Selenium Incorporation into Selenoproteins in the Se-Adequate and Se-Deficient Rat 1 2

Abstract

To rapidly discriminate between selenoproteins and Se-binding proteins, SDS + 2-mercaptoethanol treatment and then gradient SDS-slab gel electrophoresis was used to remove loosely bound Se from proteins while separating protein subunits according to molecular weight. This technique was used to study the nature and time course of ⁷⁵Se incorporation into selenoproteins. Male weanling rats were fed either a Se-adequate (0.35 ppm Se) or a Se-deficient (<0.02 ppm Se) diet for 20–41 days, injected iv with 50 μCi [⁷⁵Se]selenite (100 μCi/μg Se), and sacrificed 1, 3, 24, or 72 hr after ⁷⁵Se injection. At 1 and 3 hr, a 55-kDa plasma ⁷⁵Se protein contained the most ⁷⁵Se of any ⁷⁵Se protein observed in any tissue in either Se-adequate or Se-deficient rats. At 24 and 72 hr, a 23-kDa ⁷⁵Se protein (glutathione peroxidase subunit) in liver cytosol was the second-most labeled ⁷⁵Se protein observed in Se-adequate rats. The second-most labeled ⁷⁵Se protein in Se-deficient rats was a 17-kDa ⁷⁵Se protein in testes at 24 and 72 hr. ⁷⁵Se proteins of 10, 14, 45, and 65 kDa as well as lesser amounts of other ⁷⁵Se proteins were also detected. In a separate experiment, cycloheximide pretreatment eliminated ⁷⁵Se labeling of any of the ⁷⁵Se proteins, demonstrating that protein synthesis is required for Se incorporation. The rise and fall of various ⁷⁵Se proteins with time suggests that these selenoproteins may be important in the flux of Se between tissues.