Does inorganic arsenic exposure increase demand on one-carbon methylation capacity?

Inorganic arsenic exposure presents a distinct metabolic challenge, taxing the body's primary biochemical detoxification pathways and depleting essential methyl donor pools.

Mechanistic pathway of detoxification

• Enzymatic biotransformation: The methyltransferase enzyme AS3MT sequentially metabolizes toxic trivalent inorganic arsenic (arsenite) to facilitate its clearance.
• Methyl group consumption: This pathway relies on S-adenosylmethionine (SAM) as the primary methyl-group donor. AS3MT binds SAM to convert inorganic arsenic into monomethylarsonous acid (MAs(III)) and subsequently into dimethylarsinous acid (DMAs(III)), converting SAM into S-adenosylhomocysteine (SAH) at each step.

Metabolic and epigenetic implications

• Methyl sink activation: Chronic arsenic detoxification acts as an artificial metabolic sink for methyl groups. This process depletes cellular SAM reserves and elevates SAH, significantly reducing the SAM/SAH ratio which serves as the primary indicator of cellular methylation potential.
• Epigenetic instability: The reduction in the SAM/SAH ratio inhibits DNA methyltransferases, directly causing global and locus-specific DNA hypomethylation.
• Nutritional modulation: One-carbon capacity is highly dependent on the methionine-folate cycle. Precursors such as folate, choline, and betaine regulate the remethylation of homocysteine to replenish SAM. Dietary deficiency in these nutrients impairs arsenic detoxification and exacerbates systemic epigenetic strain.

Bottom line

• Inorganic arsenic biotransformation directly depletes S-adenosylmethionine (SAM) via AS3MT-mediated methylation. This detoxification process acts as a metabolic methyl sink that increases demand on one-carbon capacity, leading to DNA hypomethylation and epigenetic vulnerability when nutritional methyl donors like folate and choline are insufficient.