NONENZYMATIC DEGRADATION AND SALVAGE OF DIETARY-FOLATE - PHYSICOCHEMICAL FACTORS LIKELY TO INFLUENCE BIOAVAILABILITY

We investigated the oxidative degradation pathway of 5CH(3)-H(4)PteGlu, the main extracellular folate and the predominant form of the vitamin found in food and blood, BCH3-H(4)PteGlu is oxidized to 5CH(3)-5,6-H(2)PteGlu 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 ypical of the postprandial gut (pH 3.5) 5CH(3)-H(4)PteGlu is fairly stable (t(1/2) = 273.6 min), while 5CH(3)-5,6-H(2)PteGlu 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(2)PteGlu t(1/2) = 1504.6 min. Ascorbic acid was efficacious in the facile salvage of 5CH(3)-H(4)PteGlu from 5CH(3)-5,6-H(2)PteGlu 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 mu mol/liter), 5CH(3)-5,6-H(2)PteGlu and 5CH3-H(4)PteGlu 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 mu mol/liter ascorbate (moderate endogenous concentration), 5CH(3)-H(4)PteGlu is stable (t(1/2) = 334.7 min), while 5CH(3)-5,6-H(2)PteGlu is instantly salvaged to 5CH(3)-H(4)PteGlu with 43.3% efficiency, and the remaining 5CH(3)-5,6-H(2)PteGlu is degraded to P-ABG. In gastric juice with an elevated pH of 7.0 and no endogenous ascorbate, 5CH(3)-5,6-H(2)PteGlu and 5CH(3)-H(4)PteGlu are both stable, with no C-9-N-10 bond cleavage. This, for 5CH(3)-H(4)PteGlu, is in apparent contrast to findings at pH 6.4 in an artificial environment. The same gastric juice spiked to 50 mu mol/liter ascorbate did not result in 5CH(3)-H(4)PteGlu salvage from 5CH(3)-5,6-H(2)PteGlu. 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(4)PteGlu. 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(Z)PteGlu into active 5CH(3)-H(4)PteGlu. (C) 1995 Academic Press, Inc.