Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2022 Aug 9:13:946713.
doi: 10.3389/fimmu.2022.946713. eCollection 2022.

MR1-dependence of unmetabolized folic acid side-effects

Jeffry S Tang  1   2 , Alissa Cait  1 , Reuben M White  3 , Homayon J Arabshahi  3 , David O'Sullivan  1   2 , Olivier Gasser  1   2
Affiliations

MR1-dependence of unmetabolized folic acid side-effects

Jeffry S Tang et al. Front Immunol. .

Abstract

The fortification of flour with folic acid for the prevention of neural tube defects (NTD) is currently mandated in over eighty countries worldwide, hence compelling its consumption by the greater part of the world's population. Notwithstanding its beneficial impact on rates of NTD, pervasive folic acid supplementation has invariably led to additive daily intakes reaching well beyond their original target, resulting in the circulation of unmetabolized folic acid. Associated idiopathic side-effects ranging from allergies to cancer have been suggested, albeit inconclusively. Herein, we hypothesize that their inconsistent detection and elusive etiology are linked to the in vivo generation of the immunosuppressive folic acid metabolite 6-formylpterin, which interferes with the still emerging and varied functions of Major Histocompatibility Complex-related molecule 1 (MR1)-restricted T cells. Accordingly, we predict that fortification-related adverse health outcomes can be eliminated by substituting folic acid with the bioequivalent folate vitamer 5-methyltetrahydrofolate, which does not break down into 6-formylpterin.

Keywords: MAIT cell; MR1; folate; folic acid (B9); unmetabolized folic acid.

PubMed Disclaimer

Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

Figure 1
Figure 1
Oxidative degradation of folate and its relevance for MR1 immunobiology. MR1–restricted T cells have varied biological functions (A), which are blocked by 6–formylpterin (6–FP) (B), image created with BioRender. The folate vitamers folic acid (FA) and 5–methyltetrahydrofolate (5–MTHF) differ in their pterin ring and oxidation states, highlighted in blue (C). FA undergoes direct photodegradation to 6–FP and p–aminobenzoyl glutamic acid (PABA–Glu) upon UVA exposure (D, upper panel). In contrast, the photodegradation of 5–MTHF is indirect and likely mediated by reactive oxygen species (ROS). The pterin photo–metabolite of 5–MTHF has not yet been formally characterized but could hypothetically be 5–N–Methyl–6–formylpterin (5m–6–FP) (D, lower panel). Binding mode of 6–FP to MR1 (PDB ID: 4L4T) was visualized using the Maestro software suite (E). The protein was prepared using the Schrodinger protein preparation wizard and docked using CovDock. Key MR1 amino acid residues interacting with 6–FP (gray), either covalently (LYS43) or non–covalently (ARG9, ARG94, TYR7 and TYR62), are marked in yellow and non–covalent interactions represented by dashed lines. H–bond interactions between various contact points of 6–FP with ARG9, ARG94, or TYR62 (via a water molecule) are represented by dashed yellow lines. The π–stacking interaction between the pterin ring of 6–FP with the aromatic ring of TYR7 is represented by a dashed blue line (E).
Figure 2
Figure 2
Inhibition of MAIT cell activation by folic acid, but not 5–methylfolate, photometabolites. (A–C) Docked conformation of 6–FP (A), (R)–5m–6–FP (B) and (S)–5m–6–FP (C) to the MR1 binding site. Cdock affinity scores are highlighted in bold. The carbonyl of 6–FP interacts with the positively charged residues ARG9 via hydrogen bonding and ARG94 via water mediated hydrogen bonding. A π–stacking interaction with TYR7 is also observed. The (R) and (S) 5–m–6–FP enantiomers show similar positioning within the pocket but have no π–stacking interaction with TYR7. Ligand carbons are marked in grey while protein carbons are marked in orange. Hydrogen bonding and hydrophobic interactions are depicted as yellow and blue dashed lines, respectively. (D–F) Folic acid (FA) and 5–methyltetrahydrofolate (5–MTHF) solutions (80 μM in PBS) were exposed to UVA radiation in the presence or absence of equimolar amounts (80 μM) of the natural photosensitizer riboflavin. The extent of their respective photodegradation was determined by UV–Vis spectrophotometry (D). The release of inhibitory MR1 ligands was assessed by the staining intensity (as mean fluorescence intensity, MFI) of cell surface MR1 on antigen presenting (C1R.MR1) cells (E) and the associated inhibition of 5–OP–RU–mediated activation of a MAIT cell line (Jurkat.MAIT), as determined by the expression of the activation marker CD69 (F). 6–FP (5 μM and 10μM) was used as a control. Each symbol represents the mean value of an independent experiment (2–4), each run in triplicates. Statistics were calculated using one–way ANOVA with Tukey post–test, * p<0.05, ** p<0.01, *** p<0.001, **** p<0.0001, ns = non–significant.
See this image and copyright information in PMC

Similar articles

See all similar articles

Cited by

References

    1. Ducker GS, Rabinowitz JD. One–carbon metabolism in health and disease. Cell Metab (2017) 25(1):27–42. doi: 10.1016/j.cmet.2016.08.009 - DOI - PMC - PubMed
    1. MRC Vitamin Study Research Group . Prevention of neural tube defects: Results of the medical research council vitamin study. mrc vitamin study research group. Lancet (1991) 338(8760):131–7. doi: 10.1016/0140-6736(91)90133-A - DOI - PubMed
    1. Scaglione F, Panzavolta G. Folate, folic acid and 5–methyltetrahydrofolate are not the same thing. Xenobiotica (2014) 44(5):480–8. doi: 10.3109/00498254.2013.845705 - DOI - PubMed
    1. Obeid R, Kasoha M, Kirsch SH, Munz W, Herrmann W. Concentrations of unmetabolized folic acid and primary folate forms in pregnant women at delivery and in umbilical cord blood. Am J Clin Nutr (2010) 92(6):1416–22. doi: 10.3945/ajcn.2010.29361 - DOI - PubMed
    1. Sweeney MR, McPartlin J, Scott J. Folic acid fortification and public health: Report on threshold doses above which unmetabolised folic acid appear in serum. BMC Public Health (2007) 7(41):41. doi: 10.1186/1471-2458-7-41 - DOI - PMC - PubMed

Publication types

MeSH terms

Substances