Human excretory products of selenium are natural constituents of marine fish muscle.

A selenosugar (selenosugar 1, methyl-2-acetamido-2-deoxy-1-seleno-β-D-galactopyranoside) was identified in aqueous extracts of muscle tissue of three marine fish species, mackerel (Scomber scombrus), sardine (Sardina pilchardus), and tuna (Thunnus albacares), by high-performance liquid chromatography coupled to elemental and high-resolution molecular mass spectrometry. Selenoneine (2-selenyl-Nα, Nα, Nα-trimethyl-L-histidine), a known selenium compound in fish, was the major form of selenium in the aqueous extracts, and the methylated derivative of selenoneine, namely Se-methylselenoneine, was also identified as a minor natural constituent in the fish. Selenosugar 1, a major urinary excretion product of selenium often found in organs and body fluids related to selenium excretion, has so far not been reported in muscle tissue. Se-methylselenoneine has been proposed as the main urinary metabolite from selenoneine. This first report of selenosugar 1 and Se-methylselenoneine as natural constituents of fish muscle tissue opens up a new perspective on the role of these compounds in selenium metabolism and is relevant to selenium supplementation studies.