L-ALANINE ABSORPTION IN HUMAN INTESTINAL CACO-2 CELLS DRIVEN BY THE PROTON ELECTROCHEMICAL GRADIENT

In human Caco-2 intestinal epithelial layers, L-alanine absorption can be energized by a proton gradient across the brush-border membrane. A cidification of the apical medium, even in Na+-free media, is associat ed with a saturable net transepithelial absorption of L-alanine. L-Ala nine transport causes cytosolic acidification consistent with proton/a mino acid symport. L-Alanine transport in Na+-free media is rheogenic, stimulating an inward short-circuit current in voltage-clamped epithe lial monolayers. By measurement of rapid L-alanine influx across the a pical membrane, L-alanine-stimulated inward short-circuit current and intracellular acidification in the same cell batch, we estimate L-alan ine/proton stoichiometry to be 1:0.62 +/- 0.25 (SD) (short-circuit cur rent) or 1:0.73 +/- 0.19 (intracellular acidification). From competiti on studies, it is likely that L-proline, alpha-aminoisobutyric acid, a nd beta-alanine, but not L-valine and L-serine, are substrates for pro ton-linked, substrate transport in the brush border of Caco-2 cells.