However not all inhibitory profiles by metals

However, not all inhibitory profiles by metals can be a priori considered artifacts of the methodology used to quantify the activity of such enzymes. Metals can indeed interfere with cholinesterases, and the mechanisms are varied. The inhibitory effect of specific metals may derive from their ability to bind to functional groups (e.g. sulfhydryl, carboxyl and imidazole) of proteins, including cholinesterases, as suggested by Najimi et al. (1997). In addition, de Lima et al. (2013) showed that cholinesterases from the nervous system of individuals of Danio rerio, exposed to lead, cadmium, iron and copper, were significantly inhibited. Authors suggested that metals tend to denaturate specific proteins, such as enzymes, due to strong ionic interactions that alter their quaternary structure (vital for their activity), rendering them hydrolytically inactive. According to the work by Frasco et al. (2007), that aimed to characterize the interaction of cholinesterases and the metal mercury, ChE inhibition may result from a complex set of distinct effects and mechanisms. In fact, anticholinesterasic activity of mercury can result from binding to the free thiol group of cysteine residue at the active site of the enzyme (and also to histidine and Mouse side chains of this enzyme), protein aggregation, and, as suggested by de Lima et al. (2013), conformational changes. However, such hypotheses are clearly established for specific animal models, since different, species-specific cholinesterasic forms, are highly variable in terms of their comparative sensitivity towards metals. With the present approach, it was possible to ascertain that uranium by itself, and under realistic conditions, was not able to induce anticholinergic neurotoxicity on the animal models C. fluminea, D. magna, and C. auratus. If no effects were observed after exposure and following recovery periods, it is licit to conclude that uranium does not impair neurotransmission, at least involving cholinesterasic activity. In spite of being a metallic species, in vivo effects of uranium on ChE activity were not visible at environmentally high concentrations (worst case scenario in a mining effluent). These findings corroborate the viewpoint that cholinesterasic activity may not be an appropriate indicator to assess the environmental effects of uranium. In addition, and considering the cited literature, the use of ChE inhibition to address the environmental contamination by other metallic species requires the adoption of judicious criteria, since not all metals seem to be involved in anticholinesterase effects. Although our study is strictly laboratorial, it has implications in the use and interpretation of this environmental biomarker in biomonitoring studies.
Conflict of interest
Acknowledgments The present work was partly funded by Project BiOtoMetal (PTDC/AMB/70431/2006) and UraniumRisk (POCI/AMB/60899/2004), funded by the Portuguese governmental agency Fundação para a Ciência e a Tecnologia (FCT). Bruno Nunes was hired through the Investigator FCT program (IF/01744/2013). Authors wish to thank Professor Fernando Carvalho (C2TN, Instituto Superior Técnico) for supplying uranyl nitrate for part of the experiments.
Introduction Coumarins are a class of natural compounds virtually ubiquitous in the plant kingdom and consist of a 1,2-benzopyrone ring system as a basic parent moiety since the discovery of the first coumarin more than 200 years ago [1]. A high number of substituted coumarins of natural and semisynthetic origin have been isolated from plant and fungal sources and/or chemically synthesized. Such products exert a wide range of pharmacological effects, being able to act as anti-diabetic, anti-viral, anti-microbial, anticancer, anti-oxidant, anti-parasitic, anti-helminthic, anti-proliferative, anti-convulsant, anti-inflammatory, and antihypertensive agents [2]. Many of the research interests have been focused on simple coumarins or furano- or pyranocoumarins, either linear or angular, while much less attention has been dedicated to oxyprenylated ones. These are compounds of mixed metabolic origin for which the coumarin core is attached to a simple or substituted 3,3-dimethylallyl, monoterpenyl or sesquiterpenyl chains via an ethereal link. These rare secondary metabolites have displayed a promising pattern of pharmacological activities and a great potential [3]. So far, many coumarin derivatives of natural and synthetic origins have been reported to exert a noteworthy cholinesterase (ChE) inhibitory effect, [4], [5], [6], [7]. ChE family consisting of acetylcholinesterase (AChE, syn. acetycholine acetylhydrolase, E.C. 3.1.1.7) and butyrylcholinesterase (BChE, syn. pseudocholinesterase, plasma cholinesterase, E.C. 3.1.1.8) plays a key role in the pathology of Alzheimer’s Disease (AD) and, therefore, ChE inhibitors are used in clinic for the symptomatic treatment of AD since deficiency in acetylcholine/butyrylcholine levels has been shown in the brains of AD patients.