Bioactivity of [6R]-5-formyltetrahydrofolate, an unusual isomer, in humans and Enterococcus hirae, and cytochrome c oxidation of 10-formytetrahydrofolate to 10-formyldihydrofolate.
The bio-inactive C-6 isomer, [6R]-5-formyl-tetrahydrofolate (5-HCO-H(4)F), is not found in Nature. An oral dose of 13.5 micromol of [6R]-5-HCO-H(4)F in humans results in the appearance of the naturally occurring [6S]-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 c [cyt c (Fe3+)] molecules oxidize one 10-formyl-tetrahydrofolate (10-HCO-H(4)F) with second-order kinetics and a rate constant of 1.3 x 10(4) M(-1) x s(-1). The folate product of this oxidation reaction is 10-formyl-dihydrofolate (10-HCO-H(2)F), which has no C-6 asymmetric centre and is therefore bioactive. The folate-requiring bacterium, Enterococcus hirae, does not normally biosynthesize cytochromes but does so when given an exogenous source of haem (e.g. haemin). E. hirae grown in haemin-supplemented media for 3 days utilizes both [6R]- and [6S]-5-HCO-H(4)F in contrast to that grown in control medium, which utilizes only the [6S] isomer. Since known chemical reactions form 10-HCO-H(4)F from 5-HCO-H(4)F, the unusually large rate constant for the oxidation of 10-HCO-H(4)F by cyt c (Fe3+) may account for the unexpected bioactivity of [6R]-5-HCO-H(4)F in humans and in E. hirae grown in haemin-containing media. We used an unnatural C-6 folate isomer as a tool to reveal the possible in vivo oxidation of 10-HCO-H(4)F to 10-HCO-H(2)F; however, nothing precludes this oxidation from occurring in vivo with the natural C-6 isomer.
Baggott
J E
JE
Department of Nutrition Sciences, 336 Webb Building, 1675 University Blvd., University of Alabama at Birmingham, Birmingham, AL 35294-3360, USA. matlocks@uab.edu
Robinson
C B
CB
Johnston
K E
KE
D003574:Cytochrome c Group / Q000378:metabolism*; D016983:Enterococcus / Q000378:metabolism*; D005492:Folic Acid / Q000031:analogs & derivatives* / Q000378:metabolism*; D006427:Hemin / Q000378:metabolism; D006801:Humans; D002955:Leucovorin / Q000031:analogs & derivatives / Q000378:metabolism*; D010084:Oxidation-Reduction
D003574:Cytochrome c Group; C010161:10-formyltetrahydropteroylglutamic acid; C055914:10-formyldihydrofolate; D006427:Hemin; D005492:Folic Acid; D002955:Leucovorin