Enzymatic activities in cultured human lymphocytes that dephosphorylate dolichyl pyrophosphate and dolichyl phosphate

Genetic defects in dolichol metabolism

Congenital disorders of glycosylation (CDG) comprise a group of inborn errors of metabolism with abnormal glycosylation of proteins and lipids. Patients with defective protein N-glycosylation are identified in routine metabolic screening via analysis of serum transferrin glycosylation. Defects in the assembly of the dolichol linked Glc(3)Man(9)GlcNAc(2) glycan and its transfer to proteins lead to the (partial) absence of complete glycans on proteins. These defects are called CDG-I and are located in the endoplasmic reticulum (ER) or cytoplasm. Defects in the subsequent processing of protein bound glycans result in the presence of truncated glycans on proteins. These defects are called CDG-II and the enzymes involved are located mainly in the Golgi apparatus. In recent years, human defects have been identified in dolichol biosynthesis genes within the group of CDG-I patients. This has increased interest in dolichol metabolism, has resulted in specific recognizable clinical symptoms in CDG-I and has offered new mechanistic insights in dolichol biosynthesis. We here review its biosynthetic pathways, the clinical and biochemical phenotypes in dolichol-related CDG defects, up to the formation of dolichyl-P-mannose (Dol-P-Man), and discuss existing evidence of regulatory networks in dolichol metabolism to provide an outlook on therapeutic strategies.

Identification of phosphorylated oligosaccharides in cells of patients with a congenital disorders of glycosylation (CDG-I)

Protein N-glycosylation is initiated by the dolichol cycle in which the oligosaccharide precursor Glc(3)Man(9)GlcNAc(2)-PP-dolichol is assembled in the endoplasmic reticulum (ER). One critical step in the dolichol cycle concerns the availability of Dol-P at the cytosolic face of the ER membrane. In RFT1 cells, the lipid-linked oligosaccharide (LLO) intermediate Man(5)GlcNAc(2)-PP-Dol accumulates at the cytosolic face of the ER membrane. Since Dol-P is a rate-limiting intermediate during protein N-glycosylation, continuous accumulation of Man(5)GlcNAc(2)-PP-Dol would block the dolichol cycle. Hence, we investigated the molecular mechanisms by which accumulating Man(5)GlcNAc(2)-PP-Dol could be catabolized in RFT1 cells. On the basis of metabolic labeling experiments and in comparison to human control cells, we identified phosphorylated oligosaccharides (POS), not found in human control cells and present evidence that they originate from the accumulating LLO intermediates. In addition, POS were also detected in other CDG patients' cells accumulating specific LLO intermediates at different cellular locations. Moreover, the enzymatic activity that hydrolyses oligosaccharide-PP-Dol into POS was identified in human microsomal membranes and required Mn(2+) for optimal activity. In CDG patients' cells, we thus identified and characterized POS that could result from the catabolism of accumulating LLO intermediates.

Structure and synthesis of polyisoprenoids used in N-glycosylation across the three domains of life

N-linked protein glycosylation was originally thought to be specific to eukaryotes, but evidence of this post-translational modification has now been discovered across all domains of life: Eucarya, Bacteria, and Archaea. In all cases, the glycans are first assembled in a step-wise manner on a polyisoprenoid carrier lipid. At some stage of lipid-linked oligosaccharide synthesis, the glycan is flipped across a membrane. Subsequently, the completed glycan is transferred to specific asparagine residues on the protein of interest. Interestingly, though the N-glycosylation pathway seems to be conserved, the biosynthetic pathways of the polyisoprenoid carriers, the specific structures of the carriers, and the glycan residues added to the carriers vary widely. In this review we will elucidate how organisms in each basic domain of life synthesize the polyisoprenoids that they utilize for N-linked glycosylation and briefly discuss the subsequent modifications of the lipid to generate a lipid-linked oligosaccharide.

Identification and characterization of a cDNA encoding a dolichyl pyrophosphate phosphatase located in the endoplasmic reticulum of mammalian cells

The CWH8 gene in Saccharomyces cerevisiae has been shown recently (Fernandez, F., Rush, J. S., Toke, D. A., Han, G., Quinn, J. E., Carman, G. M., Choi, J.-Y., Voelker, D. R., Aebi, M., and Waechter, C. J. (2001) J. Biol. Chem. 276, 41455-41464) to encode a dolichyl pyrophosphate (Dol-P-P) phosphatase associated with crude microsomal fractions. Mutations in CWH8 result in the accumulation of Dol-P-P, deficiency in lipid intermediate synthesis, defective protein N-glycosylation, and a reduced growth rate. A cDNA (DOLPP1, GenBank accession number AB030189) from mouse brain encoding a homologue of the yeast CWH8 gene is now shown to complement the defects in growth and protein N-glycosylation, and to correct the accumulation of Dol-P-P in the cwh8Delta yeast mutant. Northern blot analyses demonstrate a wide distribution of the DOLPP1 mRNA in mouse tissues. Overexpression of Dolpp1p in yeast, COS, and Sf9 cells produces substantial increases in Dol-P-P phosphatase activity but not in dolichyl monophosphate or phosphatidic acid phosphatase activities in microsomal fractions. Subcellular fractionation and immunofluorescence studies localize the enzyme encoded by DOLPP1 to the endoplasmic reticulum of COS cells. The results of protease sensitivity studies with microsomal vesicles from the lpp1Delta/dpp1Delta yeast mutant expressing DOLPP1 are consistent with Dolpp1p having a luminally oriented active site. The sequence of the DOLPP1 cDNA predicts a polypeptide with 238 amino acids, and a new polypeptide corresponding to 27 kDa is observed when DOLPP1 is expressed in yeast, COS, and Sf9 cells. This study is the first identification and characterization of a cDNA clone encoding an essential component of a mammalian lipid pyrophosphate phosphatase that is highly specific for Dol-P-P. The specificity, subcellular location, and topological orientation of the active site described in the current study strongly support a role for Dolpp1p in the recycling of Dol-P-P discharged during protein N-glycosylation reactions on the luminal leaflet of the endoplasmic reticulum in mammalian cells.

The ins(ide) and out(side) of dolichyl phosphate biosynthesis and recycling in the endoplasmic reticulum

The precursor oligosaccharide donor for protein N-glycosylation in eukaryotes, Glc3Man9GlcNAc(2)-P-P-dolichol, is synthesized in two stages on both leaflets of the rough endoplasmic reticulum (ER). There is good evidence that the level of dolichyl monophosphate (Dol-P) is one rate-controlling factor in the first stage of the assembly process. In the current topological model it is proposed that ER proteins (flippases) then mediate the transbilayer movement of Man-P-Dol, Glc-P-Dol, and Man5GlcNAc(2)-P-P-Dol from the cytoplasmic leaflet to the lumenal leaflet. The rate of flipping of the three intermediates could plausibly influence the conversion of Man5GlcNAc(2)-P-P-Dol to Glc3Man(9)GlcNAc(2)-P-P-Dol in the second stage on the lumenal side of the rough ER. This article reviews the current understanding of the enzymes involved in the de novo biosynthesis of Dol-P and other polyisoprenoid glycosyl carrier lipids and speculates about the role of membrane proteins and enzymes that could be involved in the transbilayer movement of the lipid intermediates and the recycling of Dol-P and Dol-P-P discharged during glycosylphosphatidylinositol anchor biosynthesis, N-glycosylation, and O- and C-mannosylation reactions on the lumenal surface of the rough ER.

Trafficking of oligomannosides released during N-glycosylation: a clearing mechanism of the rough endoplasmic reticulum

The main reaction of N-glycosylation of proteins is the transfer 'en bloc' of the oligosaccharide moieties of lipid intermediates to an asparagine residue of the nascent protein. For the past 15 years, a few laboratories including ours have shown that the process was accompanied by the release of oligosaccharide-phosphates and of neutral oligosaccharides possessing one GlcNAc (OS-Gn(1)) or two GlcNAc (OS-Gn(2)) at the reducing end. The aim of this review is to gather the evidence for the different origins of these soluble oligomannosides, to examine their subcellular location and intracellular trafficking. Furthermore, using Brefeldin A we demonstrated that this released oligomannoside material could be the substrate for the Golgi glycosidases and glycosyltransferases. Indeed, released oligomannoside never reach the Golgi vesicles either because they are directly produced in the cytosol as has been demonstrated for oligosaccharide-phosphates and for neutral oligosaccharides possessing one GlcNAc at the reducing end or because they are actively transported out of the rough endoplasmic reticulum to the cytosol. One of the functions of oligomannoside trafficking between rough endoplasmic reticulum, cytosol and lysosomes could be to prevent these oligosaccharides for competing with glycosylation in the Golgi.

Purification and characterization of a polyisoprenyl phosphate phosphatase from pig brain. Possible dual specificity

Microsomal fractions from pig and calf brain catalyze the enzymatic dephosphorylation of endogenous and exogenous dolichyl monophosphate (Dol-P) (Sumbilla, C. A., and Waechter, C. J. (1985) Methods Enzymol. 111, 471-482). The Dol-P phosphatase (EC 3.1.3.51) has been solubilized by extracting pig brain microsomes with the nonionic detergent Nonidet P-40 and purified approximately 1,107-fold by a combination of anion exchange chromatography, polyethylene glycol fractionation, dye-ligand chromatography, and wheat germ agglutinin affinity chromatography. Treatment of the enzyme with neuraminidase prevented binding to wheat germ agglutinin-Sepharose, indicating the presence of one or more N-acetylneuraminyl residues per molecule of enzyme. When the highly purified polyisoprenyl phosphate phosphatase was analyzed by SDS-polyacrylamide gel electrophoresis, a major 33-kDa polypeptide was observed. Enzymatic dephosphorylation of Dol-P by the purified phosphatase was 1) optimal at pH 7; 2) potently inhibited by F-, orthovanadate, and Zn2+ > Co2+ > Mn2+ but unaffected by Mg2+; 3) exhibited an approximate Km for C95-Dol-P of 45 microM; and 4) was sensitive to N-ethylmaleimide, phenylglyoxal, and diethylpyrocarbonate. The pig brain phosphatase did not dephosphorylate glucose 6-phosphate, mannose 6-phosphate, 5'-AMP, or p-nitrophenylphosphate, but it dephosphorylated dioleoyl-phosphatidic acid at initial rates similar to those determined for Dol-P. Based on the virtually identical sensitivity of Dol-P and phosphatidic acid dephosphorylation by the highly purified enzyme to N-ethylmaleimide, F-, phenylglyoxal, and diethylpyrocarbonate, both substrates appear to be hydrolyzed by a single enzyme with an apparent dual specificity. This is the first report of the purification of a neutral Dol-P phosphatase from mammalian tissues. Although the enzyme is Mg2+-independent and capable of dephosphorylating Dol-P and PA, several enzymological properties distinguish this lipid phosphomonoesterase from PAP2.

'Glyco-deglyco' processes during the synthesis of N-glycoproteins

For the past 15 years, it has appeared increasingly evident that the N-glycosylation process was accompanied by the release of oligomannoside type oligosaccharides. This material is constituted of oligosaccharide-phosphates and of neutral oligosaccharides possessing one GlcNAc (OS-Gn1) or two GlcNAc (OS-Gn2) at the reducing end. It has been demonstrated that oligosaccharide-phosphates originated from the cleavage by a specific pyrophosphatase, of non-glycosylated cytosolic faced oligosaccharide-PP-Dol and chiefly the Man5GlcNAc2-PP-Dol. The Man5GlcNAc2-P, as the main product, is recovered in the cytosolic compartment and is further degraded to Man5GlcNAc1 by as for yet not depicted enzymes. In contrast, OS-Gn2 produced from hydrolysis of oligosaccharide-PP-Dol (presumably as a transfer reaction onto water) when the amount of protein acceptor is limiting, are generated into the lumen of rough endoplasmic reticulum (ER). They are further submitted to processing alpha-glucosidases and rough ER mannosidase and are (mainly as Man8GlcNAc2) exported into the cytosolic compartment. This material is further degraded into a single component, the Man5GlcNAc1: Man alpha 1-2Man alpha 1-2Man alpha 1-3 (Man alpha 1-6)Man beta 1-4GlcNAc by the sequential action of a cytosolic neutral chitobiase followed by cytosolic mannosidase. Furthermore, OS-Gn1 could have a dual origin: on the one hand, they originate from OS-Gn2 by the cytosolic degradation pathway indicated above; on the other hand, we will discuss a possible origin from the degradation or remodeling of newly synthesized glycoproteins. Considered first as a minor phenomenon, these observations have lead to the concept of intracellular oligomannoside trafficking, a process which results from more fundamental phenomena such as the control of the dolichol cycle, and the so-called quality-control of glycoprotein. In this review, we would like to describe the evolution of ideas on the origin, intracellular trafficking and putative roles of these oligomannosides released during the N-glycosylation process. We propose that these early stage 'glyco-deglyco' processes represent a way of control of N-glycosylation and of the fate of N-glycoproteins.

Phosphatidate phosphohydrolase catalyzes the hydrolysis of ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate

A Mg2+-independent phosphatidate phosphohydrolase was purified from rat liver plasma membranes in two distinct forms, an anionic protein and a cationic protein. Both forms of the enzyme dephosphorylated phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate. When assayed at a constant molar ratio of lipid to Triton X-100 of 1:500, the apparent Km values of the anionic phosphohydrolase for the lipid substrates was 3.5, 1.9, 0.4, and 4.0 microM, respectively. The relative catalytic efficiency of the enzyme for phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate was 0.16, 0.14, 0.48, and 0.04 liter (min x mg)-1, respectively. The hydrolysis of phosphatidate was inhibited competitively by ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate. The Ki(app) values were 5.5, 5.9, and 4.0 microM, respectively. The hydrolysis of phosphatidate by the phosphohydrolase conformed to a surface dilution kinetic model. It is concluded that the enzyme is a lipid phosphomonoesterase that could modify the balance of phosphatidate, ceramide 1-phosphate, lysophosphatidate, and sphingosine 1-phosphate relative to diacylglycerol, ceramide, monoacylglycerol, and sphingosine, respectively. The enzyme could thus play an important role in regulating cell activation and signal transduction.

Characterization and purification of a membrane-bound archaebacterial pyrophosphatase from Sulfolobus acidocaldarius

Plasma membranes of the thermoacidophilic archaebacterium Sulfolobus acidocaldarius (DSM 639) display a pyrophosphate-hydrolyzing activity [M. Lübben & G. Schäfer (1987) Eur. J. Biochem. 164, 533-540]. In our present work, we solubilized and purified this pyrophosphatase to homogeneity. It consists of a single subunit with a molecular mass of 17-18 kDa, forming an oligomer of 70 kDa under native conditions. Edman degradation revealed 30 amino acids of the N-terminus. The enzyme cleaves phosphoric-acid-anhydride bonds independently of monovalent or divalent cations. Temperature and pH optima of 75 degrees C and 3.5-3.7, respectively, characterize it as an ectoenzyme. Membrane lipids of Sulfolobus stimulate the activity. The dolichol-pyrophosphate-complexing peptide-antibiotic bacitracin inhibited growth of Sulfolobus. A possible function of the acid pyrophosphatase is the hydrolysis of dolichol pyrophosphate in connection with glycosylation reactions of membrane proteins.

Nutritional and developmental regulation of glycosylation processes in digestive organs

We review the nutritional and developmental variations of the glycosylation processes in digestive organs, since glycoproteins play a prominent part as mucins or digestive enzymes in these tissues. The biosynthesis of the glycannic chains is demonstrated to be largely sensitive to various exogenous (such as nutritional) or endogenous (such as developmental) factors. Although the metabolic regulation by dietary variations appears as rather complex, according to the variety of experimental conditions and the diversity of the organs studied, available data demonstrate that this regulation does exist, depending on the quantity or sometimes the quality of the major or minor components of the diet, which induce significant variations in the glycosylation processes. The synthesis of the internal core of N-glycans is essentially regulated by diet-induced variations of the phosphoryl-dolichol level, whereas the modulation of the biosynthesis of the external part of N-glycans or the biosynthesis of O-glycans is controlled by diet-induced variations in the systems transferring fucose, galactose, sialic acid or hexosamines. Modifications in intestinal glycosylation during post-natal development in the rat control the quality of the glycannic chains of mucins and brush-border enzymes. The post-natal maturation of the intestinal rat tissue is characterized by a shift from sialylation to fucosylation, depending on coordinate changes in glycosyltransferase activities, in sugar-nucleotide breakdown or synthesis or in the activity of regulatory proteins. These activities are largely sensitive to dietary manipulations at weaning and to hormonal stimulations before weaning. However, glucocorticoid hormones do not appear as the triggering signal for the induction of these changes.

Characterization and distribution of cis-prenyl transferase participating in liver microsomal polyisoprenoid biosynthesis

The properties of rat liver cis-prenyl transferase, mediating the synthesis of polyisoprenoid pyrophosphate from trans,trans-farnesyl pyrophosphate and [3H]isopentenyl pyrophosphate were studied. The Km values for farnesyl pyrophosphate and isopentenyl pyrophosphate were found to be 25 microM and 4.4 microM, respectively. Appropriate conditions were established to measure the condensation reaction, which was linear during the first hour using 1 mg microsomal protein. Various detergents could solubilize the enzyme, but the presence of Triton X-100 was required during the incubation to obtain full activity. There was also an absolute requirement for Mg2+ and the pH maximum was 7.0. Inorganic phosphate, especially pyrophosphate, proved to be inhibitory. cis-Prenyl transferase is associated mainly with the cytoplasmic surface of rough microsomes and, to some extent, also with smooth I microsomes, but was almost absent from smooth II microsomes. At all localizations, the product is polyprenyl pyrophosphate and to some extent, also polyprenyl monophosphate. The isoprenoids formed contain 15-18 units in the presence of detergents and 16-20 units in the absence of detergents.

Polyprenyl phosphates: synthesis and structure-activity relationship for a biosynthetic system of Salmonella anatum O-specific polysaccharide

A series of polyprenyl phosphates with modified structure of polyprenyl residue was prepared through phosphorylation of polyprenyl trichloroacetimidates with phosphoric acid. Interaction of polyprenols with tetra-n-butylammonium dihydrogen phosphate and trichloroacetonitrile was found to represent a very efficient, simple and general method for the synthesis of polyprenyl phosphates. A procedure was developed for smooth conversion of polyprenyl pyrophosphates into the monophosphates through hydrolysis in the presence of 4-dimethylaminopyridine. The polyprenyl phosphates prepared were studied as substrates for the enzymes of Salmonella anatum O-specific polysaccharide biosynthesis. Correct stereochemistry of alpha- and beta-isoprenic units was found to be essential for substrate efficiency. At the more remote positions of the hydrocarbon chain just the presence of isoprenic units of any configuration seems necessary. Some changes in position of the phosphate group may be permissible without significant loss of substrate properties.

Induction of glycoprotein biosynthesis in activated B lymphocytes

Resting murine splenic B lymphocytes (B cells) can be stimulated to proliferate by exposure to a variety of polyclonal activators. To investigate changes in glycoprotein synthesis that occur during the activation process, N-glycosylation activity was assessed by following the incorporation of [2-3H]mannose into dolichol-linked oligosaccharide intermediates and glycoprotein after B cells were exposed to anti-immunoglobulin M (anti-mu). Stimulation of B cells by anti-mu resulted in a dramatic induction of N-glycosylation activity. The incorporation of radiolabeled mannose into oligosaccharide-lipid increased 9-fold while the rate of labeling of glycoprotein increased 27-fold between 18 and 38 h after exposure to anti-mu. Maximal stimulation of N-glycosylation activity was observed at an anti-mu concentration of 20-50 micrograms/ml. Similar results were obtained when B cells were activated by bacterial lipopolysaccharide (LPS), another polyclonal activating agent. The major dolichol-bound oligosaccharide labeled during the induction period was determined to be Glc3Man9GlcNAc2 by HPLC analysis. Nearly full induction of oligosaccharide-lipid synthesis and protein N-glycosylation was also seen when DNA synthesis was suppressed by activating B cells with anti-mu in a serum-free medium, or by activating with anti-mu or LPS in the presence of hydroxyurea. The results suggest that the N-glycosylation pathway is induced during the G0 to G1 transition or during the G1 period, and that entry into S phase is not required. These studies describe a striking developmental increase in N-glycosylation activity and extend the information on biochemical changes occurring during the activation of B cells.

Glycoprotein biosynthesis by human normal platelets

Incorporation of radioactive Man, Gal, Fuc, Glc-N, and NANA into washed human normal platelets and endogenous glycoproteins has been found. Both parameters were time dependent. Analysis of hydrolyzed labeled glycoproteins by paper chromatography revealed that the radioactive monosaccharide incubated with the platelets had not been converted into other sugars. Acid hydrolysis demonstrates the presence of a glycosidic linkage. All the effort directed to the demonstration of the existence of a lipid-sugar intermediate in intact human platelets yielded negative results for Man and Glc-N used as precursors. The incorporation of these sugars into glycoproteins is insensitive to bacitracin, suggesting no involvement of lipid-linked saccharides in the synthesis of glycoproteins in human blood platelets. The absence of inhibition of the glycosylation process in the presence of cycloheximide suggests that the sugars are added to proteins present in the intact platelets. These results support the contention that glycoprotein biosynthesis in human blood platelets observed under our experimental conditions is effected through direct sugar nucleotide glycosylation.

Estrogen influences dolichyl phosphate distribution among glycolipid pools in mouse uteri

The steroid hormone 17 beta-estradiol dramatically induces uterine N-linked glycoprotein assembly [Dutt, A., Tang, J.-P., Welply, J. K., & Carson, D. D. (1986) Endocrinology (Baltimore) 118, 661-673]. To determine the role that dolichyl phosphate availability plays in this induction, we studied the effects of estrogen priming on the content of dolichyl phosphate and the distribution of dolichyl phosphate among various glycolipids in uteri. Dolichol-linked saccharides were metabolically labeled to equilibrium with either [3H]glucosamine or [3H]mannose and extracted from primary explants of uterine tissue. The amount of dolichol-linked saccharide was calculated from the specific radioactivity determined for the corresponding sugar nucleotides extracted from the tissues. The major dolichol-linked saccharides identified were mannosylphosphoryldolichol (MPD), oligosaccharylpyrophosphoryldolichol (OSL), and N,N'-diacetylchitobiosylpyrophosphoryldolichol (CBL). Estrogen increased the levels of MPD and OSL 4-fold; however, CBL levels did not change. After 3 days of treatment, the levels of these glycolipids were very similar to those in uteri from pregnant mice. Remarkably, MPD constituted 90-95% of dolichol-linked saccharides detected under all conditions. The tissue contents of total dolichyl phosphate and alkali-labile dolichyl phosphate, presumably MPD, were estimated by liquid chromatography. The levels of alkali-labile dolichyl phosphate determined in this way were in good agreement with the values estimated for MPD by metabolic labeling; moreover, alkali-labile dolichyl phosphate constituted 50-98% of the total dolichyl phosphate pool. The variations in MPD content depended upon the steroid hormone influence, most notably that of estrogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Dehydrodolichyl diphosphate synthetase from rat seminiferous tubules

Homogenates of seminiferous tubules from rat testes catalyzed the incorporation of label from [14C]isopentenyl diphosphate into a variety of polyprenyl products. Long chain polyprenyl mono- and diphosphates were formed as major products when undesirable side reactions were minimized. The long chain polyprenyl diphosphate synthetase was measured as a sum of the mono- and diphosphate derivatives formed and was dependent on the addition of t,t-farnesyl diphosphate, isopentenyl diphosphate, and divalent cation. The highest activity was associated with the membranous fractions, whereas activity was negligible in the cytosolic fraction. The products of this prenyl transferase were labile to acid and yielded petroleum ether soluble products which indicated that the alpha-isoprene unit was unsaturated. Hydrolysis of either the polyprenyl mono-or diphosphates with a testicular phosphatase in the absence of NaF yielded C75, C80, C85, and C90 polyprenols. The chain lengths of the products of the synthetase suggest that this enzyme is responsible for the de novo biosynthesis of dehydrodolichyl diphosphates which are precursors of the dolichyl derivatives found in testes.

Identification and characterization of a specific dolichylmonophosphate phosphatase activity in bovine thyroid microsomes

Bovine thyroid membranes are able to dephosphorylate exogenous [1-3H]DMP as well as endogenous prelabeled [32P]DMP. The kinetics, properties and specificity of the dolichylmonophosphatase activity have been studied by monitoring respectively the release of [1-3H]dolichol from [1-3H]DMP and the residual amount of [32P]DMP. The DMP-phosphatase activity is not linear with respect to time and exhibits a neutral pH optimum. There is only linearity in a narrow range of protein concentration when 0.25% Triton X-100 is included in the incubation mixture. Studying the enzymatic activity in function of protein concentration, the detergent requirement shows to be very critical. Triton X-100 is necessary for enzymatic activity with [1-3H]DMP (only 10% of enzymatic activity in the absence of detergent) although the detergent inhibits the hydrolysis of endogenous prelabeled [32P]DMP. Divalent cations are not essential for enzymatic activity, Ca2+-ions being even inhibitory. In accordance, EDTA (EGTA) is slightly stimulatory. The DMP-Pase activity is not influenced by the ionic strength of the incubation system and sulphydryl groups are not involved. NaF, VOS and VO4(3-) are strongly inhibitory. The inhibition by dolichol and PO3-4 can be explained as the result of product inhibition. An apparent Km-value of 2.5 X 10(-5) M is computed for [1-3H]DMP. Bacitracin inhibits DMP-phosphatase in contrast with other reports. Propylthiouracyl, cAMP, TSH and several other bio-effectors are without effect on the in vitro system. The specificity of the DMP-Pase activity is discussed, showing that the phosphatase is distinctly different from other phosphatases especially phosphatidic acid phosphohydrolase.

Distribution, metabolism and function of dolichol and polyprenols

Polyisoprenoid alcohols consisting of 9 or more isoprene units are present in all living cells. They can be fully unsaturated (polyprenols) or alpha-saturated (dolichol). Dolichol forms may have additional saturation at or near the omega-end. Some species contain ony dolichol or only polyprenols while others have nearly equal amounts of both types. Some polyisoprenoid alcohols consist entirely of trans isoprene units but most, including dolichol, contain both trans and cis units. Considerable advances in lipid methodology have occurred since the first review of polyisoprenoid alcohols by Hemming in 1974. For example, direct analysis of both dolichol and Dol-P by HPLC has replaced earlier methods which were often both insensitive and inaccurate. The availability of radiolabeled dolichol and polyprenols has facilitated studies concerning the metabolism and distribution of these compounds. Those studies suggest that only a small portion of the dolichol present in cells is likely to be involved in glycosylation. Polyisoprenoid alcohols are usually present at a family of homologues where each differs in size by one isoprene unit. Little or no size related specificity has been observed for any reaction involving dolichol or polyisoprenol intermediates. The overall length of polyisoprenoid alcohols may, however, affect the manner in which these compounds influence the physical and biochemical properties of membranes. Studies on the biosynthetic pathway leading from cis, trans Pol-PP by phosphatase action. The formation of the dolichol backbone from a polyprenol requires the action of an additional enzyme, an alpha-saturase. This enzyme does not always act at the level of a single common substrate, since Pol-PP, Pol-P, and polyprenol all appear to be utilized as substrates. The major product of the de novo pathway differs among different species. Dol-P would appear to be the most energy efficient end-product since it can participate directly in glycoprotein formation. Most often, however, Dol-P is not the major product of metabolic labeling experiments. In some cases, dolichol is formed so that rephosphorylation is required to provide Dol-P for participation in glycoprotein formation. The kinase responsible for this phosphorylation appears to bypass the considerable stores of dolichol present in tissues (i.e. sea urchin eggs) in favor of dolichol derived directly from de novo synthesis. Although HMGR is a major regulatory component of the pathway leading to polyisoprenoid alcohols and cholesterol, control is most often not co-ordinated.(ABSTRACT TRUNCATED AT 400 WORDS)

Oviduct dolichyl phosphate phosphatase: estrogen effects and a possible biosynthetic role

Estrogen-induced chick oviduct differentiation is accompanied by an increased capacity for protein glycosylation. A portion of this increase has been attributed to increased levels of dolichyl phosphate (Dol-P). Hormone withdrawal leads to an apparent decrease in Dol-P. Dol-P metabolism in the oviduct has been studied, and one of the enzymes having a direct effect on Dol-P, Dol-P phosphatase is herein described. Dol-P phosphatase has a pH optimum of 6.0, does not require a metal ion, and is inhibited by Mn2+ at concentrations greater than 5 mM. Inhibitor studies indicate that Dol-P hydrolysis is inhibited by polyprenyl phosphates having both saturated and unsaturated alpha-isoprene residues, but not by the corresponding alcohols. The enzyme is also inhibited by phosphatidic acid unless 2 mM Mn2+ is included in the incubations. Under these conditions Dol-P hydrolysis is only slightly inhibited (less than 10%), but phosphatidate inhibition is totally eliminated. Oviduct membranes also possess phosphatidate phosphatase, but this enzyme is distinct from Dol-P phosphatase based on thermolability, metal ion sensitivity, and sulfhydryl reagent sensitivity. Studies of enzyme activity in response to estrogen treatment reveal that both Dol-P phosphatase and phosphatidate phosphatase have maximal specific activity early in the differentiation process (peaking after 3 days of treatment), and low specific activity in fully differentiated oviducts, including laying hen oviduct. Hormone withdrawal elicits a small increase in specific activity of both phosphatases. The hormone effects suggest that Dol-P phosphatase may be a biosynthetic enzyme.

Dolichyl phosphate phosphatase in rat liver microsomes. Avoidance of the use of detergent in testing the effect of phospholipids on dolichyl phosphate phosphatase

A system was developed for testing the effect of phospholipids on dolichyl phosphate phosphatase, a membrane-associated enzyme. This enzyme was solubilized, delipidated, stabilized and concentrated in such a way that minimal quantities of Triton X-100 were carried by enzyme extracts to the incubation mixture. Its substrate, dolichyl phosphate, could be kept in aqueous medium as suspended particles without addition of detergent. When dolichyl phosphate phosphatase was assayed using the substrate in this detergent-free form, values for Km, pH optimum and temperature optimum were different from those obtained with detergent-solubilized substrate. This assay of dolichyl phosphate phosphatase almost free of detergent allowed testing of the effect of specific phospholipids on enzyme activity with minimal interference produced by endogenous phospholipids or exogenous detergent. Sphingomyelin, phosphatidylethanolamine or phosphatidylcholine (zwitterionic phospholipids) acted as activators, whereas phosphatidic acid and phosphatidylinositol, negatively-charged phospholipids, were inhibitors of dolichyl phosphate phosphatase.

Glycoprotein synthesis and embryonic development

One of the most striking morphogenetic events during embryonic development is gastrulation, a process that leads to formation of the primitive gut. Using sea urchin embryos, we have studied the synthesis and function of glycoproteins during gastrulation. These studies have revealed that at least three processes are induced prior to gastrulation: de novo synthesis of dolichol; phosphorylation of dolichol by dolichol kinase, which may catalyze the final step in the de novo pathway; and initiation of the synthesis of N-linked glycoproteins. Whether or not activation of the glycosylation process results merely because of the production of dolichyl monophosphate or because, in addition, proteins containing glycosylatable-Asn-X-Ser/Thr-sequences are first translated just prior to gastrulation, is currently being investigated.

Dolichyl-phosphate phosphatase and dolichyl-diphosphate phosphatase in rat-liver microsomes

Dolichyl-phosphate phosphatase and dolichyl-diphosphate phosphatase activities of a liver-cell microsomal preparation were solubilized by treatment with Triton X-100. The 100,000 X g supernatant was then passed through a column of Sepharose-4B--concanavalin A. Both enzyme activities were found in the percolate. This treatment eliminated inhibition by ATP and glucose 6-phosphate in both phosphatase activities. In each case the activities were inhibited by higher concentrations of enzyme preparation due to the presence of phospholipids. The inhibitory effects of either phosphatidylcholine or phosphatidylethanolamine were due to competition for detergent. On the other hand, the effect produced by phosphatidic acid appeared to be different, since it did not change the optimal concentration of Triton X-100 for the two enzymes. Dolichyl-phosphate phosphatase was strongly inhibited by both Pi and PPi, whereas dolichyl-diphosphate phosphatase was only slightly inhibited by Pi and not at all by PPi. Dolichyl-diphosphate phosphatase was more inhibited by divalent cations than dolichyl-phosphate phosphatase. The apparent Km of dolichyl-phosphate phosphatase for dolichyl phosphate was 0.15 mM. Dolichol also inhibited dolichyl-phosphate phosphatase, but it produced a stronger inhibition on dolichyl-diphosphate phosphatase. The inhibitory effect of dolichol was not entirely due to detergent competition.

The biosynthetic pathway of the asparagine-linked oligosaccharides of glycoproteins

This review deals with the structure and addition of the different types of oligosaccharides to asparagine residues in proteins. This process occurs in several steps, first an oligosaccharide which contains N-acetylglucosamine mannose and glucose is built up joined to dolichyl diphosphate. The oligosaccharide is then transferred to a polypeptide chain, loses its glucose, and is modified by removal of some monosaccharides and addition of others giving rise to a variety of saccharides.