دانلود رایگان مقاله فعالیت آرژنین دهیدروژناز و هدف بیولوژیکی ترکیبات جیوه

Abstract

Mercury compounds are well-known toxic environmental pollutants and potently induce severe neurotoxicological effects in human and experimental animals. Previous studies showed that one of the mechanisms of mercury compounds neurotoxicity arose from the over-activation of the N-methyl d-aspartate (NMDA)-type glutamate receptor induced by increased glutamate release. In this work, we aimed to investigate the molecular mechanisms of Hg compounds neurotoxicities by identifying their biological targets in cells. Firstly, the inhibitory effects of four Hg compounds, including three organic (methyl-, ethyl- and phenyl-mercury) and one inorganic (Hg2+) Hg compounds, on the activity of arginine decarboxylase (ADC), a key enzyme in the central agmatinergic system, were evaluated. They were found to inhibit the ADC activity significantly with methylmercury (MeHg) being the strongest (IC50 = 7.96 nM). Furthermore, they showed remarkable inhibitory effects on ADC activity in PC12 cells (MeHg > EtHg > PhHg > HgCl2), and led to a marked loss in the level of agmatine, an endogenous neuromodulatory and neuroprotective agent that selectively blocks the activation of NMDA receptors. MeHg was detected in the immunoprecipitated ADC from the cells, providing unequivocal evidence for the direct binding of MeHg with ADC in the cell. Molecular dynamics simulation revealed that Hg compounds could form the coordination bond not only with cofactor PLP of ADC, but also with substrate arginine. Our finding indicated that MeHg could attenuate the neuroprotective effects of agmatine by the inhibition of ADC, a new cellular target of MeHg, which might be implicated in molecular mechanism of MeHg neurotoxicity.

4. Discussion

The most notable toxicity of Hg compounds on human and experimental animals is neurotoxicity. ADC is responsible for the biosynthesis of agmatine and ADC’s biological importance is closely linked to the function of the agmatine. Agmatine is a neuroprotective molecule and reduces excitotoxic cell death induced by glutamate or NMDA. In the brain, agmatine is synthesized from arginine by ADC and degraded by agmatinase. Thus, increasing endogenous agmatine either by increasing ADC activity or by blocking agmatinase activity has become a novel mechanism to protect against conditions arising from higher NMDA receptor activation. In our work, a set of experiments were carried out in an attempt to establish ADC as a potential target of Hg compounds in neuron cells and to provide a novel molecular mechanism for neurotoxicity action of Hg compounds. In the in vitro ADC enzyme activity assay, all the four Hg compounds inhibited ADC activity. Their inhibition potencies were strongly dependent on the Hg species, with MeHg being the strongest and HgCl2 the weakest. Molecular dynamics simulation provided rationales for the differences in their structure-dependent inhibition potency. For the organic Hg compounds, the calculated binding energies followed the order: MeHg > EtHg > PhHg (Table 1). This trend correlates very well with the experimentally measured inhibition potency (IC50) of Hg compounds, and therefore supports the validity of the molecular dynamics simulation approach.