Role of nitric oxide synthase in mercury toxicity

在Wistar大鼠连续暴露于神经毒性金属甲基汞的过程中，研究了大脑和小脑中一氧化氮合酶（NOS）mRNA水平、蛋白质含量和酶活性的变化。皮下注射氯化甲基汞（MMC，10 mg/kg/d，8天），可使大脑和小脑的NOS活性随时间的延长而显著升高，分别为1.6-1.9倍和1.4倍。RT-PCR和免疫印迹分析表明，这种金属引起的酶活性的增加似乎是由于神经元型一氧化氮合酶（nNOS）的蛋白质水平的增加，但不是诱导型一氧化氮合酶（iNOS），因为在MMC暴露过程中很少看到可观的mRNA和蛋白质的诱导型一氧化氮合酶。使用20，000 g大鼠小脑匀浆上清液评价汞化合物对体外nNOS活性的直接影响。然而，与体内观察相反，无机汞、烷基汞和芳基汞化合物显示出对nNOS活性的有效抑制，IC 50值为11-43 μ M，而二甲基汞（DMM）对酶活性没有影响。进一步的实验表明，有机汞对nNOS的抑制作用是通过巯基修饰实现的。

Alterations in mRNA levels, protein content and enzyme activity for nitric oxide synthase (NOS) in the cerebrum and cerebellum during a continuous exposure of neurotoxic metal, methylmercury, were examined in Wistar rats. Subcutaneous (sc) administration of methylmercuric chloride (MMC, 10 mg/kg/day, 8 days) resulted in significant increases with time of NOS activities in the cerebrum (1.6-1.9-fold, 5-8 days) and cerebellum (1.4-fold, 8 days). RT-PCR and immuno-blot analyses indicated that the increase in the enzyme activity caused by this metal appears to be due to increase in protein levels of neuronal NOS (nNOS), but not inducible NOS (iNOS) because little appreciable mRNA and protein for iNOS were seen during MMC exposure. The direct effect of mercuric compounds on nNOS activity in vitro was evaluated using 20,000 g supernatant from rat cerebellum homogenate. In contrast to the in vivo observation, however, inorganic-, alkyl-, and aryl-mercuric compound showed potent inhibition of nNOS activity with IC50 values of 11-43 muM whereas dimethylmercury (DMM) was without effect on the enzyme activity. Further experiments indicated that the inhibition of nNOS by organomercurial occurred via thiol modification.