Amino acid transport in human and in sheep erythrocytes

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 Km and a twofold smaller Vmax 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 Km 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 Km values. Dibasic amino acid transport (of lysine, but not of arginine) by the C system occurs with a low affinity.