Properties of taurine transport in a human retinal pigment epithelial cell line

The characteristics of taurine transport were studied in a human retinal pigment epithelial cell line (HRPE). Uptake of taurine into monolayer cultures of the HRPE cells was markedly stimulated by the presence of NaCl in the uptake medium whereas the uptake was negligible in its absence. This NaCl-dependent uptake was an active process as the cells were able to accumulate taurine against a concentration gradient. The uptake rate of taurine was found to be many-fold greater than that of γ-aminobutyric acid (GABA). Unlabeled taurine and GABA competed with radiolabeled taurine for the uptake process, the former being more effective than the latter. However, uptake of radiolabeled GABA was not affected by unlabeled taurine and GABA. Substrate specificity studies revealed strong interaction of β-amino acids with the transport system responsible for taurine uptake. α-Amino acids failed to inhibit taurine uptake. A specific anion requirement was observed for optimal activity of the taurine transport system and CI- was the most supportive among several anions tested. Kinetic analyses showed that multiple Na+ and one CI- were involved in transfer of one taurine molecule. The transport process consisted of a single saturable system with a Michaelis-Menten constant of 2.0 ± 0.1 μM. These results show that the HRPE cell line expresses a high-affinity taurine transport system. This is the first demonstration of the presence of the taurine transporter in the human retinal pigment epithelium and the HRPE cell line may provide a useful model system for future studies involving taurine transport in the retinal pigment epithelium.