Specificity of Sialyltransferase: Structure of α1‐Acid Glycoprotein Sialylated in vitro

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Abstract
The terminal galactosyl units of desialylated α1-acid glycoprotein were selectively labeled with tritium by a galactose oxidase/NaB3H4 procedure. The 3H-labeled glycoprotein was effective as an acceptor in sialyltransferase reactions catalyzed by rat liver microsomes in vitro with unlabeled CMP-N-acetyl-neuramininic acid as sialic acid donor. Permethylation/hydrolysis of glycopeptides derived from the resialylated 3H-labeled glycoprotein yielded radioactive 2,3,4-trimethylgalactose indicating that rat liver microsomes are capable of transferring sialic acid to position C-6 of the terminal galactosyl units of desialylated α1-acid glycoprotein. No indication was obtained for transfer of sialic acid to other positions. This result is discussed in view of the multiplicity of positions of attachment of sialic acid to galactosyl residues in native α1-acid glycoprotein.