Waters Stephen B, Devesa Vicenta, Del Razo Luz Maria, Styblo Miroslav, Thomas David J
Curriculum in Toxicology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA.
Chem Res Toxicol. 2004 Mar;17(3):404-9. doi: 10.1021/tx0342161.
The postulated scheme for the metabolism of inorganic As involves alternating steps of oxidative methylation and of reduction of As from the pentavalent to the trivalent oxidation state, producing methylated compounds containing AsIII that are highly reactive and toxic. S-Adenosyl-L-methionine:AsIII methyltransferase purified from rat liver catalyzes production of methyl and dimethyl arsenicals from inorganic As. This protein is encoded by the cyt19 gene orthologous with cyt19 genes in mouse and human. The reductants dithiothreitol or tris(2-carboxylethyl)phosphine support catalysis by recombinant rat cyt19 (rrcyt19). Coupled systems containing an endogenous reductant (thioredoxin/thioredoxin reductase/NADPH, glutaredoxin/glutathione/glutathione reductase/NADPH, or lipoic acid/thioredoxin reductase/NADPH) support inorganic As methylation by rrcyt19. Although glutathione alone does not support rrcyt19's catalytic function, its addition to reaction mixtures containing other reductants increases the rate of As methylation. Aurothioglucose, an inhibitor of thioredoxin reductase, reduces the rate of As methylation by rrcyt19 in thioredoxin-supported reactions. Addition of guinea pig liver cytosol, a poor source of endogenous As methyltransferase activity, to reaction mixtures containing rrcyt19 shows that endogenous reductants in cytosol support the enzyme's activity. Methylated compounds containing either AsIII or AsV are detected in reaction mixtures containing rrcyt19, suggesting that cycling of As between oxidation states is a component of the pathway producing methylated arsenicals. This enzyme may use endogenous reductants to reduce pentavalent arsenicals to trivalency as a prerequisite for utilization as substrates for methylation reactions. Thus, cyt19 appears to possess both AsIII methyltransferase and AsV reductase activities.
无机砷代谢的假定机制涉及氧化甲基化以及砷从五价还原为三价氧化态的交替步骤，产生含三价砷的甲基化化合物，这些化合物具有高反应活性和毒性。从大鼠肝脏中纯化的S-腺苷-L-甲硫氨酸：三价砷甲基转移酶催化从无机砷生成甲基砷和二甲基砷。该蛋白质由与小鼠和人类的cyt19基因直系同源的cyt19基因编码。还原剂二硫苏糖醇或三（2-羧乙基）膦支持重组大鼠cyt19（rrcyt19）的催化作用。含有内源性还原剂（硫氧还蛋白/硫氧还蛋白还原酶/NADPH、谷氧还蛋白/谷胱甘肽/谷胱甘肽还原酶/NADPH或硫辛酸/硫氧还蛋白还原酶/NADPH）的偶联系统支持rrcyt19对无机砷的甲基化作用。虽然单独的谷胱甘肽不支持rrcyt19的催化功能，但将其添加到含有其他还原剂的反应混合物中可提高砷甲基化的速率。硫氧还蛋白还原酶抑制剂金硫葡萄糖可降低硫氧还蛋白支持的反应中rrcyt19对砷甲基化的速率。将豚鼠肝脏胞质溶胶（内源性砷甲基转移酶活性较差的来源）添加到含有rrcyt19的反应混合物中表明，胞质溶胶中的内源性还原剂支持该酶的活性。在含有rrcyt19的反应混合物中检测到含有三价砷或五价砷的甲基化化合物，这表明砷在氧化态之间的循环是产生甲基化砷的途径的一个组成部分。这种酶可能利用内源性还原剂将五价砷还原为三价，作为甲基化反应底物利用的前提条件。因此，cyt19似乎同时具有三价砷甲基转移酶和五价砷还原酶活性。
