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

Transport of a number of dipolar amino acids (and the orally active antibio tic D-cycloserine) across the apical membrane of human intestinal epithelia l (Caco-2) cell monolayers is mediated by a Na+-independent, pH-dependent t ransport 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 maint enance (rather than apical acidity alone) that is important in amino acid u ptake. Secondly, Na+-independent uptake of seven dipolar amino acids (with pK(a) (-log of acid dissociation constant) values between 1.50 and 4.23) sh owed 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 (beta-ala nine) uptake is similar irrespective of whether the cationic substrate conc entration is varied or constant, demonstrating no relationship between upta ke and concentration of the cationic form of the amino acid. These observat ions demonstrate that the transport mechanism is a H+-zwitterionic amino ac id symporter and suggest that the presence of a H+ gradient at the epical s urface of the human small intestine (in the form of the acid microclimate) may be important in driving nutrient absorption.