One-carbon metabolism is usually represented as having three canonical functions: purine synthesis, thymidylate synthesis and methylation reactions. There is however a fourth major function: the metabolism of some amino acids (serine, glycine, tryptophan and histidine), as well as choline. These substrates can provide cells with more one-carbon groups than they need for these three canonical functions. Therefore, there must be mechanisms for the disposal of these one-carbon groups (when in excess) which maintain the complement of these groups required for the canonical functions. The key enzyme for these mechanisms is 10-formyl-THF (tetrahydrofolate) dehydrogenase (both mitochondrial and cytoplasmic isoforms) which oxidizes the formyl group to CO2 with the attendant reduction of NADP(+) to NADPH and release of THF. In addition to oxidizing the excess of these compounds, this process can reduce substantial quantities of NADP(+) to NADPH.

Formate, a crucial component of one-carbon metabolism, is increasingly recognized as an important intermediate in production and transport of one-carbon units. Unlike tetrahydrofolate-linked intermediates, it is not restricted to the intracellular milieu so that circulating levels of formate can provide insight into cellular events. We report a novel isotope-dilution, GC-MS assay employing derivatization by 2,3,4,5,6-pentafluorobenzyl bromide for the determination of formate in biological samples. This assay is robust and sensitive; it may be applied to the measurement of formate in serum, plasma and urine. We demonstrate how this method may be applied by providing the first characterization of formate levels in a human population; formate levels were higher in males than in females. We also show how this procedure may be applied for the measurement of in vivo kinetics of endogenous formate production in experimental animals.

Plasma and urinary formate concentrations were recently found to be elevated during vitamin B12 and folate deficiencies. It was proposed that formate may be a valuable biomarker of impaired one-carbon metabolism. Formate is an essential intermediary metabolite in folate-mediated one-carbon metabolism and, despite its importance, our knowledge of its metabolism is limited. Formate can be produced from several substrates (e.g., methanol, branched chain fatty acids, amino acids), some reactions being folate-dependent while others are not. Formate removal proceeds via two pathways; the major one being folate-dependent. Formate is a potentially toxic molecule and we suggest that formate may play a role in some of the pathologies associated with defective one-carbon metabolism.Keywords: folic acid; formic acid; vitamin B2; vitamin B6; vitamin B12. The importance of formate in intermediary metabolismFormic acid and its conjugate base formate are essential endogenous one-carbon metabolites in virtually all living organisms. Formic acid is sometimes called methanoic acid and with its chemical formula, HCOOH, it is the simplest carboxylic acid. The name formic acid takes its origin from formica , the latin for ants from which it was first distilled by Hulse and Fisher after they observed that the ants secreted a substance that caused blue flowers to turn red [ 1 ]. For some time this acid was believed by chemists to be acetic acid, malic acid or a mixture of both. However, after thorough examination, it was demonstrated that it differed from acetic and malic acids on the basis of its specific gravity, its reaction with alkali, its metallic salts and its chemical affinities [ 2 ]. Formic acid has a pKa of 3.77 and, as a result, most of it occurs in the body as the formate anion.The importance of formate in intermediary metabolism was recognized over six decades ago when it was shown that the carbon of formate is incorporated into nucleic acids and into the glucogenic amino acid serine [ 3 -6 ]. The incorporation of formate into nucleic acids and serine requires its activation by tetrahydrofolate (THF) [ 7 ] and therefore the metabolism of formate and folate are tightly related. Our current understanding of cellular folate metabolism involves two parallel pools of folate, one mitochondrial and the other cytosolic, connected by one-carbon donors such as glycine, serine and formate [ 8 ]. For instance, mitochondria can take up serine, oxidize two of its three carbons to formate and export this to the cytosol where it re-enters the folate pool to participate in methylation reactions or nucleotide synthesis (reviewed by Tibbetts [ 8 ]). These observations, combining the roles of formate in amino and nucleic acid metabolism, illustrate the central and essential role of formate in onecarbon metabolism.Despite the importance of formate in intermediary metabolism, our knowledge of its metabolism is limited. Annison [ 9 ] showed that formate represents 10% -30% of the total volatile fatty acids in the blood of numerous anim...

Background:The mitochondrial production of formate is critical to the generation of one-carbon groups. Results: We have shown that the in vivo production of formate is markedly reduced in folate deficiency. Conclusion: Folate deficiency reduces both the production and utilization of one-carbon groups. Significance: Folate status affects the production and utilization of one-carbon groups and the role of choline metabolites as precursors of these groups.

By virtue of its role in nucleotide synthesis, as well as the provision of methyl groups for vital methylation reactions, one-carbon metabolism plays a crucial role in growth and development. Formate, a critical albeit neglected component of one-carbon metabolism, occurs extracellularly and may provide insights into cellular events. We examined formate metabolism in chronically cannulated fetal sheep (gestation days 119-121, equivalent to mid-third trimester in humans) and in their mothers as well as in normal full-term lambs. Plasma formate levels were much higher in fetal lamb plasma and in amniotic fluid (191 ± 62 and 296 ± 154 μM, respectively) than in maternal plasma (33 ± 13 μM). Measurements of folate, vitamin B12, and homocysteine showed that these high formate levels could not be due to vitamin deficiencies. Elevated formate levels were also found in newborn lambs and persisted to about 8 wk of age. Formate was also found in sheep milk. Potential precursors of one-carbon groups were also measured in fetal and maternal plasma and in amniotic fluid. There were very high concentrations of serine in the fetus (∼1.6 mM in plasma and 3.5 mM in the amniotic fluid) compared with maternal plasma (0.19 mM), suggesting increased production of formate; however, we cannot rule out decreased formate utilization. Dimethylglycine, a choline metabolite, was also 30 times higher in the fetus than in the mother.