Structure of the carbohydrate units of IgE immunoglobulin. I. Over all composition, glycopeptide isolation, and structure of the high mannose oligosaccharide unit

The total carbohydrate composition of an IgE myeloma protein was determined. Mannose, N acetylglucosamine, galactose, sialic acid, and fucose were found to be present in the amount of 16, 12, 7, 5, and 4 moles, respectively, per mole of heavy chain. Previously it had been determined that all of the carbohydrate moieties were located on the heavy chain of the IgE protein. Five major glycopeptide containing fractions were isolated after pronase degradation of the protein. The type designated B contained sialic acid, fucose, mannose, galactose, and N acetylglucosamine in the molar ratios of 1 to 2:1:3:2:4. Three of the B type oligosaccharide units were found to be present per heavy chain. In addition, a high mannose glycopeptide, termed C 1, was isolated which contained only mannose and N acetylglucosamine in the ratio of 6:2. Only one chain of this type was present per heavy chain. The complete sequence of this glycopeptide was determined using a combination of sequential enzymatic degradation, periodate oxidation, and methylation. After removal of 4 of the 6 mannose residues with α mannosidase, a core was obtained, whereby the sequence was determined. This is the first example of an α linked N acetylglucosamine to be described in a circulating glycoprotein. This glycopeptide is also unusual in that it does not have the more commonly found chitobiose core unit and gives no evidence of microheterogeneity.