Effect of folate-binding protein on intestinal transport of folic acid and 5-methyltetrahydrofolate across Caco-2 cells

Milk products are a potential matrix for fortification with synthetic folic acid or natural 5-methyltetrahydrofolate (5-CH3-H4folate) to enhance the daily folate intake. In milk, folate occurs bound to folate-binding proteins (FBP). Our previous studies with an in vitro gastrointestinal model showed that 70% of the initial FBP content of the milk product was retained in the duodenal lumen. While folic acid remained bound to FBP after gastric passage, 5-CH3-H4folate was mainly present as free folate in the duodenal lumen. To investigate the effect of FBP on the absorption of folic acid and 5-CH3-H4folate from the intestinal lumen. The transport of [3H]-folic acid and [14C]-5-CH3-H4folate across enterocytes was studied in the presence or absence of bovine FBP using monolayers of Caco-2 cells grown on semi-permeable inserts in a two-compartment model. The apparent permeability coefficients (P(app)) of folic acid and 5-CH3-H4folate were determined and compared with the permeability of reference compounds for low (mannitol) and high (caffeine) permeability. The transport from the apical to the basolateral side of the Caco-2 cells was higher (P < 0.05) for folic acid (P(app) = 1.7*10(-6) cm/s) than for 5-CH3-H4folate (P(app) = 1.4*10(-6) cm/s) after 2 h incubation to 1 microM folic acid or 5-CH3-H4folate test solutions (pH 7). The permeability of folic acid and 5-CH3-H4folate across Caco-2 monolayers appeared to be higher (P < 0.05) than that of mannitol (P(app) = 0.5*10(-6) cm/s) but lower (P < 0.05) than that of caffeine (P(app) = 34*10(-6) cm/s). The addition of FBP to the medium led to a lower (P < 0.05) intestinal transport and cellular accumulation of folic acid and 5-CH3-H4folate. Compared to the reference compounds, folic acid and 5-CH3-H4folate showed a moderate permeability across Caco-2 cells, which indicates that folate absorption from the intestinal lumen is not likely to be complete. The intestinal transport of folic acid and 5-CH3-H4folate was found to be dependent on the extent of binding to FBP at the luminal side of the cells.