The Structure of a Glycopeptide from Human Orosomucoid (a,-Acid Glycoprotein) *

Four glycopeptides were isolated from desialized orosomucoid following digestion with Pronase and chromatography on sulfoethyl cellulose. The structure of the major glycopeptide was studied with sequential hydrolysis with /3galactosidase, /3-acetylglucosaminidase, and a-mannosidase, as well as by oxidation with periodate. Evidence for a glycosylamine type linkage between aspartic acid and acetylglucosamine was provided by isolation of 2-acetamido-1-fl-(L-p-aspartamido)-1,2-dideoxyD-glucose (P-aspartyl acetylglucosaminylamine). The major glycopeptide contained 4 moles of galactose, 3 of mannose, 6 of acetylglucosamine, and 1 each of aspartic acid and threonine. The molecular weight of the glycopeptide was 2540 by vapor pressure osmometry and 2840 as measured by forming the trinitrophenyl derivative. Four moles of galactose followed by 4 moles of acetylglucosamine were released per mole of glycopeptide when it was treated sequentially with highly purified P-galactosidase from Phaseolus vulgaris and ,B-acetylglucosarninidase from bovine liver. This treatment resulted in the exposure of an inner core containing 3 moles of mannose and 2 of acetylglucosamine per mole of aspartic acid. Two moles of mannose and 1 mole of acetylglucosamine were split from the inner core of the oligosaccharide with a crude a-mannosidase preparation from ivory nut, with or without added purified or-acetylglucosaminidase from bovine liver. The intact glycopeptide and its “inner core” were oxidized with sodium periodate. The results from the oxidation and enzymatic studies revealed that the acetylglucosamine