Dimethylarsinic acid (DMAsV), the major urinary metabolite of inorganic arsenic, is weakly cytotoxic, whereas its reduced form, dimethylarsinous acid (DMAsIII), is highly toxic. Although glutathione S-transferase omega 1 (GSTO1) and arsenic methyltransferase have been shown or thought to catalyze DMAsV reduction, their role in DMAsV reduction in vivo, or in cell extracts is uncertain. Therefore, the reduction of DMAsV to DMAsIII in rats and in rat liver cytosol was studied to better understand its mechanism. To assess DMAsV reduction in rats, a novel procedure was devised based on following the accumulation of red blood cell (RBC)-bound dimethylarsenic (DMAs), which represents DMAsIII, in the blood of DMAsV-injected anesthetized rats. These studies indicated that rats reduced DMAsV to DMAsIII to a significant extent, as in 90 min 31% of the injected 50 μmol/kg DMAsV dose was converted to DMAsIII that was sequestered by the circulating erythrocytes. Pretreatment of rats with glutathione (GSH) depletors (phorone or BSO) delayed the elimination of DMAsV and the accumulation of RBC-bound DMAs, whereas the indirect methyltransferase inhibitor periodate-oxidized adenosine was without effect. Assessment of DMAsV-reducing activity of rat liver cytosol revealed that reduction of DMAsV required cytosolic protein and GSH and was inhibited by thiol reagents, GSSG and dehydroascorbate. Although thioredoxin reductase (TRR) inhibitors (aurothioglucose and SbIII) inhibited cytosolic DMAsV reduction, recombinant rat TRR plus NADPH, alone or when added to the cytosol, failed to support DMAsV reduction. On ultrafiltration of the cytosol through a 3 kDa filter, the reducing activity in the retentate was lost but was largely restored by NADPH. Such experiments also suggested that the reducing enzyme was larger than 100 kDa and was not GSTO1. In summary, reduction of DMAsV to the highly toxic DMAsIII in rats and rat liver cytosol is a GSH-dependent enzymatic process, yet its mechanism remains uncertain.
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