S-Adenosyl-L-methionine (AdoMet):arsenic(III) methyltransferase, purified from liver cytosol of adult male Fischer 344 rats, catalyzes transfer of a methyl group from AdoMet to trivalent arsenicals producing methylated and dimethylated arsenicals. The kinetics of production of methylated arsenicals in reaction mixtures containing enzyme, AdoMet, dithiothreitol, glutathione (GSH), and arsenite are consistent with a scheme in which monomethylated arsenical produced from arsenite is the substrate for a second methylation reaction that yields dimethylated arsenical. The mRNA for this protein predicts a 369-amino acid residue protein (molecular mass 41056) that contains common methyltransferase sequence motifs. Its sequence is similar to Cyt19, a putative methyltransferase, expressed in human and mouse tissues. Reverse transcription-polymerase chain reaction detects S-adenosyl-L-methionine:arsenic(III) methyltransferase mRNA in rat tissues and in HepG2 cells, a human cell line that methylates arsenite and methylarsonous acid. S-Adenosyl-L-methionine:arsenic-(III) methyltransferase mRNA is not detected in UROtsa cells, an immortalized human urothelial cell line that does not methylate arsenite. Because methylation of arsenic is a critical feature of its metabolism, characterization of this enzyme will improve our understanding of this metalloid's metabolism and its actions as a toxin and a carcinogen.In many species, including humans, exposure to inorganic arsenic results in urinary excretion of methylated and dimethylated arsenicals (1-3). Cullen and co-workers (4) summarized the conversion of inorganic arsenic into these methylated products in a reaction scheme which incorporates oxidative methylation and the cycling of arsenic between the pentavalent (As V ) 1 and trivalent (As III ) oxidation states,Because reduction of arsenic to trivalency is a prerequisite for its oxidative methylation, pentavalent arsenicals are reduced by endogenous thiols such as glutathione (GSH) (5, 6) or by As V reductases (7-9). A protein has been purified from rabbit liver cytosol that catalyzes the methylation of both arsenite and methylarsonous acid (10, 11); however, this protein has not been sequenced. These activities are designated arsenite methyltransferase (EC 2.1.1.137) and methylarsonite methyltransferase (EC 2.1.1.138), respectively. This protein (estimated molecular mass 60 kDa) uses S-adenosyl-L-methionine (AdoMet) as the methyl group donor. The methylation of arsenite by this protein is stimulated by a monothiol (GSH) and the methylation of methylarsonous acid is highly stimulated by a dithiol, dithiothreitol (DTT). The methylation of arsenic has been commonly regarded as a mechanism for its detoxification (12). However, recent research has shown that methylated arsenicals that contain As III are important intermediates in the metabolism of inorganic arsenic. Methylated arsenicals that contain As III are found in the urine of individuals who chronically consume drinking water that contains inorganic arsenic and in cells cu...

The arsenic (+3 oxidation state) methyltransferase (As3mt) gene encodes a 43 kDa protein that catalyzes methylation of inorganic arsenic. Altered expression of AS3MT in cultured human cells controls arsenic methylation phenotypes, suggesting a critical role in arsenic metabolism. Because methylated arsenicals mediate some toxic or carcinogenic effects linked to inorganic arsenic exposure, studies of the fate and effects of arsenicals in mice which cannot methylate arsenic could be instructive. This study compared retention and distribution of arsenic in As3mt knockout mice and in wild-type C57BL/6 mice in which expression of the As3mt gene is normal. Male and female mice of either genotype received an oral dose of 0.5 mg of arsenic as arsenate per kg containing [73As]-arsenate. Mice were radioassayed for up to 96 hours after dosing; tissues were collected at 2 and 24 hours after dosing. At 2 and 24 hours after dosing, livers of As3mt knockouts contained a greater proportion of inorganic and monomethylated arsenic than did livers of C57BL/6 mice. A similar predominance of inorganic and monomethylated arsenic was found in the urine of As3mt knockouts. At 24 hours after dosing, As3mt knockouts retained significantly higher percentages of arsenic dose in liver, kidneys, urinary bladder, lungs, heart, and carcass than did C57BL/6 mice. Whole body clearance of [73As] in As3mt knockouts was substantially slower than in C57BL/6 mice. At 24 hours after dosing, As3mt knockouts retained about 50% and C57BL/6 mice about 6% of the dose. After 96 hours, As3mt knockouts retained about 20% and C57BL/6 mice retained less than 2% of the dose. These data confirm a central role for As3mt in metabolism of inorganic arsenic and indicate that phenotypes for arsenic retention and distribution are markedly affected by the null genotype for arsenic methylation, indicating a close linkage between the metabolism and retention of arsenicals.

The conventional scheme for arsenic methylation accounts for methylated oxyarsenical production but not for thioarsenical formation. Here, we report that in vitro anaerobic microbiota of mouse cecum converts arsenate into oxy- and thio- arsenicals. Besides methylarsonic acid (MMA(V)), arsenate was transformed into six unique metabolites: mono-, di-, and trithio-arsenic acid, monomethyldithio- and monomethyltrithio-arsonic acid, and dimethyldithioarsonic acid. Thioarsenicals were found in soluble and particulate fractions of reaction mixtures, suggesting interactions with anaerobic microbiota. Metabolism of ingested arsenate to oxy- and thio-arsenicals before absorption across the gastrointestinal barrier could affect bioavailability, systemic distribution, and resulting toxicity.