It turns out that the black spots on salmon fillets contain melanin

It turns out that the black spots on salmon fillets contain melanin

The black spots on salmon fillets actually contain melanin

A large percentage of farmed Atlantic salmon fillets contain black or red spots. Wakamatsu and his colleagues found that black spots contain melanin, while red spots do not visibly contain melanin. The lack of biochemical continuity between red and black spots supports that they are derived from different cellular origins, i.e. red blood cells through hemorrhage and melanoma through inflammatory responses, respectively. Salmon and filet graphics taken from Image source: The figure was created by Johannes M. Dijkstra and location images were provided by Turid Mørkøre.

More than 20% of Atlantic salmon fillets may contain unattractive black and red spots, which are often larger than 1 cm and cause significant financial losses. The spots are more abundant in farmed salmon than in wild salmon, and their causes are not well understood.

Recognizing the need to elucidate the biochemistry of spots, Professor Tred Mørkur from the Norwegian University of Life Sciences turned to two leaders in the science of melanin biochemistry, Professor Kazumasa Wakamatsu and Professor Shusuke Ito from Fujita Health University, Japan.

These researchers found that black spots contain melanin called eumelanin and that red spots do not clearly contain melanin. The biochemical discontinuity between red and black spots supports that their pigments are derived from distinct cellular origins, namely red blood cells and melanocyte macrophages (dark-pigmented fish macrophages), respectively.

Although these results were within expectations, they are an important step towards understanding the problem of stains, and the study was published in the journal International Journal of Molecular Sciences.

In recent decades, focal color changes on Atlantic salmon fillets have become an increasing problem for commercial seafood farming, affecting a significant portion of fish fillets. The causes of spots are mysterious, although many plausible theories have been put forward, and there are likely multiple different causes. The spots are roughly classified as either “red spots” or “black spots” and are also known as “focal red changes” and “focal melanotic changes.”

However, proper biochemical analysis of black spots has never been performed, and the suggestion that they contain melanin has been based mostly on staining with Fontana Masson, which is relatively specific for melanin. Intermediate forms have been found between red spots and black spots, leading to the common concept among many that black spots are usually derived from red spots.

Melanin is a very large, highly irregular heteropolymer composed of monomers derived by enzymatic oxidation of the amino acid tyrosine. There are several types of melanin. Professors Wakamatsu and Ito are melanin specialists, and have set up a panel of biochemical tests to characterize multiple types of melanin from different species, ranging from human melanoma patients to fossilized animals such as dinosaurs.

By applying these tests to spots found on fillets of Atlantic salmon from Norway, they found that the black spots contained the black pigment eumelanin, which is also found in human hair and skin, while for the red spots, they were unable to infer its presence. From melanin. However, in the red spots they found some DOPA-derived products that indicate the presence of an oxidizing environment consistent with bleeding.

Importantly, the findings by Wakamatsu et al. revealed that there is no biochemical continuity between red and black spot pigments, which supports previous histology-based hypotheses that red spots result from hemorrhage, and black spots result from local accumulations of melanocyte macrophages in chronic local immune reactions.

Melanocyte macrophages are immune cells found only in ectothermic vertebrates, including fish, amphibians, and reptiles. By inference, the new study by Wakamatsu et al. This suggests that the black pigment found in melanocyte macrophages is eumelanin, which has not been properly identified before.

Bleeding can have different causes. Not every bleeding leads to chronic inflammation with melanocytes, and melanocytes can also accumulate for reasons other than bleeding. Therefore, the distinct cellular origin of red and black spots, as supported by their biochemical discontinuity shown by Wakamatsu et al, implies that researchers should anticipate a variety of possible causes for different spots and not try to find a “one-size-fits-all” explanation.

“The study by Wakamatsu and colleagues is an important element in the characterization of pigmented lesions in Atlantic salmon and is in good agreement with the study of our group,” explains Professor Erling Kupang, a Norwegian specialist in salmon spots who was not involved in this study. Previous identification of expression of tyrosinase (a gene essential for melanin production) in melanocytic changes.”

“Now we know for sure that the final product is as we expected, which is important going forward in trying to ban these pests.”

more information:
Kazumasa Wakamatsu et al., Detection of eumelanin in melanistic focal changes but not in red focal changes in fillets of Atlantic salmon (Salmo salar), International Journal of Molecular Sciences (2023). doi: 10.3390/ijms242316797

Provided by Fujita Health University

the quote: Black Spots on Salmon Fillets Contain Melanin (2024, January 8) Retrieved January 9, 2024 from

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