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

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Author: Verwei, M. · Berg, H. van den · Havenaar, R. · Groten, J.P.
Type:article
Date:2005
Institution: TNO Defensie en Veiligheid TNO Kwaliteit van Leven
Source:European Journal of Nutrition, 4, 44, 242-249
Identifier: 238514
doi: doi:10.1007/s00394-004-0516-9
Keywords: Biology · Biomedical Research · Bioavailability · Caco-2 · Folate · Folate-binding protein · Intestinal transport · 5 methyltetrahydrofolic acid · caffeine · folate binding protein · folic acid · mannitol · article · cell growth · cell membrane permeability · controlled study · human · human cell · intestine absorption · intestine cell · monolayer culture · protein binding · Biological Transport · Caco-2 Cells · Carrier Proteins · Cell Culture Techniques · Chromatography, Gel · Folic Acid · Humans · Intestinal Absorption · Intestine, Small · Receptors, Cell Surface · Tetrahydrofolates · Time Factors · Bovinae

Abstract

Background: Milk products are a potential matrix for fortification with synthetic folic acid or natural 5-methyltetrahydrofolate (5-CH 3-H4folate) to enhance the daily folate intake. In milk, folate occurs bound to folatebinding 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. Aim of the study: To investigate the effect of FBP on the absorption of folic acid and 5-CH 3-H4folate from the intestinal lumen. Methods: The transport of [3H]-folic acid and [14C]-5-CH 3-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 (Papp) of folic acid and 5-CH3-H 4folate 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 (Papp = 1.7*10-6 cm/s) than for 5- CH3-H4folate (Papp = 1.4*10-6 cm/s) after 2 h incubation to 1 μM 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 (Papp = 0.5*10-6 cm/s) but lower (P < 0.05) than that of caffeine (Papp = 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-CH 3-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-CH 3-H4folate was found to be dependent on the extent of binding to FBP at the luminal side of the cells. © Steinkopff Verlag 2004.