Radical scavenging abilities of fish MT-A and mussel MT-10 metallothionein isoforms: An ESR study

Metallothioneins (MTs) are cysteine-rich proteins involved in homeostasis of essential metals, detoxification of toxic metals and scavenging of free radicals. Scavenging of the stable 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical was measured by means of ESR spectroscopy for two recombinant MTs from aquatic species: MT-10 from the sea mussel Mytilus galloprovincialis, and MT-A from the fish Oncorhyncus mykiss. Both the zinc- and the cadmium-loaded forms (Zn(7)-MTs and Cd(7)-MTs) were analysed, using the commercial MT-II (Zn(7)-MT-II and Cd(7)-MT-II, respectively) from rabbit liver as a reference. A decrease in the scavenging ability was observed for all the three MTs passing from the Zn- to the Cd-loaded forms, because of the higher stability of the Cd-mercapto complex. The Zn(7)-MTs from aquatic species were more effective in scavenging DPPH signal than the rabbit Zn(7)-MT-II (2.8 and 4-folds, respectively). Similar results were obtained also for the Cd(7)-MTs, thus confirming the stronger antioxidant power of MTs from aquatic organisms compared with the rabbit MT-II. Moreover, mussel MT-10 was more active in DPPH scavenging than fish MT-A. When the complete release of metals from MTs was obtained by lowering the pH to 3 or, alternatively, by adding the chelating agent diethylenetriaminepentaacetic acid (DTPA), an increase in the scavenging ability of MTs was observed.