Physicochemical effect of pH and antioxidants on mono-and triglutamate forms of 5-methyltetrahydrofolate, and evaluation of vitamin stability in human gastric juice: Implications for folate bioavailability

This report examines the physico-chemical properties of mono and triglutamate forms of 5-methyltetrahydrofolate(n) (5CH(3)-H-4-PteGlu(n)) in human gastric juice and under artificial conditions, typical of the upper gastrointestinal tract. Degradation of 5CH(3)-H-4-PteGlu(n) to 5-methyldihydrofolate(n) (5CH(3)-5,6-H(2)PteGlu(n)) and on to the C-9-N-10 scission product p-aminobenzovlqlutamate (P-ABG), and the salvage of 5CH(3)-H-4-PteGlu(n) from 5CH(3)-5,6-H(2)PteGlu(n) was examined with emphasis on the influence of pH and ascorbic acid. At pH 3.5, physiological levels of ascorbic acid (50 mu Mol/l) salvage both 5CH(3)-H(4)PteGlu(1) and 5CH(3)-H(4)PteGlu(3) from acid labile 5CH(3)-5,6-H(2)PteGlu(n) and prevent loss of vitamin activity as indicated by P-ABG(n) formation. This process is less efficient at pH 7.0, although under such conditions 5CH(3)-5,6-H(2)PteGlu(n) is rendered relatively stable. Ascorbic acid (50 mu Mol/l) reduces degradation of 5CH(3)-H(4)PteGlu(1) and 5CH(3)-H(4)PteGlu(3) at both pH 3.5 and 7.0. However, irrespective of ascorbic acid, 5CH(3)-H(4)PteGlu(1) and 5CH(3)-H(4)PteGlu(3) are both more stable at pH 3.5 than pH 7.0. There is a clear differential between mono- and polyglutamate stability, since, irrespective of pH or the presence of ascorbic acid, 5CH(3)-H(4)PteGlu(1) is inherently more stable than 5CH(3)-H(4)PteGlu(3). While ascorbic acid clearly stabilises mono- and triglutamate forms of methylfolate in human gastric juice, a factor in gastric juice removes any differential instability as seen in buffer solution. We speculate that this endogenous factor could be either a binding protein or some other antioxidant. These in vitro findings provide information that may be useful in evaluating the in vivo bioavailability of natural polyglutarnate forms of the vitamin. (c) 2007 Elsevier Ltd. All rights reserved.