Effect of pH on l- and d-methionine uptake across the apical membrane of Caco-2 cells

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The transport systems involved in intestinal methionine (Met) absorption are described as Na(+)-dependent and Na(+)-independent mechanisms. However, since recent studies have suggested the importance of the H(+) gradient as a driving force for intestinal nutrient absorption, the aim of the present work was to test whether Met transport across the apical membrane of Caco-2 cells is affected by extracellular pH. The results show that l- and d-Met uptake was increased by lowering extracellular pH from 7.4 to 5.5, in both the presence and absence of Na(+). Cis-inhibition experiments revealed that inhibition of l-Met transport by 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH) or l-lysine (l-Lys) was higher at a pH of 5.5. Moreover, the BCH-insensitive component was not affected by pH, whereas the l-Lys-insensitive component was increased by lowering extracellular pH, thus suggesting the participation of system L. The contribution of another mechanism, sensitive to both BCH and l-Lys, was also considered. The inhibition obtained with taurine (Tau) was also higher at a pH of 5.5, thus suggesting the involvement of system B(0,+) on pH-stimulated component. As for d-Met uptake, the results showed higher inhibition with l-Lys and Tau at a pH of 5.5 and no effect on the l-Lys- or Tau-insensitive component. In conclusion, Met transport across the apical membrane of Caco-2 cells is increased by low extracellular pH as the result of the stimulation of two transport systems functionally identified with systems L and B(0,+) for l-Met and with system B(0,+) for d-Met.