Substrate specificity of the di/tripeptide transporter in human intestinal epithelia (Caco‐2): identification of substrates that undergo H+‐coupled absorption

Abstract

pH‐dependent transepithelial transport and intracellular accumulation of the hydrolysis‐resistant dipeptide glycylsarcosine (Gly‐Sar) have been demonstrated in the model human intestinal epithelial cell line, Caco‐2. Experiments with BCECF (2′,7′‐bis(2‐carboxyethyl)‐5(6)‐carboxyfluorescein)‐loaded Caco‐2 cells demonstrated that dipeptide (Gly‐Sar) transport across the apical membrane is coupled to proton flow into the cell. A range of postulated substrates for the intestinal di/tripeptide carrier were tested for their abilities to: (a) inhibit pH‐dependent [14C]Gly‐Sar apical‐to‐basal transport and intracellular accumulation and (b) stimulate H+ flow across the apical surface of BCECF‐loaded Caco‐2 cell monolayers. A range of compounds (including Gly‐Gly, Leu‐Leu, Gly‐Gly‐Gly, cefadroxil and cephalexin) caused marked acidification of intracellular pH when perfused at the apical surface of Caco‐2 cell monolayers. In contrast leucine and D‐Leu‐D‐Leu failed to induce proton flow. The ability to induce proton‐flow across the apical surface by these compounds, in this intestinal epithelium, was directly correlated to the relative inhibitory effects on [14C]‐Gly‐Sar transport and accumulation. The determination of substrate‐induced intracellular pH change in the Caco‐2 cell system may provide a useful rapid screen for candidate substrates for absorption via H+‐coupled transport mechanisms such as the intestinal di/tripeptide carrier in an appropriate physiological context. 1994 British Pharmacological Society