Bioactivity of [6R]-5-formyltetrahydrofolate, an unnatural isomer, in humans and Enterococcus hirae, and cytochrome c oxidation of 10-formyltetrahydrofolate to 10-formyldihydrofolate

摘要

pThe bio-inactive C-6 isomer, [6iR/i]-5-formyl-tetrahydrofolate (5-HCO-Hsub4/subF), is not found in Nature. An oral dose of 13.5iμ/imol of [6iR/i]-5-HCO-Hsub4/subF in humans results in the appearance of the naturally occurring [6iS/i]-5-methyl-tetrahydrofolate and relatively large amounts of other bioactive folates in plasma. The removal of the asymmetry at C-6 could account for these results. Two oxidized cytochrome ic/i [cyt ic/i (Fesup3+/sup)] molecules oxidize one 10-formyl-tetrahydrofolate (10-HCO-Hsub4/subF) with second-order kinetics and a rate constant of 1.3×;10sup4/sup Msup-1/sup·ssup-1/sup. The folate product of this oxidation reaction is 10-formyl-dihydrofolate (10-HCO-Hsub2/subF), which has no C-6 asymmetric centre and is therefore bioactive. The folate-requiring bacterium, iEnterococcus hirae/i, does not normally biosynthesize cytochromes but does so when given an exogenous source of haem (e.g. haemin). iE. hirae/i grown in haemin-supplemented media for 3 days utilizes both [6iR/i]- and [6iS/i]-5-HCO-Hsub4/subF in contrast to that grown in control medium, which utilizes only the [6iS/i] isomer. Since known chemical reactions form 10-HCO-Hsub4/subF from 5-HCO-Hsub4/subF, the unusually large rate constant for the oxidation of 10-HCO-Hsub4/subF by cyt ic/i (Fesup3+/sup) may account for the unexpected bioactivity of [6iR/i]-5-HCO-Hsub4/subF in humans and in iE. hirae/i grown in haemin-containing media. We used an unnatural C-6 folate isomer as a tool to reveal the possible iin vivo/i oxidation of 10-HCO-Hsub4/subF to 10-HCO-Hsub2/subF; however, nothing precludes this oxidation from occurring iin vivo/i with the natural C-6 isomer./p