| Abstract: | The selenium in mammalian glutathione peroxidase is present as a selenocysteine (Se]Cys) moiety incorporated into the peptide backbone 41-47 residues from the N-terminal end. To study the origin of the skeleton of the Se]Cys moiety, we perfused isolated rat liver with 14C- or 3H-labeled amino acids for 4 h, purified the GSH peroxidase, derivatized the Se]Cys in GSH peroxidase to carboxymethylselenocysteine (Se]Cys(Cm)), and determined the amino acid specific activity. Perfusion with 14C]cystine resulted in 14C]cystine incorporation into GSH peroxidase without labeling Se]Cys(Cm), indicating that cysteine is not a direct precursor for Se]Cys. 14C]Serine perfusion labeled serine, glycine (the serine hydroxymethyltransferase product), and Se]Cys(Cm) in purified GSH peroxidase, whereas 3-3H]serine perfusion only labeled serine and Se]Cys(Cm), thus demonstrating that the Se]Cys in GSH peroxidase is derived from serine. The similar specific activities of serine and Se]Cys(Cm) strongly suggest that the precursor pool of serine used for Se] Cys synthesis is the same or similar to the serine pool used for acylation of seryl-tRNAs. |
