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. IMAGES:

已确定斯氏梭菌（Clostridium sticklandii）硒蛋白A的完整氨基酸序列，该蛋白为甘氨酸还原酶复合物（glycine reductase complex）中含硒代半胱氨酸（selenocysteine）的蛋白组分。通过反相高效液相色谱（reversed-phase high-performance liquid chromatography）纯化完整蛋白及经金黄色葡萄球菌（Staphylococcus aureus）V8蛋白酶（V8 protease）或胰蛋白酶（trypsin）酶解得到的肽段，并分别进行电喷雾电离质谱（electrospray ionization mass spectrometry）分析与标准埃德曼降解（Edman degradation）测序。硒蛋白A由157个氨基酸残基组成，理论分子量为17011，与通过其电离质谱测得的观测分子量（17022.7）基本吻合。分离得到的天然蛋白的氨基端区域序列为：丝氨酸-精氨酸-苯丙氨酸-苏氨酸-甘氨酸-赖氨酸-赖氨酸-异亮氨酸-缬氨酸-异亮氨酸-异亮氨酸-甘氨酸-天冬氨酸-精氨酸-天冬氨酸-。由基因序列推导得到的N端甲硫氨酸（methionine）残基并未实际存在。尽管硒蛋白A在使用高碘酸-希夫试剂（periodic acid-Schiff reagent）法或商业化聚糖检测试剂盒（glycan detection kit）进行糖蛋白染色时呈阳性反应，但经酸水解（acid hydrolysis）或甲醇解（methanolysis）后的糖类分析未检测到任何糖类物质。通过实验分析得到的氨基酸序列与基因序列推导的序列完全一致，这进一步证明该蛋白不存在结合型糖类。IMAGES: