The structure of the asparagine-linked carbohydrate unit of rat alpha-lactalbumin

Developmental regulation of glycosyltransferases involved in biosynthesis of asparagine-linked glycoproteins in mouse mammary gland

Three glycosyltransferases, namely N-acetylglucosaminyl-1-phosphotransferase, mannosyltransferase and glucosyltransferase, involved in the biosynthesis of asparagine-linked glycoproteins in the mouse mammary gland, were identified by characterization of the products formed by these enzymes. The tissue capacities of the glycosyltransferases, measured as pmol product formed g tissue-1 min-1, increase during developmental cycle of the gland until, at late lactational stage, they reach more than 34, 14 and 60 times, respectively, the basal level found in the tissue of virgin animals. Among the three enzymes at late lactational stage the specific activities of glucosyltransferase and N-acetylglucosaminyl-1-phosphotransferase increase 7-fold and 4-fold respectively, whereas mannosyltransferase shows a mere 1.4-fold increase during tissue development. All of the enzymes, both in terms of tissue capacity and specific activity, return to the basal levels of the virgin gland one month after the end of lactation. The activities of the three enzymes in the lactating gland decrease precipitously following treatment of mice with bromocriptine, a drug that interferes with the release of prolactin from the pituitary gland and thereby inhibits milk synthesis and secretion. These results indicate that the three glycosyltransferases are developmentally regulated during the growth and differentiation of the mouse mammary gland.

Structural studies of glycans isolated from rat plasma hemopexin

After exhaustive pronase digestion, purification by gel filtration and affinity chromatography on concanavalin A, three glycopeptide fractions were obtained from rat hemopexin. Two fractions (I and II) were concanavalin A non-reactive and one (III) was concanavalin A reactive. On the basis of carbohydrate composition, methylation analysis and proton nuclear magnetic resonance spectroscopy, the primary structure of the glycan in fraction III is proposed as being a mixture of mono- and di-sialo-diantennae of the N-glycosidic, N- acetyllactosamine type. Hydrazinolysis of glycopeptides not binding to concanavalin A yielded mixtures of oligosaccharides for both fractions. These oligosaccharides were separated by HPLC; the molar composition of each of them is given. These data suggest that rat hemopexin contains, among others, a diantennary structure bearing three sialic acid residues.

Cell-free synthesis, proteolytic processing, core glycosylation, and amino terminal sequence of rabbit pre-alpha-lactalbumin

Two different forms of alpha-lactalbumin were isolated from rabbit milk and partially characterized. The major and the minor species had apparent molecular weights of 18000 and 14000, respectively, according to their electrophoretic mobilities on SDS polyacrylamide gels. Analyses of their amino acid compositions and amino-and carboxy-terminal sequences did not reveal any difference, but sugar analysis showed the occurrence of carbohydrates in the major species. Rabbit alpha-lactalbumin was synthesized in a cell-free translation system as a precursor with an amino terminal extension of 19 amino acid residues whose primary structure is rather different from those of its ovine and porcine counterparts, in contrast with the extensive similarity so far observed between the known signals of homologous milk proteins. When mammary microsomal membranes were added during translation, the preprotein was converted to authentic alpha-lactalbumin, as demonstrated by amino terminal sequence analyses. However, one of the two processed forms migrated more slowly than pre-alpha-lactalbumin on SDS polyacrylamide gels and this was related to the occurrence of carbohydrates: only the "slower moving" polypeptide was specifically adsorbed on concanavalin A Sepharose and its electrophoretic mobility was enhanced after treatment with endoglycosidase H, an enzyme known to remove clustered mannosyl residues linked to di-N-acetylchitobiose. It was also observed that the rate of translocation of alpha-lactalbumin across the microsomal membrane was lower than that of beta-casein.

Structure determination by 360-MHz 1H-NMR spectroscopy and methylation analysis of a biantennary glycan of the N-acetyllactosaminic type isolated from rat-liver plasma membrane

Glycopeptides obtained by exhaustive pronase digestion of delipidated rat liver plasmic membranes were purified by gel filtration on Sephadex G-25. These glycopeptides were further fractionated by affinity chromatography on a concanavalin-A--Sepharose 4B column into the following fractions: (a) glycopeptides which did not bind to the column (fraction 1); (b) glycopeptides with weak affinity for concanavalin-A--Sepharose, which could be eluted with buffer only (fraction 2); (c) glycopeptides retained on the column and which could be eluted specifically with buffer containing 0.2 M methyl alpha-glucoside (fraction 3). On the basis of the carbohydrate composition, methylation analysis and 360-MHz 1H-NMR spectroscopy, the following primary structure of a glycan in fraction 2 is proposed: (see formula in text).