Eumelanin Detection in Melanized Focal Changes but Not in Red Focal Changes on Atlantic Salmon ( Salmo salar ) Fillets

Superficial discolored spots on Atlantic salmon ( Salmo salar ) fillets are a serious quality problem for commercial seafood farming. Previous reports have proposed that the black spots (called melanized focal changes (MFCs)) may be melanin, but no convincing evidence has been reported. In this stud...

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Bibliographic Details
Published in:International Journal of Molecular Sciences
Main Authors: Kazumasa Wakamatsu, Johannes M. Dijkstra, Turid Mørkøre, Shosuke Ito
Format: Article in Journal/Newspaper
Language:English
Published: MDPI AG 2023
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Online Access:https://doi.org/10.3390/ijms242316797
https://doaj.org/article/026b83c7fba64d6884851c10cd4d57ac
Description
Summary:Superficial discolored spots on Atlantic salmon ( Salmo salar ) fillets are a serious quality problem for commercial seafood farming. Previous reports have proposed that the black spots (called melanized focal changes (MFCs)) may be melanin, but no convincing evidence has been reported. In this study, we performed chemical characterization of MFCs and of red pigment (called red focal changes (RFCs)) from salmon fillets using alkaline hydrogen peroxide oxidation and hydroiodic acid hydrolysis. This revealed that the MFCs contain 3,4-dihydroxyphenylalanine (DOPA)-derived eumelanin, whereas the RFCs contain only trace amounts of eumelanin. Therefore, it is probable that the black color of the MFCs can be explained by the presence of eumelanin from accumulated melanomacrophages. For the red pigment, we could not find a significant signature of either eumelanin or pheomelanin; the red color is probably predominantly hemorrhagic in nature. However, we found that the level of pigmentation in RFCs increased together with some melanogenic metabolites. Comparison with a “mimicking experiment”, in which a mixture of a salmon homogenate + DOPA was oxidized with tyrosinase, suggested that the RFCs include conjugations of DOPAquinone and/or DOPAchrome with salmon muscle tissue proteins. In short, the results suggest that melanogenic metabolites in MFCs and RFCs derive from different chemical pathways, which would agree with the two different colorations deriving from distinct cellular origins, namely melanomacrophages and red blood cells, respectively.