Human exposure to As and its compounds occurs primarily through ingestion of As-contaminated food and water. About 200 million people worldwide are exposed to As levels exceeding the international limit of 10 µg/L in water. Among children, exposure to As is associated with deficits in intelligence, neurobehavioral function, and development.
In the human body, As is metabolized when arsenite (AsIII) is methylated to form monomethylarsonic acid (MMAV), which is then reduced to form monomethylarsonous acid (MMAIII). Another methylation cycle converts MMAIII to dimethylarsinic acid (DMAV). These methylation reactions are catalyzed by As methyltransferase (AS3MT), wherein S-adenosylmethionine (SAM) acts as a methyl donor. Biosynthesis of SAM depends on folate for donating the methyl group.