Characterization of selenoprotein P as a selenium supply protein

Selenium (Se) is well known to be essential for cell culture when using a serum‐free medium, but not when a medium containing serum is used. This finding suggests that serum contains some usable form of Se. To identify the Se‐supplier, T‐lymphoma (Jurkat) cells were cultured for 3 days in the presence of human serum immunodepleted of Se‐containing serum protein, selenoprotein P or extracellular glutathione peroxidase. The Se‐dependent enzyme activities (glutathione peroxidases and thioredoxin reductase) and Se content within the cells markedly decreased only when cultured with selenoprotein P‐depleted serum. Compared with other Se‐containing proteins, the addition of purified selenoprotein P to the selenoprotein P‐depleted serum or a serum‐free medium was the most effective for the recovery of cellular glutathione peroxidase activity (index of Se status). These results suggest that selenoprotein P functions as a Se‐supply protein, delivering Se to the cells.