Human monomethylarsonic acid (MMA^v) reductase is a member of the glutathione-S-transferase superfamily

The drinking of water containing large amounts of inorganic arsenic is a worldwide major public health problem because of arsenic carcinogenicity. Yet an understanding of the specific mechanism(s) of inorganic arsenic toxicity has been elusive. We have now partially purified the rate-limiting enzyme of inorganic arsenic metabolism, human liver MMA^v reductase, using ion exchange, molecular exclusion, and hydroxyapatite chromatography. When SDS-βmercaptoethanol-PAGE was performed on the most purified fraction, seven protein bands were obtained. Each band was excised from the gel, sequenced by LC-MS/MS and identified according to the SWISS-PROT and TrEMBL Protein Sequence databases. Human liver MMA^v reductase is 100% identical, over 92% of sequence that we analyzed, with the recently discovered human glutathione-S-transferase Omega class hGSTO 1-1. Recombinant human GSTO1-1 had MMA^v reductase activity with K_m and V_max values comparable to those of human liver MMA^v reductase. The partially purified human liver MMA^v reductase had glutathione S-transferase (GST) activity. MMA^v reductase activity was competitively inhibited by the GST substrate, 1-chloro 2,4-dinitrobenzene and also by the GST inhibitor, deoxycholate. Western blot analysis of the most purified human liver MMA^v reductase showed one band when probed with hGSTO1-1 antiserum. We propose that MMA^v reductase and hGSTO 1-1 are identical proteins.