Glycine reductase selenoprotein A is not a glycoprotein: The positive periodic acid-Schiff reagent test is the result of peptide bond cleavage and carbonyl group generation

The complete amino acid sequence of Clostridium sticklandii selenoprotein A, a selenocysteine-containing protein component of the glycine reductase complex, has been established. Both the intact protein and peptide fragments produced by Staphylococcus aureus V8 protease or trypsin were purified by reversed-phase high-performance liquid chromatography and subjected to electrospray ionization mass spectrometric analysis and standard Edman degradation. Selenoprotein A consists of 157 amino acids with a chemical molecular weight of 17,011, in reasonable agreement with the observed molecular weight (17,022.7) determined from its ionization mass spectrum. The sequence of the amino-terminal region of the isolated native protein is Ser- Arg-Phe-Thr-Gly-Lys-Lys-Ile-Val-Ile-Ile-Gly-Asp-Arg-Asp-. An N-terminal methionine residue deduced from the gene sequence was not present. Although selenoprotein A reacted positively in a glycoprotein stain when using either the periodic acid-Schiff reagent procedure or a commercial glycan detection kit, no saccharide was detected by carbohydrate analyses after acid hydrolysis or methanolysis. Identity of the amino acid sequence determined by analysis with that deduced from the gene sequence is further evidence of the absence of bound carbohydrate.