Role of nitric oxide synthase in mercury toxicity

研究了连续接触神经毒性金属甲基汞对Wistar大鼠大脑和小脑一氧化氮合酶（NOS） mRNA水平、蛋白质含量和酶活性的影响。皮下给药氯甲基汞（MMC， 10 mg/kg/天，8天）导致大脑（1.6-1.9倍，5-8天）和小脑（1.4倍，8天）NOS活性随时间显著增加。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NOS （nNOS）蛋白水平的增加，而不是由于诱导NOS （iNOS）蛋白水平的增加，因为在MMC暴露期间几乎没有观察到明显的iNOS mRNA和蛋白。采用大鼠小脑匀浆上清液20000 g，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对nNOS活性表现出有效的抑制作用，IC50值为11-43 muM，而二甲汞（DMM）对酶活性没有影响。进一步的实验表明，有机聚合物对nNOS的抑制作用是通过巯基改性实现的。

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.