Folinic acid

Folinic acid is 5-formyl-THF (Wikipedia), or 5fTHF in short.

It’s converted to 5,10-methenyl-THF by the enzyme MTHFS (see Reactome). This reaction consumes ATP.

5,10-methenyl-THF is next converted to [5,10-methylene-THF] by the enzyme MTHFD1 and this reaction requires NADPH (see Reactome).

5-formyl-THF is considered to be an intracellular storage form of folate. (R2, R3)

Toxicity and disorders

It seems it’s possible to get to the toxic level of 5-formyl-THF: a genetic disorder - MTHFS deficiency - is presented with significantly elevated level of 5-formyl-THF and neurometabolic disorder with observed microcephaly, short stature, severe global developmental delay, progressive spasticity, epilepsy, and cerebral hypomyelination.

The case report concludes:

The direct substrate of the MTHFS reaction, 5-formyl-THF, was elevated 30-fold in patient fibroblasts compared to control, supporting the hypothesis that the pathophysiology of this disorder is a manifestation of toxicity from this metabolite. (R1)

Another case report about MTHFS deficiency explores the metabolic consequences of accumulated 5-formyl-THF:

Furthermore, it has been shown in silico that decreased MTHFS activity will lead to increased 5-formyl THF, an intracellular storage form of folate, which will go on to inhibit SHMT and AICARFT. These are both enzymes in the folic acid metabolism pathway. With decreased SHMT activity, less of the intermediate needed for thymidylate synthesis will be made. Similarly, with decreased AICARFT activity, less de novo purine synthesis will occur. Therefore, both of these effects will greatly impair DNA synthesis, affecting processes such as hematopoiesis. (R2)

Intracellular storage form of folate

“In folate-mediated one-carbon metabolism (FOCM), 5-formyltetrahydrofolate (5fTHF), a one-carbon substituted tetrahydrofolate (THF) vitamer, acts as an intracellular storage form of folate” (R3)

5fTHF is not used as a cofactor for folate-dependent biosynthetic reactions, rather it is thought to be an intracellular storage form of folate in dormant cells. (R3)

But the level of 5fTHF is regulated by the enzyme MTHFS:

“Model simulations indicate that MTHFS plays an essential role in preventing 5fTHF accumulation, which consequently averts inhibition of all other reactions in the metabolic network” (R3)

Futile cycle

5fTHF is not used as a cofactor for folate-dependent biosynthetic reactions, rather it is thought to be an intracellular storage form of folate in dormant cells.

5fTHF is regulated through a futile cycle catalyzed by serine hydroxymethyltransferase (SHMT) and methenyltetrahydrofolate synthetase (MTHFS).

5fTHF is generated from CHF in an irreversible reaction catalyzed by SHMT. 5fTHF is re-introduced to the folate cofactor pool by MTHFS, which converts 5fTHF to CHF in an irreversible, ATP-dependent reaction.

CHF is not used directly as a co-factor for biosynthetic reactions. It is formed both enzymatically from MTHFS and the cyclohydrolase activity of MTHFD1 and non-enzymatically from both 5fTHF and 10fTHF. (R3)

Note: CHF is 5,10-methenyl-THF.

Open questions

1. What are the consequences of low 5-formyl-THF?