L-Alanine absorption in human intestinal Caco-2 cells driven by the proton electrochemical gradient

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Abstract
In human Caco-2 intestinal epithelial layers, xxxl-alanine absorption can be energized by a proton gradient across the brush-border membrane. Acidification of the apical medium, even in Na+-free media, is associated with a saturable net transepithelial absorption of xxxl-alanine. xxxl-Alanine transport causes cytosolic acidification consistent with proton/amino acid symport. xxxl-Alanine transport in Na+-free media is rheogenic, stimulating an inward short-circuit current in voltageclamped epithelial monolayers. By measurement of rapid xxxl-alanine influx across the apical membrane, xxxl-alanine-stimulated inward short-circuit current and intracellular acidification in the same cell batch, we estimate xxxl-alanine/proton stoichiometry to be 1∶0.62 ±0.25 (xxxsd) (short-circuit current) or 1∶0.73 ±0.19 (intracellular acidification). From competition studies, it is likely that xxxl-proline, α-aminoisobutyric acid, and β-alanine, but not xxxl-valine and xxxl-serine, are substrates for protonlinked, substrate transport in the brush border of Caco-2 cells.