Dolichol(C55) mono- and pyrophosphatases of the rat liver

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.

Dolichol kinase activity: a key factor in the control of N-glycosylation in inner mitochondrial membranes

Inner mitochondrial membranes from liver contain a dolichol kinase which required CTP as a phosphoryl donor. Kinase activity was linear with protein concentration and unlike other reported kinases, activated almost equally well by Mg2+, Mn2+ or Ca2+. Thin-layer chromatography showed that the reaction product co-migrated with authentic dolichyl monophosphate. The phosphorylation of dolichol did not occur in presence of ATP, GTP or UTP but required exogenous dolichol for maximal activity. Newly synthesized [3H]dolichyl monophosphate has been shown to be glycosylated in the presence of GDP[14C]mannose or UDP[14C]glucose. The double labeled lipids formed by the sugar nucleotide-dependent reactions were identified respectively as [14C]mannosylphosphoryl[3H]dolichol and [14C]glucosylphosphoryl [3H]dolichol. These results are discussed in terms of regulation of N-glycosylation processes in inner mitochondrial membranes from liver.

Characterization of dolichyl diphosphate phosphatase from rat liver

Dolichyl diphosphate phosphatase (DolPPase) has been characterized in rat liver. Subcellular distribution studies indicate that the enzyme is localized in the endoplasmic reticulum. The in vitro enzymatic activity is stimulated by EDTA, due to release of inhibition by trivalent cations found in the assay tubes. All di- and trivalent cations tested were inhibitory, with the trivalent ions Al3+ and Fe3+ showing greater than 70% inhibition at a concentration of 10 microM. The assay requires the presence of a detergent for optimal activity, with Triton X-100 giving maximum activity at 0.1%. The substrate specificity of DolPPase toward polyprenyl diphosphates has been determined and indicates that there is little preference of the enzyme for substrates of different chain length, and either stereochemical orientation or degree of saturation of the alpha-isoprene unit. Km values of 11-14 microM were obtained for all substrates tested. Preliminary studies on the transmembrane topology of the DolPPase using latency assays, indicate that the active site of the enzyme may reside on the cytoplasmic face of the endoplasmic reticulum.

Cell-cycle dependence of dolichyl phosphate biosynthesis

The cell-cycle dependence of dolichyl phosphate biosynthesis has been investigated in mouse L-1210 cells fractionated by centrifugal elutriation. Dolichyl phosphate levels increased linearly through the cell cycle, reaching a value in late S phase twice that of early G1. The cell-cycle dependences of four dolichyl phosphate metabolizing enzymes have been measured: cis-prenyltransferase, CTP-dependent dolichol kinase, dolichyl phosphatase, and dolichyl pyrophosphatase. The kinase, the cis-prenyltransferase, and the pyrophosphatase showed cell-cycle variations, increasing through G1 to a maximum in S phase while the monophosphatase activity was cell-cycle independent. The rate of accumulation of dolichyl phosphate was not affected by growing the cells in mevalonolactone-supplemented media. The evidence presented is consistent with models in which either the cis-prenyltransferase or the kinase/phosphatase couple (or both) regulates the levels of dolichyl phosphate in the cell.

Characterization of dolichol kinase from bovine thyroid microsomes

Bovine thyroid microsomes are able to phosphorylate exogenous [1-3H]dolichol as well as endogenous dolichol. The properties and specificity of the dolichol kinase activity have been studied by following the phosphorylation of [1-3H]dolichol to [1-3H]DMP as well as the formation of [32P]DMP from endogenous dolichol and [gamma-32P]CTP. The dolichol kinase activity was not linear with respect to time and exhibited a neutral pH-optimum. Product formation was directly proportional to microsomal protein concentration up to 2.5 mg protein/incubation. The enzyme was found to depend on divalent cations for activity: Mg2+-ions being much more effective than Ca2+- and Mn2+-ions. In accordance, EDTA was strongly inhibitory. The enzyme exhibited specificity for CTP as phosphoryl donor and was found to be inhibited by the reaction product CDP. The apparent Km-value for exogenous dolichol amounted to 4 microM. Those for CTP were estimated to be 3.88 and 10.75 mM with exogenous [1-3H]dolichol depending on the source of CTP. With endogenous dolichol Km-values for CTP of 27.8 and 6.1 microM were calculated in respectively the absence and presence of 5 mM VO4(3-). Triton X-100 (0.15%) was necessary in the [1-3H]dolichol kinase assay (only 3% of enzymatic activity in the absence of detergent), while with [gamma-32P]CTP dolichol kinase detergent was only of minor influence (30% stimulation at 0.02% Triton X-100). The levels of the enzymatic activity could be doubled by the inclusion of 18-21 mM NaF [( 1-3H]dolichol kinase) as phosphatase inhibitor: VO4(3-) had practically no effect. In contrast with [gamma-32P]CTP dolichol kinase, the enzymatic activity could be enhanced 4-fold by addition of 5 mM VO4(3-) while F- resulted into no appreciable effect.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Changes in hepatic dolichol and dolichyl monophosphate caused by treatment of rats with inducers of the endoplasmic reticulum and peroxisomes and during ontogeny

Rats were treated with various inducers of the endoplasmic reticulum and peroxisomes and the properties and distributions of dolichol and dolichyl phosphate analyzed. The treatment of rats with carcinogenic agents 2-acetylaminofluorene, N-nitrosodiethylamine and 3-methylcholanthrene and with the compounds such as phenobarbital, terpentine, cholestyramine and di(2-ethylhexyl)phthalate have all caused changes in the microsomal or lysosomal contents of dolichol to various extents, but only the latter group influenced dolichyl-P concentration. Shortly after birth, the hepatic content of dolichyl-P reaches the adult level, whereas the level of the free alcohol is low at birth but increases continuously thereafter. Incorporation of [3H]mevalonate into dolichol was also dependent on factors other than de novo synthesis, e.g., the pool size. Rates of glycosylation reactions dependent on dolichyl-P exhibit considerable changes but are independent of the existing levels of lipid intermediate. GDP-mannosyl transferase activity increases greatly with birth, but the enzyme activity returns to the adult level within a day after birth. These results demonstrate that structural and functional modifications induced with drugs can greatly influence the content and distribution of dolichol which are independent of the existing levels of dolichyl-P.

Characterization of polyisoprenyl phosphate phosphatase activity in rat liver

The polyisoprenyl phosphate dephosphorylating activity of rat liver has been investigated with regard to substrate specificity, subcellular distribution, and transmembrane orientation. Total liver microsomes were employed as a source of enzymatic activity against a variety of 32P-labeled substrates. Susceptibility to dephosphorylation followed the order solanesyl phosphate greater than alpha-cis-polyprenyl 19-phosphate = alpha-trans-polyprenyl 19-phosphate = dihydrosolanesyl phosphate greater than (S)-dolichyl 19-phosphate = (R)-dolichyl 19-phosphate = (R,S)-dolichyl 11-phosphate. There appeared to be no major effect of chain length from 11 to 20 isoprenes. Data obtained from inhibition studies using solanesyl [32P]phosphate as substrate were consistent with the substrate specificity studies and suggested that a single activity is responsible. With dolichyl [32P]phosphate as substrate, the phosphatase specific activity of the subcellular fractions prepared from rat liver was found to follow the sequence Golgi = smooth endoplasmic reticulum greater than plasma membrane greater than lysosomes = rough endoplasmic reticulum greater than nuclei greater than mitochondria. Transmembrane topography studies, using enzyme latency as a criterion, were consistent with an orientation of the active site facing the cytoplasm.

Inhibition of dolichyl phosphate biosynthesis by compactin in cultured rat hepatocytes

Isolated rat hepatocytes were cultured in monolayer for about 24 h. During this period the cells exhibited constant protein and lipid synthesis. When the culture medium contained compactin, a competitive inhibitor of the 3-hydroxyl-3-methylglutary-coenzyme-A reductase, dolichyl-P synthesis was inhibited by 91% at the end of the incubation, as estimated by the incorporation of [3H]acetate and by 77% as estimated by the incorporation of 32Pi. These results indicate that in primary cultures of rat hepatocytes dolichyl monophosphate is mainly synthesized through a de novo process, while phosphorylation through the CTP-mediated kinase is of limited functional importance.

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.

Subcellular sites of enzymes catalyzing the phosphorylation-dephosphorylation of dolichol in the central nervous system

The subcellular locations of several enzymes involved in dolichyl monophosphate (Dol-P) metabolism in brain have been investigated. Dolichol kinase is highly enriched in a heavy microsomal fraction from calf brain, while 71% of the Dol-P phosphatase activity was recovered with the light microsomes. Lower amounts of the phosphatase activity were also found in the heavy microsomal, mitochondrial-lysosomal, and synaptic plasma membrane fractions. Since the light microsomal fraction also contained substantial acetylcholinesterase activity, an axon plasma membrane marker, an axolemma-enriched fraction, was prepared from rat brain by a second procedure. A comparison with microsomal and mitochondrial-lysosomal fractions revealed that the axolemma-enriched fraction contained the highest specific activity of Dol-P phosphatase, indicating that the enzyme was present in the axon plasma membrane. The tunicamycin-sensitive UDP-N-acetylglucosamine:Dol-P N- acetylglucosaminylphosphotransferase , glucosyl- phosphoryldolichol (Glc-P-Dol) synthase, Glc-P-Dol:oligosaccharide glucosyltransferase, and the oligosaccharyltransferase were all found predominantly in the heavy microsomes. These results indicate that the enzymes responsible for the initiation and termination of biosynthesis, as well as the transfer of dolichol-linked oligosaccharides, reside in the rough endoplasmic reticulum (ER) of central nervous tissue. Evidence that at least some Dol-P molecules formed by dolichol kinase are accessible to multiple glycosyltransferases in the rough ER of brain is also presented.

Dolichol and dolichyl phosphate levels in brain tissue from English setters with ceroid lipofuscinosis

Human ceroid lipofuscinosis (CL) is an inherited disease marked by cerebromacular degeneration and early death. We have utilized the canine model to investigate the possible role of dolichol and dolichyl phosphate in the developmental pathology of CL. We found that while brain levels of dolichol increase with age in both affected and unaffected dogs, the amount in the diseased animal was similar to that in controls. Brain levels of dolichyl phosphate ranged from 20 to 35 micrograms/g in control dogs at all ages examined, but increased substantially during development in the affected dogs, a value of 113 +/- 24 micrograms/g (mean +/- SD) being obtained in the end-stage animals. In addition to the results obtained in the canine model, dolichyl phosphate levels in human brain tissues from a 5-year-old with late infantile CL and from a 19-year-old with juvenile CL were found to be 153 and 382 micrograms/g, respectively, compared with a control that assayed 26 micrograms/g. The preliminary findings with human tissues provide further evidence for an association of elevated brain dolichyl phosphate levels with CL. Whether the increase is primary or secondary remains to be determined.

Separation, quantitation and distribution of dolichol and dolichyl phosphate in rat and human tissues

Two procedures for quantitative determination of dolichol were studied and these were applied to analyze tissue and subcellular distribution. In the first procedure the dolichols were oxidized with Cr2O3 and reduced with NaB3H4. The radioactivity in the individual dolichols was measured using reversed-phase thin-layer chromatography. In the second procedure, dolichols were analyzed by high-pressure liquid chromatography. For determination of dolichyl phosphates the lipid extract was subjected to acid and alkaline hydrolysis, and after hydrolysis with acid phosphatase the distribution was determined by high-pressure liquid chromatography. Recovery was monitored by the addition of dolichol D15 and D23 phosphate to the homogenate. Rat spleen had the highest dolichol content (114 micrograms/g) followed by lower content in rat liver and brain. The distribution pattern was similar in all organs, with 18 and 19 isoprene residues as dominating components. Human organs contain considerably higher concentrations of dolichol, with the 19 and 20 isoprene residues as the main components. In rat liver, outer mitochondrial and Golgi membranes, lysosomes and plasma membranes contain considerable amounts of dolichol. A drastic increase in dolichol content was observed in rat liver hyperplastic nodules while human liver cirrhosis and hepatocarcinoma showed a marked decrease in dolichol. In the latter case, the distribution pattern was also changed. Of the total amount of dolichol present in the tissues, 2% was phosphorylated in human liver, 10% in human testis and 18% in rat liver. In rat liver mitochondria and in microsomes 4 and 31%, respectively, of the polyprenols were in activated form. The results demonstrated that dolichyl phosphate and dolichol concentrations were regulated by different mechanisms and that the two forms possessed an independent distribution.

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.