Isolation and characterization of novel mucin-like glycoproteins from cobra venom

A high molecular weight, heavily glycosylated protein fraction was isolated from cobra venom. It consists of mucin-like glycoproteins (designated as cobra venom mucin) in noncovalent association with several lower molecular weight proteins and glycoproteins. The mucin was purified by CsBr density gradient centrifugation under dissociative conditions. The purified venom mucin comprised about 85% carbohydrate and 15% protein and was rich in Thr, Ser, Pro, Gly, Glu, Asp, and Ala. The mucin was resolved into two or more distinct classes of mucin-like glycoproteins which differ in their amino acid compositions and/or carbohydrate content. Unlike other mucins, cobra venom mucin does not form highly viscous solutions. It appears to keep several venom proteins and glycoproteins soluble by noncovalent interactions. Cobra venom mucin contains both O- and N-linked oligosaccharides; 1 N-linked chain for every 8-10 O-linked oligosaccharides. The O-linked chains are novel structures with high molar proportions of fucose, galactose, and N-acetylglucosamine relative to N-acetylgalactosamine; they have a very low sialic acid content and lack sulfate esters. The majority of the O-linked oligosaccharides are unusually large and contain 15 to as many as 50 sugar residues. The O-linked oligosaccharides are poly-N-acetyllactosaminyl chains consisting of -3Gal beta 1-4GlcNAc beta 1- and -3Gal beta 1-4(Fuc alpha 1-3)GlcNAc beta 1- repeats and thus they contain inner Le(X) antigenic determinants. These oligosaccharides terminate with novel alpha-galactosylated Le(X) and Le(a) epitopes. Due to the abundance of terminal alpha-galactosyl residues, cobra venom mucin reacts with anti-alpha-Gal antibodies that are normally present in human serum.