The α2-Macroglobulin of Human Plasma

Abstract Purified preparations of the α2-macroglobulin from human plasma were extensively digested with Pronase. Glycopeptides from these digests were fractionated by gel filtration on Sephadex G-50 and by chromatography on DEAE-cellulose. A study of the glycopeptides indicated that the average molecular weight of their carbohydrate portion was 2270. On this basis, there are 31 carbohydrate units per molecule of the α2-macroglobulin, with an average composition of 3 residues of mannose, 2 of galactose, 4.7 of N-acetylglucosamine, 1.5 of sialic acid, and 0.4 of fucose. Analysis of the glycopeptide fractions indicated considerable variation in size and composition of the individual carbohydrate units, with the smallest containing only 3 residues of mannose and 2 of glucosamine. The other units are larger and contain, in addition to these, 3 mannose and 2 glucosamine residues, variable amounts of sialic acid, fucose, and galactose, and additional glucosamine residues. Studies on the monosaccharide sequence indicated that these additional sugars are present in the form of oligosaccharide chains with the sequence sialic acid (or fucose) → galactose → N-acetylglucosamine. Variation in the carbohydrate units appeared to be primarily a function of the number and degree of completion of these oligosaccharide chains present in each carbohydrate unit. The largest carbohydrate unit present in the α2-macroglobulin would contain as many as four such chains attached to the mannose-glucosamine core. Information about the monosaccharide sequence was obtained from graded acid hydrolysis, treatment with neuraminidase, and the isolation of 4-O-β-d-galactopyranosyl-N-acetyl-d-glucosamine, as well as a mannobiose. A study of the peptide portion of the glycopeptides indicated that the linkage of the carbohydrate units to the protein is of the glycosylamine type, with asparagine being the amino acid involved in most if not all of the 31 linkages of the α2-macroglobulin.