Carrier affinity as a mechanism for the pH-dependence of folate transport in the small intestine.

A mildly acidic pH in the lumen of the small intestine markedly enhances the transport of folate. This study investigated the relationship between pH and the affinity between folic acid and the apical membrane transporter using brush border membrane vesicles from rat jejunum and differentiated monolayer cultures of the colon carcinoma cell line, CaCo-2. Uptake studies with BBMV were conducted at folic acid concentrations of 0.1 to 50 mumol/l, conditions which were suitable for analyzing uptake data based on the Michaelis-Menten equation modified to include a nonsaturable component. These analyses yielded apparent Km values of 0.6 and 12.3 microM at pH 5.5 and pH 7.4, respectively (P less than 0.05). Values for Vmax were lower at pH 5.5 than at pH 7.4 (0.8 vs. 1.6 pmol/mg protein per 10 s, P less than 0.05). The studies with CaCo-2 cells employed folic acid concentrations of 0.1 to 5 mumol/l. Under these conditions the apparent Km for folic uptake was lowest at pH 6.0, where the Km was 0.7 mumol/l. The apparent Km increased sharply as a neutral pH was approached; reaching a value of 13.9 mumol/l at pH 7.1. These data suggest that the prominent pH effect on intestinal folate transport is, in part, explained by an increased affinity of the folate substrate for its membrane transporter.