A similar pattern was observed
A similar pattern was observed for both tissues of P. harpagos, evidencing the absolute predominance of acetylcholinesterase. As observed, no effects were attributed to Iso-OMPA. The hypothesis that the major cholinesterasic form is AChE was reinforced considering the higher hydrolytic activity, which was attained with ASCh as substrate. As was observed for A. altiparanae and for P. harpagos, the set of data obtained for muscle and Nitrocefin of the fish P. pardalis showed an almost total cholinesterase inhibition by eserine and BW284C51; Iso-OMPA caused a significant, albeit not dose-dependent, inhibition of the cholinesterasic activity, which was only possible at extremely high levels of inhibitor. Again, the higher hydrolytic activity was attained when using ASCh as substrate. Similarly to what was concluded for the previous species, it is possible to point AChE as the predominant form in muscle and brain tissues of P. pardalis. Many previous studies were developed to characterize the cholinesterase forms present in test organisms to be used in ecotoxicological assays. The here obtained data present marked similarities with those from the literature. In general, fish species present similar cholinesterasic profiles in their tissues. Garcia et al. (2000) characterized the cholinesterases from muscle tissue of the fish species Poecilia reticulata (guppy), which is an important test organism in ecotoxicology, showing that the main cholinesterase isoenzyme was acetylcholinesterase. Rodrigues et al. (2011) characterized the cholinesterase forms of Lepomis gibbosus; in both tissues (brain and muscle), the main form found was acetylcholinesterase, which was responsive to organophosphate and carbamate compounds. The presence of AChE in the nervous system is common among vertebrates. As shown by Sanchez-Hernandez and Sanchez (2002), serum and brain of the lizard Gallotia galloti had different hydrolytic profiles. In serum, both AChE and BChE forms were found, whereas in the brain only AChE occurred. In addition, brain of rodents also has higher activity of AChE in comparison with other cholinesterase forms (Parmar et al., 1961). Plasma cholinesterases seem to be comprised mainly by pseudocholinesterases. The cholinesterasic characterization of three bird species (namely, Morus bassanus, Ciconia ciconia and Ardea cinerea) showed that the main cholinesterase form present was BChE (Santos et al., 2012). The increase of the phylogenetic distance between species will most likely also increase the possibility of having different cholinesterasic profiles. Ramos et al. (2012) characterized the cholinesterase of the freshwater bivalve species Corbicula fluminea and observed a cholinesterasic form with intermediate properties, which was sensitive to organophosphate compounds. Jebali et al. (2011) evaluated the ChE activity in the cockle Cerastoderma glaucum and observed that the main ChE form present in the whole animal homogenate was AChE. However, in specific tissues (e.g., gills), PChE was the most abundant cholinesterasic form. In the grass shrimp Palaemonetes pugio the most abundant ChE form was AChE (Key and Fulton, 2002). Cholinesterasic activity of the freshwater snails Potamopyrgus antipodarum and Valvata piscinalis was assessed by Gagnaire et al. (2008); this study showed that P. antipodarum had a ChE isoenzyme with intermediate characteristics between AChE and PChE; and in V. piscinalis AChE activity was predominant. These studies point to a variability of cholinesterases among taxa, and the relevance of knowing the species-specific characteristics of ChE isoenzymes to allow their use as tools in biomonitoring studies. The here-obtained results showed that carbaryl in vitro inhibited the acetylcholinesterasic activity of all fish species. Carbamates exert reversible inhibition of cholinesterases, by binding to their active site (serine group). However, this is a transient effect that may be reverted by spontaneous hydrolysis of the carbamate-enzyme complex, allowing the cholinesterasic activity to return to normal physiological values (Xiao et al., 2017). This effect was already reported for other carbamate pesticides, such as carbaryl and carbofuran in the fish species Colossoma macropomum (Assis et al., 2010). Ahmad et al. (2016) described an inhibition of AChE activity by the carbamates bendiocarp, carbofuran, carbaryl, methomyl and propoxur (at a ppm range of concentrations) in Tor tambroides. Fenobucarb and carbosulfan also inhibited brain AChE activity of the fish species Cyprinus carpio (Wang et al., 2015). Similarly, carbofuran exerted an inhibitory effect in AChE activity of Carassius auratus (Bretaud et al., 2000). Another carbamate compound (propoxur) inhibited AChE activity in brain and muscle of Astyanax jacuhiensis (Gonçalves et al., 2018). In addition, specimens of the freshwater crustacean Cherax destructor had their AChE and BChE activities inhibited after being exposed to the carbamate pesticide methomyl (Pham et al., 2017).