Biosynthetic incorporation of 75Se into GPx1 selenoprotein. Saos-2 cells were transfected with epitope-tagged GPx1 constructs as indicated in the top margin, incubated with 75Se, lysed and immunoprecipitated with antibody to human GPx1 or to the epitope tag as indicated in the lower margin, then analyzed by SDS–PAGE and autoradiography. The autoradiogram shows Lanes 1, 2: immunoprecipitated selenoprotein from non-transfected cells; Lanes 3, 4: from cells transfected with an epitope-tagged GPx1 construct with an intact SECIS element; Lanes 5, 6: from cells transfected with an epitope-tagged GPx1 construct with the SECIS element deleted. Lanes 1, 3, 5: immunoprecipitated with anti-GPx1 antibody; Lanes 2, 4, 6: immunoprecipitated with antibody to the epitope tag (“Epi”).