl- -dependent system responsible for Na+/K+ transport. Its properties closely resembled those of the gly transport system described previously in avian erythrocytes. 2. The second uptake route was transport by the Na+-dependent, Cl- -independent small neutral amino acid transport system (designated ASC). Neither NA+-dependent uptake route was present in sheep erythrocytes. 3. Two Na+-independent uptake mechanisms were also identified; first, uptake by the amino acid transport system (designated L), and secondly, SITS-sensitive uptake by the anion-exchange (band 3) transport mechanism (SITS is 4-acetamido-4'-iso-thiocyanatostilbene-2,2'-disulphonic acid, an effective inhibitor of anion transport by this route). Uptake by the latter route was increased markedly when fluxes were measured in isotonic SO42- medium or when the pH was increased. 4. At 0.2 mM extracellular glycine, the relative contributions of each of these uptake routes to the total glycine flux were 42, 11, 15 and 16% for the gly, ASC, L and band 3 systems, respectively. 5. Finally, there was a residual Na+-independent component of glycine uptake which contributed 16% of the total flux. With the exception of the gly system, all uptake routes showed a linear concentration dependence up to 2 mM-glycine.