All chemicals usedwere of analytical grade. Standards of sodiumselenite
(99%), seleno-L-methionine (Se-Met), Se-methy-selenocysteine
(Se-MeSeCys) were purchased fromSigma. All standard stock solutions
for HPLC–ESI–MS/MS analysiswere prepared to have a concentration of
100 μg/L by appropriate dilution with deionized water (18.2 MΩ).
Seleno-L-methionine and Se-methy-selenocysteine solutions were
stored at −20 °C except for sodium selenite, which was stored at
4 °C. Acetone, acetonitrile, formic acids, proteinase K, tripsin, Tris
aminomethane hydrochloride, and nitric acid (69%, v/v) were purchased
from Sigma.