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Biochemical and Molecular Actions of Nutrients

The Binding of Folic Acid and 5-Methyltetrahydrofolate to Folate-Binding Proteins during Gastric Passage Differs in a Dynamic In Vitro Gastrointestinal Model¹

Miriam Verwei^*,,2, Karin Arkbåge, Hans Mocking, Robert Havenaar^* and John Groten^**

^* Departments of Nutritional Physiology, Food and Food Supplement Analysis and ^** Biomolecular Sciences, TNO Nutrition and Food Research, 3700 AJ Zeist, The Netherlands; Division of Human Nutrition, Wageningen University, 6700 EV Wageningen, The Netherlands, Department of Food Science, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden

²To whom correspondence should be addressed. E-mail: Verwei{at}voeding.tno.nl.

Despite its low natural folate concentration, milk is responsible for 10–15% of the daily folate intake in countries with a high dairy consumption. Milk products can be considered as a potential matrix for folate fortification, e.g., with synthetic folic acid, to enhance the daily intake of folate. In untreated milk, the natural folate, 5-methyltetrahydrofolate (5-CH₃-H₄folate), is bound to folate-binding proteins (FBP). In this study, the extent of binding to FBP for folic acid and 5-CH₃-H₄folate was investigated in a dynamic in vitro model simulating human gastric passage. Protein binding of folic acid and 5-CH₃-H₄folate was characterized using gel-exclusion chromatography. Before gastric passage, folic acid and 5-CH₃-H₄folate were bound mainly to FBP (76–79%), whereas 7% was free. Folic acid remained bound to FBP to a similar extent after gastric passage. For 5-CH₃-H₄folate, the FBP-bound fraction gradually decreased from 79 to 5% and the free fraction increased from 7 to 93%. Although folic acid enters the proximal part of the intestine bound to FBP, 5-CH₃-H₄folate appears to be present mainly as free folate in the duodenal lumen. The stability of FBP was similar in both folate/FBP mixtures, i.e., 70% of the initial FBP content was retained after gastric passage. This study indicated that FBP are partly stable during gastric passage but have different binding characteristics for folic acid and 5-CH₃-H₄folate in the duodenal lumen. This could result in different bioavailability from folic acid- and 5-CH₃-H₄folate–fortified milk products.

KEY WORDS: • folate-binding protein • 5-methyltetrahydrofolate • folic acid • gastric passage • milk products

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