Demethylation Pathway of Methylmercury in the Spleen and Peripheral Organs of Bluefin Tuna─Implications for Fish Consumers

Atlantic bluefin tuna (ABFT, <i>Thunnus thynnus</i>) accumulates and breaks down methylmercury (MeHg) mainly in the spleen, with smaller amounts processed in kidneys and other peripheral organs, and only minimally in the liver. While the spleen is well-known as the body's largest immune organ, its role in detoxification is less recognized. Here, we used high-energy-resolution X-ray absorption spectroscopy to examine the chemical forms of mercury (Hg) in ABFT tissues, building on prior analysis of stable Hg isotopes. We found that Hg binds to selenium (Se) in 28 out of 29 tissues, primarily as Hg-tetraselenolate, followed by HgSe and Hg-diselenolate. The proportion of Hg-tetraselenolate decreased in the order: spleen (90 ± 5%, <i>n</i> = 9) ≈ kidneys (88 ± 5%, <i>n</i> = 4) > stomach (66 ± 7%, <i>n</i> = 4) ≈ intestine (64 ± 5%, <i>n</i> = 4) ≈ gonad (56 ± 9%, <i>n</i> = 3) > liver (17 ± 17%, <i>n</i> = 3) ≈ skeletal muscle (14 ± 10%, <i>n</i> = 2). We also observed variations in the δ²⁰²Hg values of Hg-tetraselenolate within tissues of the same fish and between individuals, unlike previous studies of marine mammals and seabirds, which showed constant δ²⁰²Hg values. This variability may be due, in part, to the mixing of two tetraselenolate species, Hg-selenoneine and Hg-selenoprotein P (SelP), each with distinct δ²⁰²Hg values. In ABFT, SelP contains 15 selenocysteine (Sec) residues, organized in five UXU amino acid patterns, which confer binding properties similar to those of metallothionein, a protein known for its role in metal detoxification. Using DeepMind's ColabFold, we modeled SelP and found that 14 Sec residues are located on flexible loops and turns, enabling the formation of Hg clusters and the growth of HgSe nanoparticles. Overall, our results suggest that the internal dynamics and metabolism of Hg differ significantly depending on whether MeHg is primarily degraded in the liver or spleen. We discuss the toxicological implications for humans consuming ABFT.