Essential micronutrient selenium is excreted into the urine and͞or expired after being transformed to methylated metabolites. Monomethylated selenium is excreted into the urine in response to a supply within the required to low-toxic range, whereas tri-and dimethylated selenium increase with excessive supply at a toxic dose. Here we show that the major urinary selenium metabolite within the required to low-toxic range is a selenosugar. The structure of 1␤-methylseleno-N-acetyl-D-galactosamine was deduced from the spectroscopic data and confirmed by chemical synthesis. This metabolite was also detected in the liver, and an additional metabolite increased with inhibition of methylation. The latter metabolite was again a selenosugar conjugated with glutathione instead of a methyl group and was assumed to be a precursor for methylation to the former metabolite. A metabolic pathway for the urinary excretion of selenium, i.e., from the glutathione-S-conjugated selenosugar to the methylated one, was proposed. Urinary monomethylated (selenosugar) and trimethylated selenium can be used as specific indices that increase within the required to low-toxic range and with a distinct toxic dose, respectively.

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The major selenium compound in an aqueous extract of the most popular mushroom in Eastern Asian countries, shiitake ( Lentinula edodes), fortified with selenium (Se) was identified by means of hyphenated techniques, i.e. HPLC-inductively coupled argon plasma mass spectrometry and HPLC-electrospray ionization mass spectrometry (HPLC-ICP MS and HPLC-ESI MS). Sixty-eight per cent of the total Se in the selenized shiitake was extracted with water, and 49.8% of the Se in the water extract was eluted in the high molecular mass fraction (>40,000 kDa) before incubation at 37 degrees C. After incubation, 40.6% of the Se in the water extract was eluted in a lower molecular mass fraction and the Se eluted in the high molecular mass fraction had decreased to 14.0%, suggesting that the major selenium compound in the water extract was initially in a form bound to macromolecule(s) and was then enzymatically liberated from the macromolecule(s). The retention time of the liberated selenium compound in HPLC-ICP MS matched that of selenomethionine (SeMet), and the masses of molecular and fragment ions detected by HPLC-ESI MS also suggested that the selenium compound was SeMet. The selenized shiitake accumulated Se as SeMet, and SeMet might be bound to the water extractable high molecular mass protein(s).

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