Arsenic (+3 oxidation state) methyltransferase (As3MT) catalyzes the methylation of trivalent arsenic (As(III)) to monomethylarsonate (MMA(V)) and dimethylarsinic acid (DMA(V)), and plays an important role in the detoxification of arsenicals. Here, we report the identification of two splicing variants of the human A53MT gene. One splicing variant was an exon-3 skipping (Delta 3) form which produced a premature stop codon, and the other was an exon-4 and -5 skipping (Delta 4,5) form which produced a 31.1 kDa As3MT protein. In addition to the full-length mRNA of As3MT, Delta.4,5 mRNAs were detected in HepG2, A549, HL60, K562, and HEK293 cells. The methyltransferase activity of the recombinant Delta 4,5 A53MT and wild-type As3MT proteins purified from Escherichia coli was determined. Speciation analysis by HPLC-ICP-MS showed a clear peak of MMA(V) after incubation of As(III) with the wild-type As3MT protein, but not with the Delta 4,5 A53MT protein. In addition, COS-7 cells transfected with Delta 4,5 A53MT cDNA did not convert As(III) to MMA(V) or DMA(V). The lack of methyltransferase activity of Delta 4,5 As3MT seems to be related to the deletion of an S-adenosylmethionine-binding site and a critical cysteine residue. These data suggest that the expression pattern of splicing variants of the As3MT gene may affect the capacity for arsenic methylation in cells. (C) 2011 Elsevier Inc. All rights reserved.