Selenocysteine on glutathione peroxidase may be converted from phosphoserine on the apo-enzyme synthesized with an opal suppressor phosphoseryl-tRNA.

There are two possible mechanisms (co- or post-translational) for incorporation of Se into glutathione peroxidase in which selenocysteine presents at the active site of the enzyme and corresponds to UGA on the mRNA. We studied the above mechanisms using opal suppressor tRNA in mammals. Opal suppressor tRNA did not accept any selenocysteine and phosphoseryl-tRNA did not change to selenocysteyl-tRNA. Meanwhile, phosphoprotein changed to a protein containing selenocysteine by the incubation with H2Se and some enzymes. From these results, we propose that phosphoserine on glutathione peroxidase (apo-enzyme), which is synthesized with phosphoseryl-tRNA, is converted to selenocysteine in the mature enzyme, by a posttranslational mechanism. Opal suppressor tRNA may play a role to synthesize the apo-enzyme of glutathione peroxidase.