Metallothionein functions: metal chelation and antioxidant activity

• Autor(a) principal: Branco, Diana Patrícia Rodrigues
• Data de Publicação: 2011
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/7317
• topic: Biologia molecular; Metais - Toxicidade; Ecotoxicologia; Ecossistemas aquáticos; Berbigão; Metabolismo celular; Antioxidantes; Stresse oxidativo; Quelação

It is generally accepted that the principal roles of metallothioneins (MTs) lie in the detoxification of toxic metals and regulation of the metabolism of essential trace metals. However, there is increasing evidence that it can act as a free radical scavenger. Although the great number of studies on the antioxidant activity of MTs, the effective physiological role of this protein is still unclear. In order to understand the role of MTs in the protection against metal contamination and oxidative stress, the bivalve Cerastoderma edule was used to evaluate the response of MTs in both situations. Cadmium, a widely reported MT inducer, was used to simulate metal contamination whereas H2O2, an oxidizing compound, was used to simulate oxidative stress. In the first approach, cockles were exposed to a range of Cd and H2O2 concentrations and MTs and TBARS were quantified. Results showed that both treatments induced MT synthesis, confirming the involvement of MTs in metal contamination and oxidative stress. Indeed, the use of MTs as biomarkers for metal pollution was questioned due to the similar synthesis of MT in the two highest concentration used. At last, one concentration of Cd (10 μM) and of H2O2 (20 μM) were selected and cockles were exposed again. TBARS concentration and the intracellular amount of H2O2 were determined. Metal-MT complexes in the two conditions and control were isolated by size exclusion chromatography, and the binding of Zn and Cd to MTs and other cytosolic proteins was evaluated. Furthermore, MTs were quantified and their content in each treatment and control was compared to the amount of Zn associated to them. Results showed that the >H2O2 treatment induced high levels of oxidative stress, demonstrated by the high lipid peroxidation and intracellular concentration of H2O2. Data also indicated that Cd was mainly associated with MTs pool in the Cd treatment, confirming that the protective role of MTs in metal contamination in this bivalve species was due to the binding of MTs to Cd ions. Additionally, the percentage of Zn bound to MTs decreased in the H2O2 treatment, indicating Zn release in oxidative stress. Also, MTs molecules were not as metalated as in the control, confirming Zn release from MTs in oxidative stress and indicating that MTs were needed for demands other than Zn distribution. Further studies on the redox status of MTs are needed to determine the redox status of MTs in the oxidative stress, and understand if, in this bivalve, MTs are acting as ROS scavengers.