Oxidation of 10-formyltetrahydrofolate to 10-formyldihydrofolate by complex IV of rat mitochondria

We hypothesized that the unanticipated bioactivity of orally administered Unnatural carbon-6 isomers, (6R)-5-formyltetrahydrofolate (5-HCO-THF) and (6S)-5, 10-methenyltetrahydrofolate (5,10-CH-THF), in humans [Baggott, J. E., and Tamura, T. (1999) Biochim. Biophys. Acto 1472, 323-32] is explained by the rapid oxidation of (6S)-10-formyltetrahydrofolate (10-HCO-THF), which is produced by in vivo chemical processes from the above folates. An oxidation of 10-HCO-THF produces 10-formyldihydrofolate (10-HCO-DHF), which no longer has the asymmetric center at carbon-6 and is metabolized by aminoimidazole carboxamide ribotide (AICAR) transformylase forming bioactive dihydrofolate. Since cytochrome c (Fe3+) rapidly oxidizes both (6R)- and (6S)-10-HCO-THF [Baggott et al. (2001) Biochem. J. 354, 115-22], we investigated the metabolism of 10-HCO-THF by isolated rat liver mitochondria. We found that 10-HCO-THF supported the respiration of mitochondria without uncoupling ATP synthesis. The site of electron donation was identified as complex IV, which contains cytochrome c; the folate product was 10-HCO-DHF, and the reaction was saturable with respect to 10-HCO-THF. Both (6S)(unnatural) and (6R)-10-HCO-THF supported the respiration of mitochondria, whereas (6S)-5-formyltetrahydrofolate (5-HCO-THF) was inactive. To our knowledge, this cytochrome c oxidation of 10-HCO-THF to 10-HCO-DHF in the mitochondrial intermembrane space represents a possible folate metabolic pathway previously unidentified and would explain the bioactivity of unnatural carbon-6 isomers, (6R)5-HCO-THF and (6S)-5,10-CH-THF, in humans.