Tryptophan transport through transport system T in the human erythrocyte, the Ehrlich cell and the rat intestine.

We studied the transport of tryptophan through transport system T in the human red cell, the Ehrlich ascites-tumour cell and in everted sacs of rat intestine. In red cells we confirmed earlier results on Na+-independence and aromatic amino acid specificity (Rosenberg, R., Young, J.D. and Ellory, J.C. (1980) Biochim. Biophys. Acta 598, 375-384). In addition we observed that N-methylation or N-acetylation did not reduce the affinity of the substrates for system T, hydroxylation could increase or decrease substrate affinity, and system T was insensitive to pH changes in the medium. These results characterized reactive differences between system T and other known amino acid transport systems. We also found that D-isomers were about 1/3 as effective as L-isomers to inhibit L-tryptophan uptake. D-Tryptophan competitively inhibited L-tryptophan uptake, but was not taken up by system T. L-Tryptophan produced trans-stimulation of the uptake (influx) and trans-inhibition of the release (efflux) of L-[3H]tryptophan; D-tryptophan produced trans-inhibition of the efflux but did not affect significantly the uptake. These results show that in red cells the transport properties of transport system T are asymmetric. Transport system T seems to be absent in the other two preparations studied, the Ehrlich ascites-tumour cell and the rat intestine.