Isolation of Glycopeptides from Rat Liver Microsomes Involved in the Biosynthesis of Plasma Glycoprotein

Two radioactive glycopeptides (Glycopeptides I and II) were isolated from the deoxycholate-soluble fraction of rat liver microsomes 90min after injection of glucosamine-1-14C. These two glycopeptides were rich in mannose and N-acetylglucosamine but relatively poor in galactose and sialic acid. Glycopeptide I contained sialic acid, galactose, N-acetylglucosamine, mannose, aspartic acid, serine, glutamic acid, proline, and glycine in the proportion 1:1:4:5:3:1:2:1:1, resulting in a calculated minimum molecular weight of 2950. Glycopeptide II consisted only of aspartic acid, N-acetylglucosamine, and mannose. This glycopeptide gave a single symmetrical peak with a sedimentation constant of 0.6 S when examined by ultracentrifugation. The minimum molecular weight of this glycopeptide is 2360, calculated from the molar ratio for aspartic acid, amide, N-acetylglucosamine, and mannose of 1:1:4:9. By using various glycosidases, oligosaccharide chains in Glycopeptide I were found to contain sialic acid and mannose at the nonreducing ends, while mannose and N-acetylglucosamine separately occupy the nonreducing termini of the sugar chains in Glycopeptide II. Possible sequential arrangements of various monosaccharide units in these two glycopeptides are suggested. Comparison of the sequential arrangement of monosaccharide units in Glycopeptides I and II with that of well characterized plasma glycoproteins suggests that the oligosaccharide chains in these glycopeptides lack sialic acidgalactose-N-acetylglucosamine chains, which occupy the peripheral position of the sugar units in many plasma glycoproteins. It is therefore suggested that these glycopeptides isolated from the microsomal membrane may carry mainly the sugar units which occupy the inner core of the mature plasma glycoprotein.