A candidate mammalian glycoinositol phospholipid precursor containing three phosphoethanolamines

The glycoinositol phospholipid (GPI) anchors of mammalian proteins contain linear ethanolamine (EthN)-P-6ManManManGlcN glycan sequences that bear additional EthN-P substituents and in some cases include a fourth Man and a GalNAc or sialic acid-GalGalNAc. Precursors of these anchoring structures are preassembled in the endoplasmic reticulum by sequential glycosylation of inositol phospholipid. In previous studies (Hirose, S., Prince, G. M, Sevlever, D., Ravi, L., Rosenberry, T. L., Ueda, E., and Medof, M. E. (1992) J. Biol. Chem. 267, 16968-16974), a series of putative intermediates of this assembly process were isolated from human HeLa cells and murine lymphomas, and several of the more polar products were found to contain a second EthN-P attached to the Man residue (Man 1) proximal to GlcN. In this study, the most polar HeLa cell GPI species was purified by normal phase HeLa cell GPI species was purified by normal phase Iatrobead high performance liquid chromatography, and its glycan was characterized. Dionex anion exchange chromatographic analyses of fragments produced by nitrous acid deamination, hydrofluoric acid dephosphorylation, and trifluoroacetic acid hydrolysis in conjunction with biosynthetic labeling studies indicated a structure containing a third EthN-P substituent linked to the 6-position of Man 2. The polar GPI product exhibited a ManManManGlcN core lacking additional Man or GalNAc. The implications of the identification of this triply phosphoethanolamine-substituted species to mammalian GPI anchor biosynthesis are discussed.

Purification and characterization of dolichyl-P-mannose:Man5(GlcNAc)2-PP-dolichol mannosyltransferase

The dolichyl-P-mannose:dolichyl-PP-heptasaccharide alpha-mannosyltransferase (2.4.1.130), which catalyzes the transfer of mannose from dolichyl-P-mannose to the Man5(GlcNAc)2-PP-dolichol acceptor glycolipid, was solubilized from pig aorta microsomes with 0.5% NP-40 and purified 985-fold by a variety of conventional methods. The partially purified enzyme had a pH optimum of 6.5 and required Ca2+, at an optimum concentration of 8-10 mM, for activity. Mn2+ was only 20% as effective as Ca2+, and Mg2+ was inhibitory. The mannosyltransferase activity was also inhibited by the addition of EDTA to the enzyme, but this inhibition was fully reversible by the addition of Ca2+. The enzyme was quite specific for dolichyl-P-mannose as the mannosyl donor and Man5(GlcNAc)2-PP-dolichol as the mannosyl acceptor. The Km values for dolichyl-P-mannose and the acceptor lipid Man5(GlcNAc)2-PP-dolichol were 1.8 and 1.6 microM. On Bio-Gel P-4 columns and by HPLC, the radiolabeled oligosaccharide formed during incubation of dolichyl-P-[14C]mannose and unlabeled Man5(GlcNAc)2-PP-dolichol with the purified enzyme behaved like Man6(GlcNAc)2. This octasaccharide was susceptible to digestion by endoglucosaminidase H, indicating that the newly added mannose was attached to the 6-linked mannose in an alpha 1,3-linkage. This linkage was further confirmed by acetolysis of the oligosaccharide product [i.e., Man6(GlcNAc)2], which gave a labeled disaccharide as the major product (greater than 90%).

Isolation of a crustacean N-acetyl-D-glucosamine-1-phosphate transferase and its activation by phospholipids

The N-acetyl-D-glucosamine-1-phosphate:dolichol phosphate transferase from Artemia has been partially purified and characterized. The enzyme is solubilized from crude microsomes using Triton X-100, and after detergent removal appears to be associated with phospholipids. Using dolichol phosphate and UDP-N-acetyl-D-glucosamine as substrates, the enzyme catalyzes the formation of dolichol-pyrophosphate-N-acetyl-D-glucosamine. The product identity has been verified by TLC and paper chromatography following mild acid hydrolysis. Under the incubation conditions used only one product is made, i.e., Dol-P-P-GlcNAc. The formation of product is linear with increasing amounts of added protein and with time of incubation. The enzyme requires magnesium ions for activity. Activity of the enzyme is stimulated 6-fold by exogenous dolichol phosphate and is also stimulated by added phospholipids, with optimal activity being obtained in the presence of mixtures of phosphatidylcholine and phosphatidylglycerol. Enzymatic activity is not increased upon addition of GDP-mannose or dolichol phosphate mannose. The enzyme is rapidly inactivated by exposure to several detergents, including Triton X-100 and deoxycholate. The activity is inhibited by tunicamycin and by the purified B2 homologue of this antibiotic. Other antibiotic inhibitors such as diumycin and polyoxin D have little effect on the enzyme. Both the microsomal and solubilized enzyme preparations are inactivated by 70% upon treatment with phospholipase A2; activity may be restored by addition of phospholipids. Following hydrophobic interaction chromatography on Phenyl Sepharose, gel filtration chromatography on Sepharose CL-4B indicated that the enzyme, purified 81-fold, contained phophatidylcholine and phosphatidyl-ethanolamine.

Separation of dolichol monophosphate mannose and dolichol monophosphate glucose by thin-layer chromatography

Dolichol monophosphate mannose and dolichol monophosphate glucose from mammalian or mold cells were separated by thin-layer chromatography. This separation was not due to a difference in the size of the lipid moieties. It was found that the lipid moieties of dolichol monophosphate mannose and dolichol monophosphate glucose are the same size when synthesized by the same cell. The thin-layer chromatographic separation appeared to be due, therefore, to the saccharide part of the molecules.

Characterization of a mannosyl-lipid compound of microsomal fractions of rat pancreas and influence of diet

1. High-starch diet induces an activation of rat pancreatic microsomal mannosyl-transferase activity as compared with a standard diet or a high-fat diet. 2. This increase is found in a mannose-containing lipid which is identified as a dolichyl-phosphoryl-mannose on the criteria of DEAE-cellulose chromatography, alkaline and acid hydrolysis, thin-layer chromatography and identity of this endogenous product with the [14C]mannose-containing product synthesized in presence of exogenous dolichyl-monophosphate added to the incubation medium. 3. Kinetic parameters (Km and Vmax) of the enzyme versus polyprenic acceptor are not modified by the diet. 4. The results indicate that the activation of the mannose transfer is principally due to an increase of the polyprenic endogenous acceptor by the high-starch diet.

Isolation and identification of a mannolipid intermediate in the synthesis of pulmonary glycoproteins

In order to demonstrate the role of lipid-linked sugars as intermediates in the synthesis of pulmonary glycoproteins, membranous preparations of rabbit lung were incubated with GDP-14C-mannose and selectively extracted with chloroform-methanol (2:1) and chloroform-methanol-water (1:1:0.3). The chloroform-methanol extract was found to contain a lipid-linked sugar which had chemical and chromatographic properties similar to mannosyl-phosphoryl dolichol. The chloroform-methanol-water extract contained an oligosaccharide-lipid and the remaining precipitate contained a labeled protein. Transfer of mannose from the labeled mannolipid to the endogenous protein was demonstrated. These studies indicate a glycolipid intermediate is present in the synthesis of oligosaccharides attached to pulmonary glycoproteins.

Dietary effects on the formation of dolichyl monophosphate mannose by microsomal preparations of rat adiopose tissue

Microsomal preparations from rat adipose tissue catalyse the transfer of [(14)C]mannose from GDP-[(14)C]mannose to an endogenous acceptor forming a [(14)C]mannosyl lipid. The mannosyl lipid co-chromatographs with hen oviduct dolichyl monophosphate beta-mannose on three solvent systems. It is stable to mild alkaline hydrolysis, but strong alkaline treatment yields a compound that co-migrates with mannose 1-phosphate. The mannosyl lipid is labile to mild acid hydrolysis, yielding [(14)C]mannose. Formation of the compound is reversible by GDP, but not GMP, and is stimulated by exogenous dolichyl phosphate. The kinetics of transfer of [(14)C]mannose from GDP-[(14)C]mannose to form dolichyl monophosphate mannose were studied by using preparations derived from rats fed on one of four diets: G (high glucose), L (high lard), F (fructose) or GC (high glucose, 0.9% cholesterol). The K(m) and V(max.) values for transfer from GDP-mannose were virtually indistinguishable in the four preparations.In the absence of exogenous dolichyl phosphate, the largest amount of transfer of [(14)C]mannose into the mannosyl lipid was observed with preparations from fructose-fed animals. Preparations from glucose-fed animals showed about 60% as much transfer, whereas membranes from rats fed the other diets showed intermediate values between the fructose- and glucose-fed animals. The inclusion of cholesterol in the glucose diet elicited an increase in transfer of mannose. Under conditions of saturating exogenous dolichyl phosphate, preparations from lard-fed animals have 1.5 times as much enzyme activity as do preparations from animals fed the other three diets.

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.

Effects of exogenous retinol and retinoic acid on the biosynthesis of 14C-mannose labelled glycolipids and glycoproteins in rat liver

Studies were conducted to investigate the in vivo and in vitro effects of retinol and retinoic acid on the synthesis of mannolipids and mannopeptides in rat liver. The incorporation of 14C-mannose into glycolipids and glycoproteins showed a decrease in vitamin A-depleted rats as compared with vitamin A-fed rats. By means of DEAE-cellulose, silicic acid and thin-layer chromatography, the mannose-containing lipids were separated into mannosyl retinyl phosphate (MRP, Rf 0.2) and dolichyl mannosyl phosphate (DMP, Rf 0.4), respectively. A rapid increase in the synthesis of labelled MRP was observed, exhibiting a peak between 25 and 60 min after intraperitoneal administration of retinol to vitamin A-depleted rats. Similarly, administration of retinoic acid brought about elevation of 14C-mannolipid (Rf 0.2) synthesis with a peak at 60 min after injection. On the other hand, the incorporation of 14C-mannose into DMP (Rf 0.4) remained unchanged by such treatment. In vitro addition of retinyl phosphate, but not retinoyl phosphate, markedly stimulated the synthesis of 14C-mannolipid (Rf 0.2), using crude membrane of rat liver and GDP-14C-mannose as the donor. These findings strongly suggest that not only retinol but also retinoic acid plays an important biological role in mannosyl transfer reaction in rat liver. However, the molecular participation of a metabolite of retinoic acid in the formation of a mannolipid and the structure of such a metabolite remain to be established.