Autentication of fish species by isoelectric focusing and two dimensional electrophoresis : application to freshwater fish commercially labelled "perch"

According the United Nation’s FAO the average consumption of fish per inhabitant is rapidly increasing, and predicted to multiply 7 fold in the next decade. The growing consumer demand together with the opening of new international markets have boosted the global trade of fisheries products and the variety of fish species that regularly arrive on the worldwide markets. The seafood safety will be therefore a major challenge for veterinary inspectors. A serious risk for human health is the mislabelling of seafood products. For example it is indispensable to unambiguously identify species that may contain toxins or can cause allergic reactions, or during an official block of the import of a particular species due to sanitary reasons. In addition, difficulties in identification can encourage fraudulent practices such as replacing species with different commercial values. Morphological criteria are commonly used for seafood identification, but in doubtful cases or when the fish are sold without clear morphology (i.e. filleted) biochemical analyses are fundamental to ensure accuracy (1-2). In the present paper isoelectric focusing (IEF) and two-dimensional electrophoresis (2-DE) were applied to identify four fish species commercially sold under the generic label of “perch”, which arrive on the fish markets already filleted in view of their numerous bones. The four species are: 1) the European perch (Perca fluviatilis); 2) the Nile perch (Lates niloticus); 3) the European pikeperch (Stizostedion lucioperca); 4) the Sunshine bass (Morone chrysops x saxatilis). These fish have significantly different commercial values, the European perch being the most appreciated by consumers and commanding the highest prices. Hygienical and sanitary problems arise from the treatment of Nile perch, which is filleted in the country of origin (Egypt and Central Africa countries) and then exported to Europe. Many cases of sanitary blocks of import of products coming from Africa, occurred in recent years. Each species showed characteristic IEF pattern and 2-DE map, with differences making for easy discrimination. The IEF patterns of the water-soluble proteins extracted from the white muscle of the four species were species-specific, and, when analysed with suitable software and compared with standard patterns archived in a data-base, permitted a quick and correct identification. Interestingly, none of the IEF bands was common to the four species. The 2- DE maps showed numerous spots, distributed mainly in the acidic part of the IPG pH gradient. Numerous spots are species-specific, although some were similar in the four species. These similarities allowed to assign some spots to specific proteins, an thus allowed a first insight into the proteome of these poorly characterized species. In conclusion, the simple and quick native IEF analysis of the water-soluble proteins extracted from the white muscle serves to distinguish the four species. Thus, the method results a suitable tool for the authority whose task is to guarantee quality control in order to reduce or even avoid mislabelling and frauds. The more sophisticated, expensive and timeconsuming 2-DE, may have major application to deepen the knowledge of fish proteome. Moreover, the proteomic approach has been recently shown to be suitable to identify makers of health status of an organism, contamination levels and post-mortem changes (2).