SUMMARY1. 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/tM) 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 < 20 % by the loop diuretics, bumetanide and furosemide (10-4M), 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 ihibitor 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 160% for the gly, ASC, L and band 3 systems, respectively. 5. Finally, there was a residual Na+-independent component of glycine uptake which contributed 160 of the total flux. With the exception of the gly system, all uptake routes showed a linear concentration dependence up to 2 mM-glycine.

Amino acid transport was compared in human and in sheep erythrocytes. Kinetic studies established that human cells have three discrete amino acid transport systems, designated L, Ly + and ASC. The L system is partially stereospecific, with a preference for large neutral amino acids. L-leucine has a threefold lower apparent K m and a twofold smaller V max than D-leucine. Alanine, cysteine and possibly dibasic amino acids are transported by this route, but with a low affinity. The Ly + system is highly stereoselective, and specific for dibasic amino acids, including arginine. The ASC system is Na-dependent and selective for neutral amino acids of intermediate size. It has a particularly low apparent K m for cysteine and is stereospecific. Sheep erythrocytes lack these systems. Instead they possess an additional system (C system) responsible for the transport both of neutral and of dibasic amino acids, with cysteine as the optimal substrate. Although the substrate specificities of the human ASC and sheep C systems are similar, the sheep system does not require Na and has considerably higher apparent K m values. Dibasic amino acid transport (of lysine, but not of arginine) by the C system occurs with a low affinity.

No abstract

No abstract