Abundance and tissue distribution of selenocysteine-containing proteins in the rat

The form and distribution of selenium (Se) in proteins from selected tissues of the rat were studied by measuring ⁷⁵Se radioactivity in animals provided for 5 months with [^75Se]selenite as the main dietary source of Se. Equilibration of the animals to a constant specific activity of ⁷⁵Se allowed the measurement of ⁷⁵Se to be used as a specific elemental assay for Se. Skeletal muscle, liver and blood accounted for 73% of the whole-body Se and 95% of the total Se-dependent glutathione peroxidase activity. Over 80% of the whole-body Se was in protein in the form of the selenoamino acid, selenocysteine. All other forms of Se that were measured accounted for less than 3% of the whole-body Se. The Se in protein was distributed in seven subunit sizes and nine chromatographic forms. The Se in glutathione peroxidase accounted for one-third of the whole-body Se. These results show that the main use of dietary Se, as selenite, in rats is for the synthesis of selenocysteine-containing proteins. Furthermore, the presence of two-thirds of the whole-body Se in nonglutathione peroxidase, selenocysteine-containing proteins suggests that there may be other important mammalian selenoenzymes besides glutathione peroxidase.