Nonenzymatic Degradation and Salvage of Dietary Folate: Physicochemical Factors Likely to Influence Bioavailability

Abstract

We investigated the oxidative degradation pathway of 5CH 3-H 4PteGlu, the main extracellular folate and the predominant form of the vitamin found in food and blood, BCH 3-H 4PteGlu is oxidized to 5CH 3-5,6-H 2PteGlu which subsequently undergoes C 9-N 10 bond cleavage yielding a pteridine residue and P-ABG, the latter step resulting in irreversible loss of vitamin activity. Under moderately acid conditions typical of the postprandial gut (pH 3.5) 5CH 3-H 4PteGlu is fairly stable ( t 1 2 = 273.6 min), while 5CH 3-5,6-H 2PteGlu is rapidly degraded ( t 1 2 = 16.9 min). In a neutral environment (pH 6.4) stability is reversed; 5CH(3)-H(4)PteGlu t 1 2 = 12.0 mins, 5CH 3-5,6-H 2PteGlu t 1 2 = 1504.6 min. Ascorbic acid was efficacious in the facile salvage of 5CH 3-H 4PteGlu from 5CH 3-5,6-H 2PteGlu which occurred rapidly and with significant efficiency (100% conversion) under acid (pH 3.5) conditions, t 1 2 = 1.3 min (1 mmol/liter ascorbate), but was less efficient under neutral (pH 6.4) conditions t 1 2 = 273.6 min (36% conversion). The presence of zinc and iron broadly maintains the pattern of effect, but increases all reaction rates. PteGlu was stable under all conditions studied. These results obtained in an artificial environment were supported by findings in human gastric juice: at a gastric pH of 1.47 with low endogenous ascorbate (7.0 μmol/liter), 5CH 3-5,6-H 2PteGlu and 5CH 3-H 4PteGlu both degrade instantly via C 9-N 10 bond cleavage to yield an equimolar amount of P-ABG. If the same gastric juice is spiked at 58.0 μmol/liter ascorbate (moderate endogenous concentration), 5CH 3-H 4PteGlu is stable ( t 1 2 = 334.7 min), while 5CH 3-5,6-H 2PteGlu is instantly salvaged to 5CH 3-H 4PteGlu with 43.3% efficiency, and the remaining 5CH 3-5,6-H 2PteGlu is degraded to P-ABG. In gastric juice with an elevated pH of 7.0 and no endogenous ascorbate, 5CH 3-5,6-H 2PteGlu and 5CH 3-H 4PteGlu are both stable, with no C 9-N 10 bond cleavage. This, for 5CH 3-H 4PteGlu, is in apparent contrast to findings at pH 6.4 in an artificial environment. The same gastric juice spiked to 50 μmol/liter ascorbate did not result in 5CH 3-H 4PteGlu salvage from 5CH 3-5,6-H 2PteGlu. These results may represent evidence of a biological role for gastric ascorbic acid in maintaining efficient folate bioavailability from dietary sources which reportedly contain significant amounts of folate as 5CH 3-5,6-H 4PteGlu. The clinical aspects of such an intimate relationship between gastric ascorbate and dietary folate are discussed, as are the implications for interpretation of bioavailability studies using nondiscriminatory folate assays which incorporate ascorbate into the assay procedure and thus convert metabolically inactive 5CH 3-5,6-H 2PteGlu into active 5CH 3-H 4PteGlu.