Investigating the Presence of Selenoneine, Ergothioneine, and Selenium-Containing Biomolecules in Fish and Fish-Derived Commercial Products

Two potent antioxidants, selenoneine and ergothioneine, have been determined simultaneously for the first time in the muscle of highly consumed fish and in fish-derived products. The extraction methods were optimized, and their subsequent analysis was performed by HPLC-ESI-MS/MS. Best extraction efficiency (106 ± 3 and 104 ± 5%) was attained by performing a simple aqueous extraction containing 100 mmol L^–1 ammonium acetate or ammonium formate as extractants. Selenoneine and ergothioneine were simultaneously detected in tuna (0.306 ± 0.006 and 3.5 ± 0.2 μg g^–1) and swordfish (0.14 ± 0.02 and 1.1 ± 0.4 μg g^–1) muscle tissues, while in fish roe and in farmed salmon, only ergothioneine was detected (with contents ranging 0.03–3.3 μg g^–1). Moreover, the presence of their respective methylated forms was also considered. Additionally, Se-containing biomolecule profiles from water-soluble proteins were obtained by SEC-ICP-MS analysis. Results revealed similar profiles between fish and fish roe, whereas for ultraprocessed fish products, Se seems to be mostly associated with low-molecular-weight biomolecules. The results included in this work may contribute to a better understanding of the relations between selenoneine and ergothioneine in fishery products as well as their fate and potential modifications of Se-containing biomolecule profiles during fish processing.