The contradictory effects exerted by metals were
The contradictory effects exerted by metals were reviewed by Nunes (2011) that stated that the concentration and the species of metal can cause specific effects in ChE activity. Being environmentally exposed, the organism can accumulate metals in tissues, increasing their concentrations. Some authors suggest that some metals can bind to the muscarinic receptor antagonists receptors and cause an impairment on ChE activity, as previously elucidated (De Lima et al., 2012). Another factor that can be considered for the interpretation of the neurotoxicity by metals is that some metallic compounds can exert neurotoxic effects by inhibition of cholinesterase activity through the induction of oxidative stress (Nunes et al., 2014). In this sense, metals can promote oxidative stress due to the interference on oxygen metabolism, leading to the exhaustion of the antioxidant enzymes, increasing free reactive oxygen species (ROS), that may cause impairment of structure and function of ChE enzymes (Jebali et al., 2006, Nunes et al., 2014). However, metal effects on cholinesterases is a disputable issue. Frasco et al. (2005) showed that in vitro inhibitory effects by metals may be due to their reaction with the compounds used in the methodology used for the quantification of cholinesterase activity, namely DTNB, reducing their availability. This effect may be interpreted as an inhibitory effect.
Conclusion The present study fully characterized the main and predominant ChE isoenzymes in brain and muscle tissues of three tropical fish species, in order to allow their future use as test organisms in ecological monitoring and ecotoxicological assays in tropical environments. It was possible to observe that acetylcholinesterase was present in both tissues (brain and muscle), and at much higher levels than other cholinesterasic forms. Indeed, the evaluation of the in vitro ChE sensitivity towards metals and pesticides showed strong inhibitory profiles by common anticholinesterasics. Furthermore, the already described profiles of cholinesterasic inhibition by specific metals were also registered in the analyzed fish species. This set of data demonstrated that the three selected species were highly sensitive to the tested compounds, and they can be used as test organisms in tropical ecotoxicology, to diagnose the presence and exposure to common contaminants that compromise cholinergic neurotransmission.
Acknowledgments Bruno Nunes was hired through the Investigator FCT program (IF/01744/2013). Thanks also due to CESAM (UID/AMB/50017) for financial support and FCT through national funds and co-funding FEDER, within the PT2020 Partnership Agreement. Thanks are due to the program PVE (Pesquisador Visitante Especial), financed by CAPES (Fundação Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), Brazil, with the project entitled “Avaliação dos efeitos ecotoxicológicos de drogas terapêuticas com relevância ambiental em espécies de peixes autóctones brasileiros: uso de biomarcadores de stress oxidative” (process no. – 88881.068122/2014-01). We are also thankful to Dr. George Mattox for the identification of the fish species.
Introduction Cholinesterases (ChEs) belong to a superfamily of serine hydrolases which play a vital role through participating in termination of acetylcholine based signal transmission in neurosynaptic cleft (Ballard et al., 2005; Khaled et al., 2010). On the basis of substrate and inhibitor specificity, two types of ChEs are found in vertebrates; Acetylcholinesterase (AChE, EC 18.104.22.168) and Butyrylcholinesterase (BChE, EC 22.214.171.124). These enzymes are identical (>65%) in their structure with similar catalytic site and contain three amino acids (i.e. serine, glutamate and histidine) which are essential for enzymatic activity (Giacobini, 2001). AChE is present in conducting tissues (motor neurons) of higher organisms and hydrolyzes acetylcholine (ACh) quickly than other ChEs. BChE is actually pseudocholinesterase enzyme which is mainly found in liver and preferentially hydrolyzes butyrylthiocholine chloride (Sturm et al., 1999; Huang et al., 2010). Moreover, BChE is involved in hydrolysis of carboxylic or phosphoric acid ester in detoxification of many compounds like succinylcholine, cocaine etc. it acts as endogenous scavenger of anticholinesterase compounds thus protect AChE function (Çokuğraş, 2003).