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H+-zwitterionic amino acid symport at the brush-border membrane of human intestinal epithelial (Caco-2) cells

Lookup NU author(s): Professor David Thwaites


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Transport of a number of dipolar amino acids (and the orally active antibiotic D-cycloserine) across the apical membrane of human intestinal epithelial (Caco-2) cell monolayers is mediated by a Na+-independent, pH-dependent transport mechanism. Relatively little is known about the mode of action of this transport system so to differentiate between pH dependence and proton coupling three experimental protocols were designed and tested. The results demonstrate, firstly, that it is the transapical pH gradient and its maintenance (rather than apical acidity alone) that is important in amino acid uptake. Secondly, Na+-independent uptake of seven dipolar amino acids (with pK(a) (-log of acid dissociation constant) values between 1.50 and 4.23) showed a similar dependence on apical pH (half-maximal uptake being observed at pH 5.99-6.20). Thirdly, the pattern of pH-dependent amino acid (β-alanine) uptake is similar irrespective of whether the cationic substrate concentration is varied or constant, demonstrating no relationship between uptake and concentration of the cationic form of the amino acid. These observations demonstrate that the transport mechanism is a H+-zwitterionic amino acid symporter and suggest that the presence of a H+ gradient at the apical surface of the human small intestine (in the form of the acid microclimate) may be important in driving nutrient absorption.

Publication metadata

Author(s): Thwaites DT; Stevens BC

Publication type: Article

Publication status: Published

Journal: Experimental Physiology

Year: 1999

Volume: 84

Issue: 2

Pages: 275-284

Print publication date: 01/03/1999

ISSN (print): 0958-0670

ISSN (electronic): 1469-445X

Publisher: Wiley-Blackwell Publishing Ltd.


DOI: 10.1111/j.1469-445X.1999.01843.x

PubMed id: 10226170


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