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

Abstract

A unique enzyme, MMA(V) 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 K(m) and V(max) values were 2.16 x 10(-)(3) M and 10.3 micromol h(-)(1) (mg of protein)(-)(1). When DMA(V) or arsenate was tested as a substrate, the K(m) was 20.9 x 10(-)(3) or 109 x 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 K(m) values of rabbit liver arsenite methyltransferase (5.5 x 10(-)(6) M) and MMA(III) methyltransferase (9.2 x 10(-)(6)) were compared to that of MMA(V) reductase (2.16 x 10(-)(3) M), it can be concluded that MMA(V) reductase was the rate-limiting enzyme of inorganic arsenite biotransformation. MMA(V) reductase was also present in surgically removed human liver.

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

“…AdoMet served as the methyl group donor for both reactions. These results are consistent with evidence that trivalent arsenicals are the preferred substrate for this enzyme and that the pentavalent arsenicals must be reduced enzymatically or nonenzymatically before methylation (7,11,41). Based on estimated molecular weights, the protein purified from rat liver cytosol (ϳ42.000) is probably not identical with the enzyme purified from rabbit liver cytosol (ϳ60,000) that catalyzes the methylation of arsenite and methylarsonous acid (10,11).…”

“…Omission of DTT from reaction mixtures that contain rat liver S-adenosyl-L-methionine:arsenic(III) methyltransferase sharply reduces the conversion of arsenite to methylated products. 2 In cells, this reduction may depend on pentavalent arsenic reductase (7,8) (20). AdoMet served as the methyl group donor for both reactions.…”

“…Vahter (55) has suggested that genotypic differences in the activities of the enzyme that methylates arsenic or in enzymes involved in the recycling of homocysteine used in the synthesis of AdoMet could be the sources of variability among individuals in the urinary output of methylated arsenicals. To this list could be added genotypic differences in the activity of the reductases that convert arsenate and methylarsonic acid to arsenite and methylarsonous acid (7,8). Additional studies of polymorphisms of S-adenosyl-L-methionine:arsenic(III) methyltransferase will be required to determine the contribution of this factor to differences among individuals in metabolism, retention, and clearance of arsenic.…”

“…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 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)(8)(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.…”

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

“…In rats and hamsters, DMAs is further methylated to yield trimethylarsine oxide (TMAs V O) (Yamauchi and Yamamura, 1985;Yoshida et al, 1998). Two classes of enzymes are involved in the methylation of iAs, including As V -reductases (Gregus and Nemeti, 2002;Radabaugh et al, 2002;Zakharyan and Aposhian, 1999;Zakharyan et al, 2001) and As III -methyltransferases Zakharyan et al, 1995). The metabolic conversion of iAs is generally recognized to be a determinant of the toxic and carcinogenic effects of this metalloid.…”