The effects of triton X-100 on the transfer of mannose, glucose and n-acetylglusomine phosphate to dolichol monophosphate by preparations of rough and smooth endoplasmic reticulum and of mitochondria of rat liver

Activity of N-acetylglucosamine-1-phosphate transferase in sheep liver microsomes: in vivo and in vitro inhibition by tunicamycin

The neurological disease of livestock known as annual ryegrass toxicity, caused by ingestion of bacterial toxins called corynetoxins, has been shown to be produced experimentally by injection of tunicamycin, a related antibiotic. In this study the effects of tunicamycin inhibition on the activity of the enzyme, N-acetylglucosamine-1-phosphate transferase, in sheep liver rough microsomes were measured in vitro and in vivo. Enzyme activity was dependent on Triton X-100 and exogenous dolichyl phosphate for maximal activity, although there was measurable activity in their absence. The transferase enzyme was very sensitive to in vitro (inhibition can be detected below 10 ng ml(-1)). In vivo, sheep treated parenterally with a single dose of tunicamycin showed a time and dose-dependent decrease in enzyme activity, which was almost completely inhibited for up to 14 days after a sublethal dose of toxin. In addition, the yield of rough microsomes was lower from toxin-treated sheep than from control animals.

Mitochondrial dolichyl-phosphate mannose synthase. Purification and immunogold localization by electron microscopy

Mitochondrial dolichyl-phosphate mannose synthase has been purified to homogeneity using an original procedure, reconstitution into specific phospholipid vesicles and sedimentation on a sucrose gradient as final step. The enzyme has an apparent molecular mass of 30 kDa on an SDS/polyacrylamide gel. Increased enzyme activity could be correlated with this polypeptide band. A specific antibody was raised in rabbits against this transferase. Specific IgG obtained from the immune serum removed enzymatic activity from a detergent extract of mitochondrial outer membrane and reacted specifically with the 30-kDa band on immunoblots. Furthermore, an immunocytochemical experiment proved the localization of dolichyl-phosphate mannose synthase on the cytosolic face of the outer membrane of mitochondria.

Investigation of glycosylation processes in mitochondria and microsomal membranes from human skeletal muscle

Glycoconjugates are directly involved in major skeletal muscle functions. As little is known about glycosylation processes in muscle, we investigated glycoconjugate synthesis in subcellular fractions from human skeletal muscle tissue. Mitochondria and microsomal membranes were prepared from muscle biopsies by thorough mechanical disruption and differential centrifugations. This procedure resulted in the isolation of intact mitochondria (1 mg protein/g muscle) and of a microsomal fraction (1.5 mg protein/g muscle). Glycosyltransferases were studied in both subcellular fractions using either dolichylmonophosphate as a polyprenic acceptor or chemically modified fetuin as a glycoprotein substrate. Our results provide evidence for high rates of glycosylation in muscle. The highest activities were obtained with GDP-mannose: dilichylmonophosphate mannosyltransferase, a key enzyme in glycosylation process (220 pmol/mg per h in mitochondria and 1,550 pmol/mg per h in microsomal membranes). Substantial individual variations were observed for dolichol pathway glycosyltransferases but low individual variations were found for glycosyltransferases involved in maturation of glycoproteins. The role which glycosylation defects may play in muscle dysfunction has yet to be defined.

Organization of mitochondrial UDP-glucose:dolichylmonophosphate glucosyltransferase in the outer membrane

Previous studies have shown the existence of an autonomous mitochondrial UDP-glucose: dolichylmonophosphate glucosyltransferase, located in mitochondrial outer membrane of liver cells. To improve our knowledge about the topographical aspects of glycosylation in mitochondria, we have investigated the organization of this enzyme in intact mitochondria, using controlled proteolysis with trypsin and sensitivity towards amino-acid specific reagents. Our data provides evidence: --for a mitochondrial glucosyltransferase facing the cytoplasmic side of the outer membrane --and for the involvement of histidine and tryptophan residues as well as sulfhydryl groups in the catalytic activity of the enzyme.

Characterization of the submitochondrial compartments: study of the site of synthesis of dolichol and dolichol-linked sugars

The fractionation of mitochondrial membranes on discontinuous sucrose gradient leads to the obtaining of free outer membranes, free inner membranes and two distinct membrane contact site populations characterized as follows. Only outer membrane contact sites and inner membrane contact sites bind hexokinase. Outer membranes and outer membrane contact sites are cholesterol-rich fractions. The endogenous dolichol content is twice fold higher in outer membranes and outer membrane contact sites than in inner membranes and inner membrane contact sites, only the biosynthesis of dolichol in inner membrane contact sites is not stimulated by addition of exogenous [14C]-IPP and FPP. The glycosylation of endogenous dolichol from labeled nucleotide-sugars (UDP-GlcNAc, GDP-Man and UDP-Glc) leads to the synthesis of dolichol-pyrophosphoryl-sugars and dolichol-monophosphoryl-sugars with the rate of synthesis proportional to the dolichol content of each submitochondrial fraction.

The effect of dietary linoleic acid on rat platelet sialic acid

The total sialic acid content of blood platelets from rats raised for 8 weeks or 12 months on a diet containing 1% linoleic acid (1LA) was significantly lower (by over 30%) than that from those raised on an isocalorific diet containing 6% linoleic acid (6LA). The transfer of sialic acid to endogenous glycoprotein acceptor was also significantly lower (up to almost 4-fold) in 1LA platelet and megakaryocyte-rich preparations but the transfer to exogenous glycoprotein acceptor was similar in both 1LA and 6LA platelets. The megakaryocyte-rich fraction of 1LA animals showed a reduced phosphodolichol-sensitive N-acetylglucosaminyl (but not mannosyl) transfer to endogenous glycoprotein compared with 6LA animals. No significant difference was found between the megakaryocytes of 1LA and 6LA animals in the incorporation of radioactive mannose and glucosamine into the glycoprotein of the whole cells. It was concluded that the decreased transfer of sialic acid to glycoproteins of platelets and megakaryocyte of animals on the 1LA diet was due to the decreased availability of sialyl acceptor. The formation of N-linked oligosaccharide was the same in both 1LA and 6LA megakaryocytes, and thus any differences in phosphodolichol-mediated N-glycosylation did not account for this decreased availability of sialyl acceptor.

Galactosyltransferase activities in mitochondria outer membrane: biosynthesis of galactosylated proteins

1. Mitochondria outer membranes prepared from mouse livers were purified on a discontinuous sucrose gradient. Control in electron microscopy and marker enzymes assays confirmed purity and homogeneity of this fraction. 2. Purified mitochondria outer membranes exhibited significant UDP-galactose: glycoprotein galactosyltransferase activities when incubated with endogenous or exogenous glycoprotein acceptors in presence of detergent (Nonidet P40). 3. Some properties of two distinct mitochondrial galactosyltransferases, acting respectively on ovomucoid and ovine asialo-mucin were investigated. 4. Transfer of galactose on ovomucoid was maximal for a pH of 7.6 at 33 degrees C whereas asialo-mucin galactosyltransferase exhibited an optimum pH of 5.6 for an optimal temperature of 46 degrees C. 5. These two distinct membrane-bound enzymes were both inhibited by diacylglycerophospholipids whereas lysophospholipids modulated both enzymes in a different way: at 5 mM lysophosphatidylcholine, asialo-mucin galactosyltransferase was slightly stimulated while ovomucoid galactosyltransferase was markedly activated. 6. The most important activating effect on ovomucoid galactosyltransferase was obtained with a phospholipid containing a long aliphatic side chain linked by an ester bond in sn-1 of glycerol, an hydroxyl group or hydrogen atoms in sn-2 and a phosphorylcholine head group in sn-3.

Study of the N-glycoprotein biosynthesis through dolichol intermediates in the mitochondrial membranes

1. In the mitochondria, the biosynthesis of N-glycoprotein products, through the dolichol intermediates pathway, appears in the outer and in the inner membranes. 2. The biosynthesis of dolichol-pyrophosphoryl-N-acetyl-glucosamine, dolichol-pyrophosphoryl-di-N-acetylchitobiose, dolichol-phosphoryl-glucose and dolichol-phosphoryl-mannose is effective in both membranes. 3. The lipid-linked oligosaccharides biosynthesized in both membranes contain high mannose-type oligosaccharides ranging in size from Man9-GlcNac2 to Man4-GlcNac2. 4. The assembly of the dolichol-pyrophosphoryl-oligosaccharides on the trimannosidic core begins by the elongation of the alpha-1,3 mannose branch in the outer membrane and of the alpha-1,6 mannose branch in the inner membrane.

Characterization and partial purification of two enzymes transferring N-acetylglucosamine to dolichyl monophosphate and ribonuclease A

Two N-acetylglucosamine (GlcNAc) transferases which catalyze the incorporation of GlcNAc into GlcNAc-P-P-dolichol (dolichol enzyme) and into bovine pancreatic ribonuclease A (RNAseA enzyme) were solubilized from the rat liver microsomes in a non-ionic detergent, Triton X-100. Both enzyme activities were adsorbed on activated CH-Sepharose 4B, and could be eluted with a linear KCl gradient. Two enzyme activities were separated by this column with the dolichol enzyme eluting before the RNAseA enzyme. A 49-fold and 136-fold purification was achieved for the dolichol and the RNAseA enzyme, respectively. The addition of exogeneous dolichyl phosphate resulted in a 3-5-fold stimulation of the purified dolichol enzyme, but did not affect the purified RNAseA enzyme. The addition of RNAseA stimulated only the RNAseA enzyme. Whereas, tunicamycin could inhibit only the dolichol enzyme. The purified dolichol enzyme had a Km of 14 X 10(-6) M for UDP-GlcNAc and the reaction was saturated with about 0.25 M dolichyl phosphate. The purified RNAseA enzyme had a Km of 4.55 X 10(-6) M for UDP-GlcNAc and was saturated with about 0.36 mM RNAseA. The pH optima and the metal ion requirement for the two enzymes were different. These results suggest that because of the different properties of these two enzymes they may have distinct functions regarding the core glycosylation of N-linked glycoproteins. It is well established that the dolichol enzyme catalyzes the formation of the first dolichol-linked intermediate GlcNAc-P-P-dolichol, whereas according to the present finding, the RNAseA enzyme may catalyze the transfer of GlcNAc directly from UDP-GlcNAc into acceptor protein.

Effect of dexamethasone on mannolipid synthesis by hepatocytes prepared from control and inflamed rats

Hepatocytes were prepared from control and inflamed rats. Mannose incorporation into dolichol monophosphate mannose in homogenate and microsomal fraction of the hepatocytes was increased 2-fold over the controls 24 h after induction of inflammation by turpentine injection. Incubation of hepatocytes from both control and inflamed rats with 0.1-10 microM-dexamethasone produced a 1.5-fold increase of dolichol phosphate mannose formation, whereas, 100 microM-dexamethasone decreased its formation. The increase in the ratio of dolichol phosphate mannose formation in inflamed over controls was virtually eliminated when the cell homogenate assay mixtures included 30 nmol of exogenous dolichol phosphate. This supports the earlier suggestion that the increase in the enzyme activity in inflammation could be due to higher concentrations of endogenous dolichol phosphate [ Coolbear & Mookerjea (1981) J. Biol. Chem. 256, 4529-4535]. In contrast, the increase in the ratio of dolichol phosphate mannose formation between dexamethasone-treated and untreated hepatocytes remained unchanged when increasing concentrations of exogenous dolichol phosphate were added to the assays. This suggests that the increase in glycosylation of dolichol phosphate in dexamethasone-treated hepatocytes is probably due to the increased mannosyltransferase activity, rather than due to higher concentrations of endogenous dolichol phosphate in these cells.

Intramembranous arrangement of the glycosylating systems in rough and smooth microsomes from rat liver

The distribution of mannosyl-, glucosaminyl- and glucosyltransferases in rough and smooth microsomes isolated from rat liver homogenate has been investigated. Amphomycin and tunicamycin were used as inhibitors of dolichol-mediated glycosylation, and diazobenzene sulfonate and proteolytic enzymes were used as nonpenetrating surface probes. Under in vitro conditions only 20-30% of the proteins glycosylated are of the secretory type. Nonpenetrating surface probes, which interact with components on the outer surface of rough microsomal vesicles, decrease glycosylation of both secretory and membrane proteins to a great extent. Inhibitor sensitive glycosylation is present in both the outer and inner compartments of the microsomal membranes. In contrast, the surface probes and the inhibitors of dolichol-mediated glycosylation do not significantly affect protein glycosylation in smooth microsomes. When dolichol phosphate sugars were used as substrates, instead of nucleotide sugars, the probes used inhibited protein glycosylation in both subfractions. Glycosylation of externally added Lipidex-bound dolichol monophosphate and of ovalbumin were in agreement with the above results. It appears that both rough and smooth microsomes may possess several types of glycosylating pathways. The most prominent of these in rough microsomes under the conditions used is the dolichol mono- and pyrophosphate-mediated glycosylation of endogenous proteins, where the enzymes involved in the initial steps are distributed at the outer surfaces of the microsomal vesicles. The dominating pathway in smooth microsomes appears to function in completion of the oligosaccharide chain of the protein and this process does not involve lipid intermediates and cannot be influenced by nonpenetrating surface probes.

Identification and characterisation of two N-acetylglucosaminyltransferases associated with Trypanosoma Brucei microsomes

Microsomes forom Trypanosoma brucei contain glycosyltransferases able to incorporate N-[14C]acetylglucosamine into two different types of acceptors. A first transferase catalyzes the transfer of N-[14C]acetylglucosamine 1-phosphate from uridine diphosphate N-[14C]acetylglucosamine into dolichol monophosphate. The enzymatic activity requires Mn2+, is time and temperature dependent, has an optimum pH of 7.4 and is completely inhibited by the antibiotic tunicamycin. Exogenous dolichol monophosphate enhances the glycosyltransferase activity. The kinetics of incorporation are characterized by a Km of 2.6 microM for uridine diphosphate N-acetylglucosaminyltransferase are comparable to those reported for the first enzyme of the dolichol cycle described in several eukaryotes. N-Acetyl-glucosaminylpyrophosphoryl-dolichol is essentially the only product of the reaction. A second type of activity which is responsible for the direct transfer of N[14C]acetylglucosamine from uridine diphosphate N[14C]acetylglucosamine into several endogenous polypeptide acceptors, is also associated with T. brucei microsomes. The reaction, which might be due to more than one enzyme, is dependent on Mn2+, but differs from the other transferase in all other characteristics. Time course and optimal temperature are different, and the optimum pH is 6.5. The reaction is independent of the external addition of dolichol monophosphate and tunicamy cin has no inhibitory effect on the enzymatic activity. AKm of 1.6 microM was calculated fr uridine diphosphate N-acetylglucosamine.

Quantitative assay and subcellular distribution of enzymes acting on dolichyl phosphate in rat liver

To establish on a quantitative basis the subcellular distribution of the enzymes that glycosylate dolichyl phosphate in rat liver, preliminary kinetic studies on the transfer of mannose, glucose, and N-acetylglucosamine-1-phosphate from the respective (14)C- labeled nucleotide sugars to exogenous dolichyl phosphate were conducted in liver microsomes. Mannosyltransferase, glucosyltransferase, and, to a lesser extent, N- acetylglucosamine-phosphotransferase were found to be very unstable at 37 degrees C in the presence of Triton X-100, which was nevertheless required to disperse the membranes and the lipid acceptor in the aqueous reaction medium. The enzymes became fairly stable in the range of 10-17 degrees C and the reactions then proceeded at a constant velocity for at least 15 min. Conditions under which the reaction products are formed in amount proportional to that of microsomes added are described. For N- acetylglucosaminephosphotransferase it was necessary to supplement the incubation medium with microsomal lipids. Subsequently, liver homogenates were fractionated by differential centrifugation, and the microsome fraction, which contained the bulk of the enzymes glycosylating dolichyl phosphate, was analyzed by isopycnic centrifugation in a sucrose gradient without any previous treatment, or after addition of digitonin. The centrifugation behavior of these enzymes was compared to that of a number of reference enzymes for the endoplasmic reticulum, the golgi complex, the plasma membranes, and mitochondria. It was very simily to that of enzymes of the endoplasmic reticulum, especially glucose-6-phosphatase. Subcellular preparations enriched in golgi complex elements, plasma membranes, outer membranes of mitochondira, or mitoplasts showed for the transferases acting on dolichyl phosphate relative activities similar to that of glucose- 6-phosphatase. It is concluded that glycosylations of dolichyl phosphate into mannose, glucose, and N-acetylglucosamine-1-phosphate derivatives is restricted to the endoplasmic reticulum in liver cells, and that the enzymes involved are similarly active in the smooth and in the rough elements.

Dolichyl phosphate phosphatase from Tetrahymena pyriformis

A soluble dolichyl phosphate phosphatase from Tetrahymena pyriformis was purified about 68-fold. The enzyme appeared to be specific for dolichyl phosphate and existed in two interrelated forms, one of mol.wt. about 500000 and the other of mol.wt. about 63000. The enzyme was strongly inhibited by 5 mM-Mn2+ and was strongly stimulated by Mg2+. Tetrahymena in the exponential growth phase contained more of this enzymic activity than cells in stationary or lag phase. The dolichyl phosphate phosphatase may be loosely bound to mitochondrial membranes. Two roles proposed for this enzyme are (1) that of releasing dolichol from its phosphorylated biosynthetic form for its use in the cell as unesterified dolichol or dolichyl ester and/or (2) that of regulation of synthesis of glycoproteins or some other glycosylated compound.

Steps in the biosynthesis of mosquito cell membrane glycoproteins and the effects of tunicamycin

A cultured cell line of the mosquito, Aedes aegypti, is sensitive to tunicamycin as expected from the ability of crude membrane preparations to catalyse the formation of N-acetylglucosamine-linked dolichyl pyrophosphate. Formation of dolichylphosphomannose was also detected and this reaction was totally insensitive to tunicamycin. Incorporation of radioactive mannose into total acid-precipitable glycoproteins was inhibited greater than 90% in whole cells by tunicamycin, while the incorporation of leucine and glucosamine was less affected. Separation of the radioactive hexosamines from acid hydrolysates of cells incubated with [14C]glucosamine and tunicamycin showed predominant labelling of galactosamine, whereas in control cells not treated with the drug both glucosamine and galactosamine were labelled equally. Evidently, mosquito cells synthesise N-glycosidically linked carbohydrate chains assembled through tunicamycin-sensitive steps involving dolichyl pyrophospho-oligosaccharides, and O-glycosidically linked chains rich in N-acetylgalactosamine, the assembly of which is unaffected by tunicamycin. These results support structural evidence (Butters, T.D. and Hughes, R.C. (1981) Biochim. Biophys. Acta 640, 655-671) for the presence of high mannose N-glycans and N-acetylgalactosamine-rich O-glycans in mosquito cell glycoproteins. The absence of complex N-glycans was confirmed by the demonstration of negligible activities of N-acetylglucosaminyl-, galactosyl- and sialyltransferases responsible for assembly of the terminal sequences of N-glycans of mature mammalian glycoproteins.