Spatial aspects of mannosyl phosphoryl retinol formation

Rat liver microsomes catalyze the transfer of mannose from GDPmannose to both retinyl phosphate and dolichyl phosphate to form mannosylphosphorylretinol, mannosylphosphoryldolichol and GDP. The two reactions differ in term of reversibility. In fact, a 200-fold isotopic dilution of GDP[14C]mannose by unlabeled GDPmannose causes mannosylphosphoryldolichol labeling to disappear almost completely, while mannosylphosphorylretinol labeling remains at the same level. The same observation can be made if the mannose donor is removed by centrifugation and replaced by excess GDP; again mannosylphosphorylretinol is stable, but mannosylphosphoryldolichol drops down to one-third of its initial level, as expected for, respectively, a non-reversible and a reversible reaction. Placed in an aqueous medium, mannosylphosphorylretinol releases mannose 1-phosphate (beta configuration) whereas it is quite stable when kept in a membranous environment. These results strongly suggest that mannosylphosphorylretinol as soon as it is formed is segregated in such a way that it is no longer available to the back-reaction; the functional consequence of this segregation would be the possibility for mannosylphosphorylretinol to mannosylate some non-polar regions of certain protein chains.     

Mannosylation of endogenous and exogenous phosphatidic acid by liver microsomal membranes. Formation of phosphatidylmannose

Hamster liver post-nuclear membranes catalyze the transfer of mannose from GDP-mannose to endogenous dolichyl phosphate and to a second major endogenous acidic lipid. This mannolipid was believed to be synthesized from endogenous retinyl phosphate and was tentatively identified as retinyl phosphate mannose (Ret-P-Man) (De Luca, L. M., Brugh, M. R. Silverman-Jones, C. S. and Shidoji, Y. (1982) Biochem. J. 208, 159-170). To characterize this endogenous mannolipid in more detail, we isolated and purified the mannolipid from incubations containing hamster liver membranes and GDP-[14C]mannose and compared its properties to those of authentic Ret-P-Man. We found that the endogenous mannolipid was separable from authentic Ret-P-Man on a Mono Q anion exchange column, did not exhibit the absorbance spectrum characteristic of a retinol moiety, and was stable to mild acid under conditions which cleave authentic Ret-P-Man. The endogenous mannolipid was sensitive to mild base hydrolysis and mannose was released from the mannolipid by snake venom phosphodiesterase digestion. These properties were consistent with the endogenous acceptor being phosphatidic acid. Addition of exogenous phosphatidic acid, but not phospholipids with a head group blocking the phosphate moiety, to incubations containing hamster liver membranes and GDP-[14C]mannose resulted in the synthesis of a mannolipid with chromatographic and physical properties identical to the endogenous mannolipid. A double-labeled mannolipid was synthesized in incubations containing hamster liver membranes, GDP-[14C]mannose, and [3H]phosphatidic acid. Mannosyl transfer to exogenous phosphatidic acid was saturable with increasing concentrations of phosphatidic acid and GDP-mannose and specific for glycosyl transfer from GDP-mannose. Class E Thy-1-negative mutant mouse lymphoma cell membranes, which are defective in dolichyl phosphate mannose synthesis, also fail to transfer mannose from GDP-mannose to exogenous phosphatidic acid or retinyl phosphate. Amphomycin, an inhibitor of dolichyl phosphate mannose synthesis, blocked mannosyl transfer to the endogenous lipid, and to exogenous retinyl phosphate and phosphatidic acid. We conclude that the same mannosyltransferase responsible for dolichyl phosphate mannose synthesis can also utilize in vitro exogenous retinyl phosphate and phosphatidic acid as well as endogenous phosphatidic acid as mannosyl acceptors.     

Control of carbohydrate processing: increased beta-1,6 branching in N-linked carbohydrates of Lec9 CHO mutants appears to arise from a defect in oligosaccharide-dolichol biosynthesis.

A correlation between increased beta-1,6 branching of N-linked carbohydrates and the ability of a cell to metastasize or to form a tumor has been observed in several experimental models. Lec9 Chinese hamster ovary (CHO) mutants exhibit a drastic reduction in tumorigenicity in nude mice, and this phenotype directly correlates with their ability to attach an increased proportion of beta-1,6-branched carbohydrates to the G glycoprotein of vesicular stomatitis virus (J. Ripka, S. Shin, and P. Stanley, Mol. Cell. Biol. 6:1268-1275, 1986). In this paper we provide evidence that cellular carbohydrates from Lec9 cells also contain an increased proportion of beta-1,6-branched carbohydrates, although they do not possess significantly increased activity of the beta-1,6 branching enzyme (GlcNAc-transferase V). Biosynthetic labeling experiments show that a substantial degree of underglycosylation occurs in Lec9 cells and that this affects several classes of glycoproteins. Lec9 cells synthesize ca. 40-fold less Glc3Man9GlcNAc2-P-P-lipid and ca. 2-fold less Man5GlcNAc2-P-P-lipid than parental cells do. In addition, Lec9 cells possess ca. fivefold less protein-bound oligosaccharide intermediates, and one major species is resistant to release by endo-beta-N-acetylglucosaminidase H (endo H). Membranes of Lec9 cells exhibit normal mannosylphosphoryldolichol synthase, glucosylphosphoryldolichol synthase, and N-acetylglucosaminylphosphate transferase activities in the presence of exogenous dolichyl phosphate. However, in the absence of exogenous dolichyl phosphate, mannosylphosphoryldolichol synthase and glucosylphosphoryldolichol synthase activities are reduced in membranes of Lec9 cells, indicating that membranes of Lec9 cells are deficient in lipid phosphate. This was confirmed by analysis of lipids labeled by [3H]mevalonate, which showed that Lec9 cells have less lipid phosphate than parental CHO cells. Mechanisms by which a defect in the synthesis of dolichol-oligosaccharides might alter the degree of beta-1,6 branching in N-linked carbohydrates are discussed.     

Regulation of glycosylation. Three enzymes compete for a common pool of dolichyl phosphate in vivo

We examined changes in the levels of the dolichol forms in Chinese hamster ovary cells containing alterations in the levels of activity of two enzymes in the oligosaccharyl-P-P-dolichol biosynthetic pathway, namely UDP-GlcNAc:dolichyl phosphate:GlcNAc-phosphotransferase (GlcNAc-1-phosphotransferase) and mannosylphosphoryldolichol (Man-P-Dol) synthase. Under normal conditions in wild type cells, Glc3Man9GlcNAc2-pyrophosphoryldolichol was the most abundant form. Of the other anionic forms of dolichols, dolichyl phosphate, Man-P-Dol, glucosylphosphoryldolichol, and Man5GlcNAc2-pyrophosphoryl dolichol were approximately equally abundant. When 3E11 cells (a tunicamycin-resistant Chinese hamster ovary line containing 15 times more GlcNAc-1-phosphotransferase activity than wild type), B4-2-1 cells (a mutant lacking Man-P-Dol synthase activity), and wild type cells incubated with or without tunicamycin were utilized, significant changes in the levels of most of the anionic dolichol derivatives, with the exception of dolichyl phosphate, were found. Since changes in dolichyl phosphate levels were not detected under a variety of conditions where the levels of enzyme activity utilizing this substrate were varied, all three enzymes appear to have access to the same pool of dolichyl phosphate, and further, to have similar Km values for dolichyl phosphate.     

Mannosyl carrier functions of retinyl phosphate and dolichyl phosphate in rat liver endoplasmic reticulum

Of the subcellular fractions of rat liver the endoplasmic reticulum was the most active in GDP-mannose: retinyl phosphate mannosyl-transfer activity. The synthesis of retinyl phosphate mannose reached a maximum at 20-30 min of incubation and declined at later times. Retinyl phosphate mannose and dolichyl phosphate mannose from endogenous retinyl phosphate and dolichyl phosphate could also be assayed in the endoplasmic reticulum. About 1.8 ng (5 pmol) of endogenous retinyl phosphate was mannosylated per mg of endoplasmic reticulum protein (15 min at 37 degrees C, in the presence of 5 mM-MnCl2), and about 0.15 ng (0.41 pmol) of endogenous retinyl phosphate was mannosylated with Golgi-apparatus membranes. About 20 ng (13.4 pmol) of endogenous dolichyl phosphate was mannosylated in endoplasmic reticulum and 4.5 ng (3 pmol) in Golgi apparatus under these conditions. Endoplasmic reticulum, but not Golgi-apparatus membranes, catalysed significant transfer of [14C]mannose to endogenous acceptor proteins in the presence of exogenous retinyl phosphate. Mannosylation of endogenous acceptors in the presence of exogenous dolichyl phosphate required the presence of Triton X-100 and could not be detected when dolichyl phosphate was solubilized in liposomes. Dolichyl phosphate mainly stimulated the incorporation of mannose into the lipid-oligosaccharide-containing fraction, whereas retinyl phosphate transferred mannose directly to protein.     

Rat liver microsomes catalyse mannosyl transfer from GDP-d-mannose to retinyl phosphate with high efficiency in the absence of detergents

In the absence of detergent, the transfer of mannose from GDP-mannose to rat liver microsomal vesicles was highly stimulated by exogenous retinyl phosphate in incubations containing bovine serum albumin, as measured in a filter binding assay. Under these conditions 65% of mannose 6-phosphatase activity was latent. The transfer process was linear with time up to 5min and with protein concentration up to 1.5mg/0.2ml. It was also temperature-dependent. The microsomal uptake of mannose was highly dependent on retinyl phosphate and was saturable against increasing amounts of retinyl phosphate, a concentration of 15mum giving half-maximal transfer. The uptake system was also saturated by increasing concentrations of GDP-mannose, with an apparent K(m) of 18mum. Neither exogenous dolichyl phosphate nor non-phosphorylated retinoids were active in this process in the absence of detergent. Phosphatidylethanolamine and synthetic dipalmitoylglycerophosphocholine were also without activity. Several water-soluble organic phosphates (1.5mm), such as phenyl phosphate, 4-nitrophenyl phosphate, phosphoserine and phosphocholine, did not inhibit the retinyl phosphate-stimulated mannosyl transfer to microsomes. This mannosyl-transfer activity was highest in microsomes and marginal in mitochondria, plasma and nuclear membranes. It was specific for mannose residues from GDP-mannose and did not occur with UDP-[(3)H]galactose, UDP- or GDP-[(14)C]glucose, UDP-N-acetyl[(14)C]-glucosamine and UDP-N-acetyl[(14)C]galactosamine, all at 24mum. The mannosyl transfer was inhibited 85% by 3mm-EDTA and 93% by 0.8mm-amphomycin. At 2min, 90% of the radioactivity retained on the filter could be extracted with chloroform/methanol (2:1, v/v) and mainly co-migrated with retinyl phosphate mannose by t.l.c. This mannolipid was shown to bind to immunoglobulin G fraction of anti-(vitamin A) serum and was displaced by a large excess of retinoic acid, thus confirming the presence of the beta-ionone ring in the mannolipid. The amount of retinyl phosphate mannose formed in the bovine serum albumin/retinyl phosphate incubation is about 100-fold greater than in incubations containing 0.5% Triton X-100. In contrast with the lack of activity as a mannosyl acceptor for exogenous dolichyl phosphate in the present assay system, endogenous dolichyl phosphate clearly functions as an acceptor. Moreover in the same incubations a mannolipid with chromatographic properties of retinyl phosphate mannose was also synthesized from endogenous lipid acceptor. The biosynthesis of this mannolipid (retinyl phosphate mannose) was optimal at MnCl(2) concentrations between 5 and 10mm and could not be detected below 0.6mm-MnCl(2), when synthesis of dolichyl phosphate mannose from endogenous dolichyl phosphate was about 80% of optimal synthesis. Under optimal conditions (5mm-MnCl(2)) endogenous retinyl phosphate mannose represented about 20% of dolichyl phosphate mannose at 15min of incubation at 37 degrees C.     

A mutant of Chinese hamster ovary cells with a reduction in levels of dolichyl phosphate available for glycosylation

F2A8, a glycosylation mutant of Chinese hamster ovary cells, was isolated without prior enrichment or selective procedures by screening colonies for reduced [3H]mannose incorporation into macromolecules. F2A8 cells incubated with [3H]mannose synthesized 70% the amount of labeled GDP-mannose found in parental cells, and the same oligosaccharides attached to lipid and protein as did parental cells, but in reduced amounts. Incorporation of radioactivity from labeled mannose into saccharide-lipids and into total glycopeptides of F2A8 was reduced 7-fold compared to parental cells. In addition, glycosylation of the vesicular stomatitis virus glycoprotein was reduced in F2A8 cells as assessed by a mobility intermediate between normally glycosylated and unglycosylated protein during sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In vitro assays using membrane preparations showed that F2A8 had parental levels of glucosylphosphoryldolichol synthase and of UDP-GlcNAc:dolichyl phosphate:GlcNAc-phosphotransferase when the enzymatic determinations were done in the presence of exogenous dolichyl phosphate. However, 5-fold less glucosylphosphoryldolichol synthase activity was detected in membranes of F2A8 compared to membranes of parental cells in assays relying on endogenous lipid substrate. F2A8 appears to have reduced amounts of dolichyl phosphate available for its glycosylation reactions.     

Dolichol metabolism in Chinese hamster ovary cells.

The addition of oligosaccharide to asparagine residues of soluble and membrane-associated proteins in eukaryotic cells involves a polyisoprenoid lipid carrier, dolichol. In Chinese hamster ovary cells, the major isomer of this polyisoprenol has 19 isoprenyl units, the terminal one being saturated. Our laboratory has developed a procedure to analyze the levels and nature of the cell's dolichyl derivatives. Chinese hamster ovary cells contain predominately activated, anionic dolichol derivatives, such as oligosaccharyl pyrophosphoryldolichol, monoglycosylated phosphoryldolichols, and dolichyl phosphate. Our studies show that in growing cells there is continual synthesis of total dolichol. Also, preliminary data suggest there is no catabolism or secretion of this lipid. The level of dolichyl phosphate did not change significantly under a variety of conditions where the levels of enzyme activities utilizing dolichyl phosphate did change. These results suggested that these enzymes had access to the same pool of dolichyl phosphate and had similar Km values for this lipid.     

The role of vitamin A in the glycosylation reactions of glycoprotein synthesis in an ''in vitro'' system.

Microsomal membrane preparations from rat livers, when incubated with labelled sugar-nucleotides, were shown to synthesize labelled oligosaccharide-lipids in the presence of excess exogenous dolichyl phosphate. Under the incubation conditions defined in the present study, dolichyl pyrophosphoryl(DolPP)GlcNAc2-Man5, DolPPGlcNAc2Man9 and DolPPGlcNAc2Man9Glc3 were the principal oligosaccharide-lipids formed by both control and vitamin A-deficient membranes. However, deficient membranes synthesized 3.2 +/- 0.8 times as much oligosaccharide-lipids and 2.6 +/- 0.7 times as much dolichyl phosphate mannose (DolPMan) and dolichyl phosphate glucose (DolPGlc) as the controls. The transfer of the oligosaccharide chain from the dolichol carrier to the endogenous protein acceptors in vitamin A-deficient microsomes (microsomal fractions) was only 57.5 +/- 9.5% of that of controls. After endo-beta-N-acetylglucosaminidase treatment, only one oligosaccharide species was isolated from both control and vitamin A-deficient microsomal glycoproteins, and was characterized as GlcNAcMan9Glc3. We conclude that the decreased incorporation of labelled mannose and glucose from sugar-nucleotides into the glycoproteins must be due to decreased transfer of GlcNAc2Man9Glc3 from the dolichol carrier to the protein acceptors. This conclusion was further substantiated by the finding that control membranes transferred 4-6 times as much labelled oligosaccharides from exogenously added dolichol-linked substrate (DolPPGlcNAc2Man9Glc3) to endogenous microsomal protein acceptors as compared with the vitamin A-deficient membranes. Attempts to reverse this defect by addition of retinol or retinyl phosphate (a source of retinyl phosphate mannose) to the incubations were unsuccessful.     

Effect of exogenous dolichyl monophosphate on a developmental change in mannosylphosphoryldolichol biosynthesis

The initial rate of mannosylphosphoryldolichol formation by pig brain white matter is 2.9 to 3.3-fold higher in membranes from actively myelinating animals as compared to similar preparations from adults. Exogenous dolichyl monophosphate stimulated mannolipid synthesis in both preparations indicating that the level of the acceptor lipid was rate-limiting. The relative enhancement, however, was higher in membranes from adult animals reducing the ratio of initial rates for young/adult. Exogenous dolichyl monophosphate also stimulated the labeling of a mannosylated oligosaccharide lipid and mannoproteins, including a polypeptide (apparent molecular weight of 100,000) not labeled by gray matter membranes.     

Characterization of three kinetically distinct forms of glutamate decarboxylase from pig brain.

Hamster liver microsomal membranes catalyse the synthesis of retinyl phosphate mannose (Ret-P-Man) from GDP-mannose and exogenous retinyl phosphate (Ret-P). We have previously shown that maximal Ret-P-Man synthesis occurs in vitro at 20-30 min, followed by a subsequent loss of mannose from Ret-P-Man, suggestive of an intermediary function of Ret-P-Man and/or Ret-P-Man breakdown [Shidoji, Silverman-Jones & De Luca (1982) Biochem. J. 208, 865-868; Creek, Morre, Silverman-Jones, Shidoji & De Luca (1983) Biochem. J. 210, 541-547). To monitor Ret-P-Man synthesis and breakdown carefully, we developed a chromatographic system in which mannose, Ret-P-Man, mannose phosphate and GDP-mannose are separated in a single analysis on a Mono Q column eluted with a gradient of NaCl. Using this chromatographic system, we have determined that 80-90% of the Ret-P-Man made in vitro by hamster liver membranes in 30 min is recovered with the membranes upon centrifugation. Subsequent incubation of Ret-P-Man-loaded membranes at 37 degrees C results in a non-enzymic breakdown of Ret-P-Man to beta-mannopyranosyl phosphate and anhydroretinol. However, incubation of the Ret-P-Man-loaded hamster liver membranes with GDP, but not GMP, ADP, CDP or UDP, results in a loss of mannose from Ret-P-Man and the formation of GDP-mannose and Ret-P. These results demonstrate that Ret-P-Man synthesized in vitro is subject to non-enzymic breakdown to beta-mannopyranosyl phosphate and anhydroretinol and that the GDP-mannose:retinyl phosphate mannosyltransferase reaction is reversible.     

A clonal derivative of tunicamycin-resistant Chinese hamster ovary cells with increased N-acetylglucosamine-phosphate transferase activity has altered asparagine-linked glycosylation

A population of Chinese hamster ovary (CHO) cells resistant to the antibiotic tunicamycin (TM) had previously been isolated (Criscuolo, B.A., and Krag, S.S. (1982) J. Cell Biol. 94:586-591) by a stepwise selection procedure using progressive increments of TM added to the medium. TM inhibits asparagine-linked glycoprotein biosynthesis by blocking the transfer of N-acetylglucosamine-1-phosphate from the sugar nucleotide UDP-N-acetylglucosamine to the isoprenoid lipid carrier, dolichyl phosphate. Four clonal derivatives were isolated from the TM-resistant population in the presence of 27 micrograms TM/ml and were found to overproduce the N-acetylglucosamine-phosphate transferase activity to the same extent (approximately 15-fold compared to wild-type cells). One of these clones, 3E11, was greater than 550-fold more resistant to TM than wild-type cells. The resistance phenotype remained during at least 2.5 months of growth in the absence of TM. 3E11 cells exhibited chromosomal translocations, but no homogeneously staining regions (HSR) or double minute chromosomes. The N-acetylglucosamine-phosphate transferase activity in 3E11 cells was membrane-associated and was inhibited by TM. A 140,000-dalton membrane protein and at least four other membrane proteins were enriched in 3E11 cells. Mannosylphosphoryldolichol synthase and glucosylphosphoryldolichol synthase activities were not elevated in membranes prepared from 3E11 cells. Asparagine-linked glycosylation was altered such that 3E11 cells synthesized primarily a truncated oligosaccharide, Man5GlcNAc2, perhaps due to the reduced amount of mannosylphosphoryldolichol relative to wild-type cells.     

A developmental change in dolichyl phosphate mannose synthase activity in pig brain

The initial rate of dolichyl phosphate mannose biosynthesis was measured in white-matter membranes from pig brain at various ages from before birth throughout the period of most rapid brain development. Dolichyl phosphate mannose synthase activity increased from prenatal values to a maximum in 3 week-old animals, and gradually decreased to adult values after 8 weeks of age. The nature of the developmental change was investigated by enzymic and biochemical comparisons of the membrane preparations from the most active age (3 weeks) and adult controls. The specific activity of dolichyl phosphate mannose synthase in preparations from actively myelinating animals was approx. 3-fold higher than adults when mannolipid formation was assayed with saturating concentrations of GDP-[¹⁴C]mannose and utilizing only endogenous acceptor lipid. No major variations were found in the apparent K_m values for GDP-mannose or exogenous dolichyl monophosphate. However, the ratio of dolichyl phosphate mannose synthase activity for myelinating animals/adult animals decreased significantly when large amounts of exogenous dolichyl monophosphate were added to the incubation mixtures. Dolichyl phosphate mannose synthase activity was also compared in white-matter membranes depleted of endogenous dolichyl monophosphate by enzymic mannosylation or treatment with butanol. When these preparations were assayed with identical amounts of exogenous dolichyl monophosphate, the dolichyl monophosphate-depleted membranes from actively myelinating animals contained only 20–30% more dolichyl phosphate mannose synthase activity. Overall, these studies strongly suggest that the developmental change in dolichyl phosphate mannose synthase activity is due primarily to the presence of a relatively lower amount of endogenous dolichyl monophosphate being accessible to the mannosyltransferase in the white-matter membranes from adult animals.     

Inhibition of lipid-linked oligosaccharide synthesis by aryl phosphates.

Our previous work has shown that phenyl phosphate acts as an exogenous substrate for GDP-mannose:dolichyl phosphate mannosyltransferase in rat liver microsomal fractions to give rise to phenyl phosphate beta-D-mannose, a compound which, unlike Dol-P-Man (dolichyl phosphate beta-D-mannose), cannot act as mannose donor for further mannose-adding reactions in microsomal fractions. The study has now been extended to the action of various aryl phosphates and structurally related compounds on several other glycosyltransferase systems in the microsomal fractions. (1) Examination of the ability of these compounds to accept sugars from various sugar nucleotides indicated that the individual compounds have specificity as sugar acceptors. Thus phenyl phosphate acted as an effective acceptor for both mannose and glucose, whereas benzenephosphonic acid was active only in accepting mannose. p-Nitrophenyl phosphate was a more effective glucose acceptor than phenyl phosphate, but had only 8% of the mannose-accepting activity of phenyl phosphate. (2) Phenyl phosphate had an inhibitory effect on the transfer of mannose form GDP-mannose to lipid-linked oligosaccharides and to glycoproteins in rat liver microsomal fractions. The inhibition depended on the concentration of phenyl phosphate and on the extent of inhibition of Dol-P-Man synthesis. It is proposed that phenyl phosphate has a direct effect on the synthesis of Dol-P-Man and that its inhibition of synthesis of lipid-linked oligosaccharides and glycoproteins could be a consequence of this effect.     

Relationships among dolichyl phosphate, glycoprotein synthesis, and cell culture growth

Following treatment of Chinese hamster ovary cells with inhibitors of mevalonate biosynthesis in the presence of exogenous cholesterol, the cellular concentration of phosphorylated dolichol and the incorporation of [3H]mannose into dolichol-linked saccharides and N-linked glycoproteins declined coincident with a decline in DNA synthesis. Addition of mevalonate to the culture medium increased rates of mannose incorporation into lipid-linked saccharides and restored mannose incorporation into N-linked glycoproteins to control levels within 4 h. After an additional 4 h, synchronized DNA synthesis began. Inhibition of the synthesis of lipid-linked oligosaccharides and N-linked glycoproteins by tunicamycin prevented the induction of DNA synthesis by mevalonate, indicating that glycoprotein synthesis was required for cell division. The results suggest that the rate of cell culture growth may be influenced by the level of dolichyl phosphate acting to limit the synthesis of N-linked glycoproteins.     

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.     

A concanavalin A-resistant Chinese hamster ovary cell line is deficient in the synthesis of [3H]glucosyl oligosaccharide-lipid.

In this report we present an initial determination of the biochemical defect present in a Chinese hamster ovary cell line selected for resistance to concanavalin A. Membranes of this mutant, B211, incorporated at least 10-fold less mannose from GDP-[14C]mannose into oligosaccharide-lipid than membranes of the wild type. In the presence of dolichol phosphate, membranes of the mutant and wild type exhibited similar rates of synthesis of number of early intermediates, namely, mannosylphosphoryldolichol, N-acetylglucosaminyl- and N,N'-diacetylchitobiosylpyrophosphoryldolichol, glucosylphosphoryldolichol, and mannosyloligosaccharide-lipid. The membranes of B211 did not incorporate glucose from UDP-[3H]glucose into oligosaccharide-lipid or protein. Comparison by gel filtration chromatography of oligosaccharides derived from the oligosaccharide-lipids of B211 and wild type cells labeled with [2-3H]mannose revealed that B211 cells incorporated little if any label into an oligosaccharide corresponding to the most excluded oligosaccharide labeled by wild type cells. This concanavalin A-resistant cell line appears to lack the ability to glucosylate oligosaccharide-lipid.     

EVIDENCE FOR THE BIOSYNTHESIS OF MANNOSYLPHOSPHORYLDOLICHOL AND N-ACETYLGLUCOSAMINYLPYROPHOSPHORYLDOLICHOL BY AN AXOLEMMA-ENRICHED MEMBRANE PREPARATION FROM BOVINE WHITE MATTER

An axolemma-enriched membrane fraction prepared by an improved procedure from bovine white matter catalyzes the enzymatic transfer of [¹⁴C]mannose and N-acetyl[¹⁴C]glucosamine from their nucleotide derivatives into a mannolipid and an N-acetylglucosaminyl lipid in the presence of exogenous dolichyl monophosphate. The labeled glycolipid products have the chemical and chromatographic characteristics of mannosylphosphoryldolichol and N-acetylglucosaminylpyrophosphoryldolichol. The initial rates of synthesis of the glycolipids by the axolemma-enriched membrane fraction have been compared with the initial rates of glycolipid formation catalyzed by a microsomal preparation and myelin in the presence or absence of dolichyl monophosphate. Essentially no glycolipid synthesis was observed when either GDP-[¹⁴C]mannose or UDP-N-acetyl[¹⁴C]glucosamine were incubated with myelin in the presence or absence of exogenous dolichyl monophosphate. A comparison of the initial rates of synthesis of the glycolipids using endogenous acceptor lipid revealed that the rate of formation of mannolipid was 7 times faster for the microsomal membranes than the axolemma-enriched membranes. In the presence of an amount of dolichyl monophosphate approaching saturation the initial rate of glycolipid synthesis was markedly enhanced for both membrane preparations. However, due to a more dramatic enhancement in the axolemma-enriched membranes the initial rate of mannolipid synthesis was only approx. 2.5 times greater in the microsomal membranes. A similar observation was made when the initial rates of N-acetylglucosaminyl lipid synthesis were compared for axolemma-enriched and microsomal preparations in the presence and absence of exogenous dolichyl monophosphate. These studies indicate that the axolemma-enriched membranes have a relatively lower content of dolichyl monophosphate than the microsomal membranes although the difference in the amount of mannosyltransferase is only two to three-fold lower. The presence of a sugar nucleotide pyrophosphatase activity capable of degrading GDP-mannose and UDP-N-acetylglucosamine has also been demonstrated in the axolemma-enriched membrane fraction.     

Biosynthesis of lipid-linked oligosaccharides by microsomes from virus induced Hepatoma MC-29. Dependence of some glycosyl transferases on dolichyl phosphates of different chain length

Dolichyl phosphates of various chain length ranging from 7 to 22 isoprene units were tested as lipid acceptors in transglycosylation reactions in chicken liver and Hepatoma MC-29. In the presence of exogenous dolichyl phosphate mixture (18 and 19 isoprene units) the synthesis of dolichyl pyrophosphate N-acetylglucosamine and dolichyl phosphate mannose increased 3 times both in the liver and Hepatoma MC-29, while the formation of dolichyl phosphate glucose was 4 fold higher in the liver and 6-fold higher in Hepatoma MC-29. In liver microsomes the maximum rate of the stimulation of glycosylation was achieved by exogenous dolichyl phosphates, containing 18 and 19 isoprene units, while glycosyl transferases in microsomes from Hepatoma MC-29 did not show any structural requirements to the chain length of dolichyl phosphates.     

The mechanism and regulation of dolichyl phosphate biosynthesis in rat liver

Rat liver slices were pulse labeled for 6 min with [3H]mevalonolactone and then chased for 90 min with unlabeled mevalonolactone in order to study the mechanism of dolichyl phosphate biosynthesis. The cholesterol pathway was also monitored and served to verify the pulse-chase. Under conditions in which radioactivity in the methyl sterol fraction chased to cholesterol, radioactivity in alpha-unsaturated polyprenyl (pyro)-phosphate chased almost exclusively into dolichyl (pyro)phosphate. Lesser amounts of radioactivity appeared in alpha-unsaturated polyprenol and dolichol, and neither exhibited significant decline after 90 min of incubation. The relative rates of cholesterol versus dolichyl phosphate biosynthesis were studied in rat liver under four different nutritional conditions using labeled acetate, while the absolute rates of cholesterol synthesis were determined using 3H2O. From these determinations, the absolute rates of dolichyl phosphate synthesis were calculated. The absolute rates of cholesterol synthesis were found to vary 42-fold while the absolute rates of dolichyl phosphate synthesis were unchanged. To determine the basis for this effect, the rates of synthesis of cholesterol and dolichyl phosphate were quantitated as a function of [3H]mevalonolactone concentration. Plots of nanomoles incorporated into the two lipids were nearly parallel, yielding Km values on the order of 1 mM. In addition, increasing concentrations of mevinolin yielded parallel inhibition of incorporation of [3H]acetate into cholesterol and dolichyl phosphate. The specific activity of squalene synthase in liver microsomes from rats having the highest rate of cholesterol synthesis was only 2-fold greater than in microsomes from rats having the lowest rate. Taken together, the results suggest that the maintenance of constant dolichyl phosphate synthesis under conditions of enhanced cholesterogenesis is not due to saturation of the dolichyl phosphate pathway by either farnesyl pyrophosphate or isopentenyl pyrophosphate but coordinate regulation of hydroxymethylglutaryl-CoA reductase and a reaction on the pathway from farnesyl pyrophosphate to cholesterol.