Identification of carbonylated protein in frozen rainbow trout (Oncorhynchus mykiss) fillets and development of protein oxidation during frozen storage

Frozen storage of fish is known to enhance lipid oxidation, resulting in the development of an unpleasant rancid taste and odor. Frozen storage of fish is also known to reduce protein solubility, and proteins are expected to be oxidatively modified; however, these oxidative mechanisms are poorly understood. Generally, protein oxidation leads to a wide range of modifications; the most studied being the formation of carbonyl groups. The present work shows, by UV spectrophometric determination of protein carbonyl groups in rainbow trout muscle, that storage at -20 C resulted in a 2-fold increase in protein carbonylation compared to storage at -30 or -80 degrees C. Furthermore, low-salt-soluble proteins in fish that were either fresh or stored for 3 years at -80 degrees C were found to have similar extents of carbonylation. Proteome analysis and two-dimensional immunoblotting of rainbow trout low-salt- and high-salt-soluble proteins gave a detailed description of the protein carbonylation pattern. Several carbonylated proteins were identified by LC-MS/MS, such as nucleoside diphosphate kinase, adenylate kinase, pyruvate kinase, actin, creatine kinase, tropomyosin, myosin light chains 1 and 2, and myosin heavy chain. Furthermore, the results showed a reduced solubility of nucleoside diphosphate kinase in fish stored at -20 degrees C for 2 years compared to fish stored at -80 degrees C. It was observed that low-abundant proteins could be relatively more carbonylated than high-abundant proteins, thereby indicating that some proteins are more susceptible to oxidation than others, due to either their cellular localization, amino acid sequence, or biochemical function.