AMINO-ACID-TRANSPORT SYSTEM Y(- SPECIFICITY AND CATION DEPENDENCE OF THE TRANSLOCATION STEP()L OF HUMAN ERYTHROCYTES )

The transport specificity of system y(+)L of human erythrocytes was in vestigated and the carrier was found to accept a wide range of amino a cids as substrates. Relative rates of entry for various amino acids we re estimated from their trans-effects on the unidirectional efflux of L-[C-14]-lysine. Some neutral amino acids, L-lysine and L-glutamic aci d induced marked trans-acceleration of labeled lysine efflux; saturati ng concentrations of external L-leucine and L-lysine increased the rat e by 5.3 +/- 0.63 and 6.2 +/- 0.54, respectively. The rate of transloc ation of the carrier-substrate complex is less dependent on the struct ure of the amino acid than binding. Translocation is slower for the bu lkier analogues (L-tryptophan, L-phenylalanine); smaller amino acids, although weakly bound, are rapidly transported (L-alanine, L-serine). Half-saturation constants (+/-SEM) calculated from this effect (L-lysi ne, 10.32 +/- 0.49 mu M and L-leucine, 11.50 +/- 0.50 mu M) agreed wit h those previously measured in cis-inhibition experiments. The degree of trans-acceleration caused by neutral amino acids did not differ sig nificantly in Na+, Li+ or K+ medium, whereas the affinity for neutral amino acids was dramatically decreased if N-a+ or Li+ were replaced by K+. The observation that specificity is principally expressed in subs trate binding indicates that the carrier reorientation step is largely independent of the forces of interaction between the carrier and the transport site.