Enzymatic Reduction of Arsenic Compounds in Mammalian Systems: The Rate-Limiting Enzyme of Rabbit Liver Arsenic Biotransformation Is MMAV Reductase

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Abstract
A unique enzyme, MMAV reductase, has been partially purified from rabbit liver by using DEAE-cellulose, carboxymethylcellulose, and red dye ligand chromatography. The enzyme is unique since it is the rate-limiting enzyme in the biotransformation of inorganic arsenite in rabbit liver. The Km and Vmax values were 2.16 × 10-3 M and 10.3 μmol h-1 (mg of protein)-1. When DMAV or arsenate was tested as a substrate, the Km was 20.9 × 10-3 or 109 × 10-3 M, respectively. The enzyme has an absolute requirement for GSH. Other thiols such as DTT or l-cysteine were inactive alone. At a pH below the physiological pH, GSH carried out this reduction, but this GSH reduction in the absence of the enzyme had little if any value at pH 7.4. When the Km values of rabbit liver arsenite methyltransferase (5.5 × 10-6 M) and MMAIII methyltransferase (9.2 × 10-6) were compared to that of MMAV reductase (2.16 × 10-3 M), it can be concluded that MMAV reductase was the rate-limiting enzyme of inorganic arsenite biotransformation. MMAV reductase was also present in surgically removed human liver.