Selenium incorporation into selenoproteins in the Se-adequate and Se-deficient rat

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 (less than 0.02 ppm Se) diet for 20-41 days, injected iv with 50 microCi [75Se]selenite (100 microCi/micrograms 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 seleno-proteins may be important in the flux of Se between tissues.