Structures of sialylated O-glycosidically and N-glycosidically linked oligosaccharides in a monoclonal immunoglobulin light chain.

Abstract

The complete structures of the oligosaccharides in a human X-type immunoglobulin light chain have been determined.Four glycopeptides were isolated by chromatography on Bio-Gel P-6 columns following digestion of the glycoprotein with pronase.The 0-glycosidically linked chains were also isolated as reduced oligosaccharides by treating the glycoprotein with alkaline borohydride.Analysis of the purified glycopeptides and oligosaccharides by exclusion and paper chromatography, methylation, and gas chromatography after sequential hydrolysis with specific exoglycosidases showed that the most complete oligosaccharide chains had the following structures:B13 la2,6 NeuAc a2,6 NeuAc -Gal -GlcNAc -Man 81.4 81 2 a1,6 I Man -GlcNAc -GlcNAc + Asn B L ~ 81 4 a2 6 131.4 81.2 7 T a1.6 NeuAc Gal -GlcNAc -Man a1,3 FucSmaller amounts of two other glycopeptides with similar structures lacking one sialic acid residue were also present.The precise location of the 0-glycosidically and N-glycosidically linked oligosaccharides in the polypeptide chain was established by amino acid sequence analysis of glycopeptides prepared from protein Sm X by digestion with specific proteases.Based on the primary structure of protein Sm X and composition data obtained in the present study, the native glycoprotein contains only one 0-serine-and one N-asparagine-linked oligosaccharide chain.These units are attached to serine-21 and asparagine-26 in the polypeptide chain.This human X-type immunoglobulin light chain has a deletion of 81 amino acids in the variable reg%on and both oligosaccharide chains are located in the hypervariable region just preceding the deletion.The two oligosaccharide chains are separated by only four amino acids, and this close proximity of two different types of chains has not been previously observed in immunoglobulins.Another unique feature of this myeloma light chain is the presence of an 0-serine-linked disialylated tetrasaccharide chain.These types of chains are normally found in mucin and cell-surface glycoproteins.The complete amino acid and oligosaccharide structure of this immunoglobulin light chain has now been established.The heavy chains of normal immunoglobulins are glycoproteins, whereas the light chains are usually devoid of covalently linked carbohydrate (1, 2).However, the light chains of some myeloma immunoglobulins found in serum and the corresponding Bence Jones proteins secreted into the urine do contain carbohydrate (3-5).Abnormal immunoglobulins are commonly found in patients with heavy chain disease.In this syndrome, the protein in the serum or urine consists of deleted dimers of the heavy chains but lacking light chains.In some cases, the heavy chain disease proteins have internal deletions