Structural characterization of substrates for the anion exchange transporter in Caco-2 cells

The present study was conducted to characterize the structural specificity of an anion exchange transporter in intestinal epithelial cells. The transport of carboxylic acids with hydroxyl group(s) at the 2, 3, 4, and/or 5 positions with respect to carboxylate was examined by using Caco-2 cells in the presence of bicarbonate ions on the basolateral side to enhance the activity of the anion-exchange transporter. In the presence of the bicarbonate ion gradient, transport of L-lactic acid consisted of a saturable process and a nonsaturable process as judged from the Eadie-Hofstee plot. The transport of L-lactic acid at 1 microM was reduced by sodium azide, dinitrophenol, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). It was also reduced by 2-, 4-, and 5-hydroxycarboxylic acids such as hydroxyacetic acid, 4-hydroxybutyric acid, and 5-hydroxydecanoic acid, but not by 3-hydroxycarboxylic acids such as 3-hydroxypropionic acid and 3-hydroxybutyric acid. Transport of both 2- and 4-hydroxybutyric acids involved saturable and nonsaturable processes, whereas that of 3-hydroxybutyric acid was nonsaturable and was not inhibited by DIDS. These results indicate that 3-hydroxycarboxylic acids might not be substrates for this anion exchange transporter in intestinal epithelial cells, suggesting that the position of hydroxylation is significant for molecular recognition by the transporter.