The role of the proton electrochemical gradient in the transepithelial absorption of amino acids by human intestinal Caco-2 cell monolayers

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Abstract
We determined the extent of Na+-independent, proton-driven amino acid transport in human intestinal epithelia (Caco-2). In Na+-free conditions, acidification of the apical medium (apical pH 6.0, basolateral pH 7.4) is associated with a saturable net absorption of glycine. With Na+-free media and apical pH set at 6.0, (basolateral pH 7.4), competition studies with glycine indicate that proline, hydroxyproline, sarcosine, betaine, taurine, β-alanine, α-aminoisobutyric acid (AIB), α-methylaminoisobutyric acid (MeAIB), τ-amino-n-butyric acid and l-alanine are likely substrates for pH-dependent transport in the brush border of Caco-2 cells. Both d-serine and d-alanine were also substrates. In contrast leucine, isoleucine, valine, phenylalanine, methionine, threonine, cysteine, asparagine, glutamine, histidine, arginine, lysine, glutamate and d-aspartate were not effective substrates. Perfusion of those amino acids capable of inhibition of acid-stimulated net glycine transport at the brush-border surface of Caco-2 cell monolayers loaded with the pH-sensitive dye 2′,7′-bis(2-carboxyethyl-5(6)-carboxyfluorescein) (BCECF) caused cytosolic acidification consistent with proton/amino acid symport. In addition, these amino acids stimulate an inward short-circuit current (I sc) in voltage-clamped Caco-2 cell monolayers in Na+-free media (pH 6.0). Other amino acids such as leucine, isoleucine, phenylalanine, tryptophan, methionine, valine, serine, glutamine, asparagine, d-aspartic acid, glutamic acid, cysteine, lysine, arginine and histidine were without effect on both pHi and inward I sc. In conclusion, Caco-2 cells express a Na+-independent, H+-coupled, rheogenic amino acid transporter at the apical brush-border membrane which plays an important role in the transepithelial transport of a range of amino acids across this human intestinal epithelium.