Methylation of arsenic by recombinant human wild-type arsenic (+ 3 oxidation state) methyltransferase and its methionine 287 threonine (M287T) polymorph: Role of glutathione

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• 作者：Lan Ding ; R. Jesse Saunders ; Zuzana Drobn¨¢ ; Felecia S. Walton ; Pencheng Xun ; David J. Thomas ; Miroslav Styblo
• 关键词：AAS ; atomic absorption spectrometry ; AdoMet ; S-adenosylmethionine ; ANOVA ; analysis of variance ; As ; arsenic ; AsIII ; trivalent As ; AsV ; pentavalent As ; AS3MT or As3mt ; arsenic (+  ; 3 oxidation state) methyltransferase ; iAs ; inorganic arsenic ; iAsIII ; ar
• 刊名：Toxicology and Applied Pharmacology
• 出版年：2012
• 出版时间：1 October, 2012
• 年：2012
• 卷：264
• 期：1
• 页码：121-130
• 全文大小：1171 K

文摘

Arsenic (+ 3 oxidation state) methyltransferase (AS3MT) is the key enzyme in the pathway for methylation of arsenicals. A common polymorphism in the AS3MT gene that replaces a threonyl residue in position 287 with a methionyl residue (AS3MT/M287T) occurs at a frequency of about 10 % among populations worldwide. Here, we compared catalytic properties of recombinant human wild-type (wt) AS3MT and AS3MT/M287T in reaction mixtures containing S-adenosylmethionine, arsenite (iAsp>IIIp>) or methylarsonous acid (MAsp>IIIp>) as substrates and endogenous or synthetic reductants, including glutathione (GSH), a thioredoxin reductase (TR)/thioredoxin (Trx)/NADPH reducing system, or tris (2-carboxyethyl) phosphine hydrochloride (TCEP). With either TR/Trx/NADPH or TCEP, wtAS3MT or AS3MT/M287T catalyzed conversion of iAsp>IIIp> to MAsp>IIIp>, methylarsonic acid (MAsp>Vp>), dimethylarsinous acid (DMAsp>IIIp>), and dimethylarsinic acid (DMAsp>Vp>); MAsp>IIIp> was converted to DMAsp>IIIp> and DMAsp>Vp>. Although neither enzyme required GSH to support methylation of iAsp>IIIp> or MAsp>IIIp>, addition of 1 mM GSH decreased K_m and increased V_max estimates for either substrate in reaction mixtures containing TR/Trx/NADPH. Without GSH, V_max and K_m values were significantly lower for AS3MT/M287T than for wtAS3MT. In the presence of 1 mM GSH, significantly more DMAsp>IIIp> was produced from iAsp>IIIp> in reactions catalyzed by the M287T variant than in wtAS3MT-catalyzed reactions. Thus, 1 mM GSH modulates AS3MT activity, increasing both methylation rates and yield of DMAsp>IIIp>. AS3MT genotype exemplified by differences in regulation of wtAS3MT and AS3MT/M287T-catalyzed reactions by GSH may contribute to differences in the phenotype for arsenic methylation and, ultimately, to differences in the disease susceptibility in individuals chronically exposed to inorganic arsenic.