Mannose transfer from mannolipid to endogenous acceptors in hamster liver

Study of nuclear mannosyl-transferase: lipids intermediates

Mannosyl-transferase of rat liver nuclei catalyzed the transfer of mannose, from GDP-mannose, to endogeneous lipids and proteins. Two solubility-different glycolipids were characterized: a mannosyl-phosphoryl-dolichol and an oligosaccharide-lipid. The labeling of the two lipids was consistent with a role in mannose transfer to nuclear glycoproteins, but not according to the Lennarz' scheme.

Incorporation of mannose and glucose into prenylphosphate sugars in isolated human platelet membranes

Isolated platelet membranes synthesize mannosylretinyl phosphate and dolichylmannosyl phosphate from GDP-[14C]mannose, but only dolichylglucosyl-phosphate is synthesized from UDP-[14C]glucose. Addition of exogenous retinylphosphate specifically stimulates the biosynthesis of mannosylretinylphosphate.

Cell surface mannosyl transferase activity in the liver of embryonic chick. A mannose containing glycolipid as intermediate in glycoprotein synthesis

Trypsin dissociated intact cells of embryonic chick liver catalyze the transfer of mannose from exogenous GDP-mannose to glycoprotein in vitro. In cells of 8 day old embryos a surface bound mannosyltransferase-system forms several mannose containing isoprenoid-lipids in a primary step. One of these compounds serves as a substrate in the highly specific second step of the overall reaction, the transfer of mannose to glycoprotein. The isoprenoic intermediate has been isolated and its effectivity as substrate for the incorporation of mannose into glycoprotein has been examined.

Cell surface glycosyl transferase activities in liver cells of developing chicken embryos

The cell surface of embryonic chick liver cells contains transferases for mannose, fucose, galactose, N-acetyl-glucosamine and N-acetyl-neuraminic acid. Liver cells obtained by trypsin-dissociation of the tissue use the corresponding exogenous sugar nucleotides as substrates. The activities of the enzymes tested do not depend neither no the dissociation procedure nor on de novo protein synthesis. They vary considerably during development of the embryos, reaching maximal values at the 8th+/-1 day and at the 12th+/-1 day. Glycoproteins are the final stable endogenous acceptors for all sugars. Mannose transfer proceeds via a two or multistep reaction sequence. In a first step labile lipophilic intermediates are formed. Mannose can be liberated by treating the intermediates with 0.1 N HCl at 100 degrees C. In a second reaction step mannose becomes attached to glycoproteins. From embryonic chick liver cells a glycopeptide fraction has been obtained by pronase digestion followed by several purification steps. The purified glycopeptides inhibit all transferase systems and act as exogenous acceptors for mannose transfered from exogenous GDP-mannose.