Does arsenic detoxification increase one-carbon methylation demand?

Exposure to inorganic arsenic and other heavy metals heavily taxes the body's primary metabolic pathways, specifically targeting cellular methylation capacity.

Biochemical mechanisms of arsenic clearance

• Arsenic detoxification relies on the enzyme arsenite methyltransferase (AS3MT), which catalyzes the sequential methylation of trivalent inorganic arsenic to monomethylated and dimethylated species.
• This enzymatic reaction requires an $S_N2$ attack on the activated methyl group of S-adenosylmethionine (SAM), directly consuming SAM and generating S-adenosylhomocysteine (SAH) as a byproduct.
• Crucially, accumulated SAH acts as a potent product inhibitor of AS3MT, feeding back to suppress further arsenic methylation and stalling the clearance process.

Systemic impact on one-carbon metabolism

• The direct, iterative consumption of SAM during detoxification rapidly depletes cellular methyl-donor pools and lowers the critical SAM/SAH ratio (the cellular methylation index).
• This metabolic strain is compounded by other toxic metals (such as lead, mercury, and cadmium) that generate reactive oxygen species. These species oxidize the cobalamin (vitamin B12) cofactor of methionine synthase, halting the remethylation of homocysteine to methionine.
• Furthermore, metals like lead and copper inhibit cystathionine beta-synthase, disrupting transsulfuration and restricting the de novo synthesis and recycling of SAM needed for essential DNA and histone methylation.

Bottom line

• Arsenic clearance directly consumes SAM and produces SAH—which feedback-inhibits further detoxification—while co-exposure to other heavy metals impairs critical folate- and B12-dependent enzymes, severely escalating overall one-carbon methylation demand.