Taurine uptake in apical membrane vesicles from the bovine retinal pigment epithelium

Characteristics of taurine uptake were investigated in apical membrane vesicles isolated from bovine retinal pigment epithelium. Uptake of taurine into these vesicles was stimulated markedly by the presence of an inwardly directed NaCl gradient across the membrane. The time course of the NaCl gradient-dependent uptake showed an overshoot, indicating a transient accumulation of taurine in the vesicles against a concentration gradient. Monovalent cations other than Na+ did not support taurine uptake. There was also an anion requirement for the uptake system, and Cl- was the most potent among several monovalent anions tested. The NaCl-dependent taurine uptake was stimulated by inside-negative, K+, and H+ diffusion potentials, demonstrating the electrogenic nature of the system. Stoichiometric analysis revealed that two Na+ and one Cl- ions were involved in the translocation of one taurine molecule. The system had an apparent dissociation constant (Kt) of 96 +/- 17 microM and a maximal velocity of 267 +/- 24 pmol/mg of protein over 15 sec. Uptake of radiolabeled taurine was inhibited by excess amounts of unlabeled taurine, hypotaurine, beta-alanine, and gamma-aminobutyric acid. The relative potencies of the amino acids to inhibit taurine uptake were compared in three tissues, the bovine retinal pigment epithelium, the rat kidney, and the human placenta. In the apical membrane vesicles of the pigment epithelium, gamma-aminobutyric acid was many times more potent than taurine itself in inhibiting radiolabeled taurine uptake. However, in the brush border membrane vesicles of the kidney and the placenta, taurine was many times more potent than gamma-aminobutyric acid.(ABSTRACT TRUNCATED AT 250 WORDS)