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  • In addition to carbamates organophosphates are classic inhib

    2020-10-16

    In addition to carbamates, organophosphates are classic inhibitors of cholinesterases. Previous studies with aquatic organisms have demonstrated their sensitivity to such chemicals. For example, individuals of the freshwater fish Colossoma macropomum exposed to the organophosphate dichlorvos, chlorpyrifos and tetraethilpyrophosphate had their ChEs activity inhibited (Assis et al., 2010). A similar result was observed in the Cy5 EGFP and muscle of the marine fish species Prionace glauca exposed to chlorpyrifos-oxon (Alves et al., 2015). In vivo exposure of individuals of C. carpio to malathion and triazophos resulted in a decrease in brain AChE activity (Wang et al., 2015). Individuals of the crustacean species Cherax destructor showed a similar response, since their AChE and BChE activities were Cy5 EGFP significantly inhibited after the exposure to organophosphates (chlorpyrifos and malation) insecticides (Pham et al., 2017). Rodrigues et al. (2011) observed the same pattern of cholinesterasic inhibition after exposing the fish Lepomis gibbosus to the same compound. Not only fish seem to respond according to these patterns. The bivalve Corbicula fluminea, after being exposed to the organophosphate chlorfenvinphos, exhibited an inhibition of cholinesterase activity (Ramos et al., 2012). Gagnaire et al. (2008) showed that the snail Potamopyrgus antipodarum was sensitive to the organophosphate pesticide chlorpyrifos. Despite being recognized as an irreversible cholinesterase inhibitor (namely of acetylcholinesterase), dimethoate did not show this effect in our study. The absence of effects may be attributed to the fact that dimethoate needs to be metabolized in the organism prior to the exertion of its inhibitory effect, being the metabolite dimethoxon the biologically active molecule that exerts the inhibitory effect (Pereira et al., 2013). A similar conclusion was drawn from the study carried out by Silva et al. (2016) in which the freshwater mollusk Corbicula fluminea was in vitro exposed to the same pesticide. From the results of this study, an almost negligible inhibitory effect was reported, a fact that was related to the absence of bioactivation of dimethoate to its active metabolite dimethoxon. Metals are essential and natural compounds, but in high concentrations, they can impair cholinesterase activity. Guilhermino et al. (1998) suggested that metals can react with the active site of cholinesterases, causing an impairment in the function of the enzyme. The here obtained results showed that copper could cause an in vitro inhibition of muscle acetylcholinesterase for all organisms tested, while lead did not cause this effect. Although lead caused in vivo and in vitro cholinesterasic inhibition in D. rerio (De Lima et al., 2012), the same effect was not observed in Anguilla anguilla cholinesterases, as described by Nunes et al. (2014). In the already mentioned study by De Lima et al. (2012), copper had no in vitro inhibitory effect on cholinesterase activity; on the contrary, this metal was capable of inducing a stimulatory effect on the enzyme activity. However, copper is well known due to its in vivo inhibitory effects of cholinesterases. De Lima et al. (2012) reported an inhibition of cholinesterase activity after 7 days of in vivo exposure to copper in D. rerio. The authors suggest that in vivo inhibition may occur through physiologically mechanisms that can indirectly inhibit cholinesterase activity. During the first phases of the intoxication process, metals can directly bind to acetylcholine receptors. This immediate effect may cause an increase of AChE activity due to the accumulation of acetylcholine in the synaptic cleft caused by the obstruction/blockade of acetylcholine receptors by metals (de Lima et al., 2012). However, with prolonged exposure to metals, the enzymatic activity may decrease due the lack of acetylcholine release. Nunes et al. (2014) did not observe changes in cholinesterasic activity after a chronic exposure (28 days) of the fish Anguilla anguilla to copper. The in vivo assay conducted by Cunha et al. (2007) with marine gastropods species Monodonta lineata and Nucella lapillus exposed to copper for 96 h did not show any cholinesterasic inhibition; however, the in vitro assay described in the same study indicated that copper can inhibit cholinesterasic activity.