A Novel S-Adenosyl-l-methionine:Arsenic(III) Methyltransferase from Rat Liver Cytosol

Abstract

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...

Metabolism and toxicity of arsenic in human urothelial cells expressing rat arsenic (+3 oxidation state)-methyltransferase

“…We have also shown that UROtsa cells do not express AS3MT . Importantly, comparison of our previous work (Drobná et al, 2003;Lin et al, 2002;) with results of the present study suggests that serum starvation for 16 h has no effect on AS3MT expression or on the metabolic pattern of iAs in UROtsa cells.…”

Arsenic in the human food chain, biotransformation and toxicology – Review focusing on seafood arsenic

“…An arsenic methyl transferase (As3MT) existing in a number of polymorphic forms has been identified [66][67][68]. This enzyme is crucial as a reducing agent in the methylation of iAs, and thus is at least one of the enzymes involved in this process [66,69,70].…”

The Role of Biomethylation in Toxicity and Carcinogenicity of Arsenic: A Research Update

“…As V reductases may enzymatically reduce these arsenicals in mammalian tissues (4,5). Methylation of trivalent arsenicals [arsenite (iAs III ) and methylarsonous acid (MAs III )] is catalyzed by As III -methyltransferases that use S-adenosylmethionine (AdoMet) as the methyl group donor (6,7). Because MAs V and DMAs V are not toxic in acute lethality assays, methylation of iAs has long been considered a detoxification mechanism.…”