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


OBJECTIVE
To investigate the effect of FBP on the absorption of folic acid and 5-CH3-H4folate from the intestinal lumen.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Verwei
Miriam
M
Physiological Sciences Dept., TNO Nutrition and Food Research, 360, 3700 AJ, Zeist, The Netherlands. verwei@voeding.tno.nl
van den Berg
Henk
H
Havenaar
Robert
R
Groten
John P
JP
D001692:Biological Transport / Q000187:drug effects; D018938:Caco-2 Cells; D002352:Carrier Proteins / Q000378:metabolism / Q000494:pharmacology*; D018929:Cell Culture Techniques; D002850:Chromatography, Gel; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000378:metabolism*; D006801:Humans; D007408:Intestinal Absorption; D007421:Intestine, Small / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000378:metabolism; D013763:Tetrahydrofolates / Q000378:metabolism*; D013997:Time Factors
D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D011956:Receptors, Cell Surface; D013763:Tetrahydrofolates; D005492:Folic Acid; C005984:5-methyltetrahydrofolate