Constitutive expression of the taurine transporter in a human colon carcinoma cell line

The human colon carcinoma cell line HT-29, when grown to confluence, was found to take up taurine and accumulate it against a concentration gradient from a NaCl-containing uptake medium. Replacement of NaCl with choline chloride almost totally abolished the uptake. Taurine uptake was dependent not only on Na+ but also on Cl-, because other anions failed to support the uptake in the presence of Na+. The uptake process was specific for beta-amino acids such as taurine, hypotaurine, and beta-alanine. Apparently, a single transport system with a Michaelis-Menten constant of 10.6 +/- 0.3 microM was responsible for the uptake. Stoichiometric analyses revealed that the Na+:taurine coupling ratio was 2:1, whereas the Cl-:taurine coupling ratio was 1:1. Culture of the cells in the presence of taurine caused downregulation of the uptake system. These cells were also capable of accumulating beta-alanine against a concentration gradient in the presence of NaCl. Beta-Alanine uptake occurred via a single transport syst...