Recent studies on the involvement of retinyl phosphate as a carrier of mannose in biological membranes

Rat liver microsomes synthesized [14C]mannosylretinylphosphate and dolichyl [14C]mannosylphosphate from guanosinedisphosphate [14C]mannose, retinylphosphate and dolichylphosphate. Two distinct enzyme activities were shown to be responsible for the biosynthesis of the two mannolipids. A higher affinity mannosyl transferase (EA I), responsible for dolichylmannosylphosphate synthesis, displayed a Km for GDP-mannose of 1.7 microM; while a lower affinity enzyme (EA II), responsible for mannosylretinylphosphate synthesis, displayed a Km for GDP-mannose of 12.5 microM. These Km values were unaffected by the addition of either dolichylphosphate for EA II, or retinylphosphate for EA I. The same Km values were found before and after solubilization of the enzyme activity with 1% Triton X-100. Differential solubilization of EA I and EA II was demonstrated, utilizing different concentrations of Triton X-100. Triple-labeled mannosylretinylphosphate was prepared from [3H]retinylphosphate, retinyl[32P]phosphate and GDP-[14C]mannose from incubations containing rat liver microsomes. This compound was shown to donate [14C]mannose to endogenous acceptors of rat liver microsomes.

The dependence of glycosyltransferases in Dictyostelium discoideum on the structure of polyisoprenols

Mannosyltransferases in plasma membranes of Dictyostelium discoideum synthesize polyisoprenylphosphomannosides from exogenous polyisoprenylphosphates and GDP-mannose. The specificity of the enzymes depends on the chain length and the saturation of the polyisoprenols. Maximum activity is reached by a alpha-saturated C-55 polyisoprenylphosphate.

Enzymatic glucosylation of dolichyl monophosphate formed via cytidine triphosphate in calf brain membranes

When calf brain membrane preparations containing endogenous dolichyl [32P]monophosphate (Dol-32P), prelabeled enzymatically by [gamma-32P]-CTP, are incubated with unlabeled UDP-glucose, the formation of a mild acid-labile [32P]phosphoglucolipid is observed. The biosynthesis of the [32P]phosphoglucolipid is dependent on the concentration of UDP-glucose added, and no [32P]phosphoglycolipid appeared when UDP-glucose was replaced by ADP-glucose, UDP-xylose, UDP-galactose, UDP-mannose, or UDP-glucuronic acid. The 32P-labeled product formed by the UDP-glucose-dependent reaction is chemically and chromatographically identical to glucosylphosphoryldolichol. Several enzymatic parameters of the glucosylation of the specific pool of Dol-P, synthesized by the CTP-mediated kinase, and the total available pool of Dol-P have been compared by a double-label assay utilizing endogenous, prelabeled Dol-32P and UDP-[3H]glucose as substrates.

Dolichyldiphosphoryloligosaccharide--protein oligosaccharyltransferase; solubilization, purification, and properties

Dolichyldiphosphoryloligosaccharide-protein oligosaccharyltransferase was solubilized from hen oviduct rough endoplasmic reticulum by extraction with 0.2% Nonidet P40. Oligosaccharyltransferase activity was assayed in an incubation mixture containing Glc(n)-Man(x)-GlcNAc(2)-diphosphoryldolichol as an oligosaccharyl donor and the (125)I-labeled tryptic peptide consisting of residues 29-58 from bovine alpha-lactalbumin as acceptor. The transferase was purified approximately 2000-fold by fractionation on a bovine alpha-lactalbumin-Sepharose column; the active material bound quantitatively to the gel and was eluted by removal of divalent cation from the wash buffer. The product of the transferase activity, (125)I-glycopeptide, was determined as concanavalin A-agarose-adsorbed radioactivity by a filter disc assay method. (125)I-Labeled concanavalin A-agarose-bound product was characterized as a glycopeptide as follows: (i) gel filtration behavior on Sephadex G-50; (ii) elution from concanavalin A-agarose with 1% alpha-methyl mannoside; (iii) absence of affinity for ricin-Sepharose and loss of affinity for concanavalin A-agarose after treatment with endo-beta-N-acetylglucosaminidase H; (iv) enzymatic synthesis of identical product upon using [(3)H]oligosaccharyldiphosphoryldolichol and unlabeled peptide acceptor; and (v) digestion of (3)H-labeled peptide with Pronase, resulting in the formation of lower molecular weight glycopeptide. Oligosaccharyltransferase activity exhibited an absolute requirement for divalent cations (3 mM Mn(2+); Mg(2+) was 30% as effective), complete dependence on exogenously supplied peptide acceptor (1.33 mug/ml) and oligosaccharyldiphosphoryldolichol (approximately 10 nmol/ml), and an optimum pH between 7 and 7.5.

Yeast mannosyl transferases requiring dolichyl phosphate and dolichyl phosphate mannose as substrate. Partial purification and characterization of the solubilized enzyme

The first mannosyl unit of manno-oligosaccharides of fungal mannoproteins is transferred in a dolichyl-phosphate-dependent reaction sequence to serine/threonine residues of the protein. The two membrane-bound enzymes catalyzing this transfer in the yeast Saccharomyces cerevisiae have been solubilized by detergents. The enzyme transferring mannose from guanosine diphosphate mannose to dolichyl phosphate has been purified 18-fold when based on membrane protein and 140-fold when based on total cell protein. The enzyme transferring mannose from dolichyl phosphate mannose to protein has been purified 48-fold and 380-fold, respectively. A HCl-treated cell-wall mannoprotein from yeast served as acceptor protein for the second enzyme. The solubilized enzyme catalyzing the formation of dolichyl diphosphate mannose has a Km for guanosine diphosphate mannose of 7 x 10(-6) M and is saturated with about 0.15 mM yeast dolichyl phosphate. The metal requirement, pH-optima, and the detergent concentration necessary for optimal activity have been determined for both solubilized enzymes.

Glucosylation of phosphorylpolyisoprenol and sterol at the plasma membrane of soya-bean (Glycine max) protoplasts

Protoplasts were prepared from cells of soya-bean (Glycine max) suspension cultures and the plasma membrane was labelled with diazotized [G-3H]sulphanilic acid. Homogenates were fractionated by differential and isopycnic centrifugation, and membrane fractions in a density gradient were characterized by enzymic markers and the radioactive label. When fractions containing a large amount of protein were incubated with UDP-[U-14C]glucose, radioactive material soluble in chloroform/methanol was formed and this separated into acidic and neutral fractions on ion-exchange chromatograms of DEAE-cellulose. The acidic fraction was shown to consist of dolichol phosphate glucose, and the neutral fraction sterol glucosides and acylsterol glucosides. Optimum conditions for glucosylation of dolichol phosphate were established as 5 mM-MgCl2, pH 6.0, and the enzyme had a Michaelis constant of 1.5 x 10(-5) m-UDP-glucose. Optimum conditions for glucosylation of sterol were 5 mM-MgCl2, pH 8.0 GDP-[U-14C]glucose was a poor substrate for the synthesis of both acidic and neutral lipids. Although the synthesis of dolichol phosphate glucose and sterol glucosides occurred throughout the sucrose gradient, the specific activities of both glucosyltransferases were greatest in a fraction coincident with the radioactively labelled plasma membrane. Results are discussed in relation to the likely role fo these transglucosylase activities.

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

Enzymatic activities which dephosphorylate dolichyl phosphate (Dol-P) and dolichyl pyrophosphate (Dol-P-P) have been observed in membranes from cultured human lymphocytes. Neither activity requires divalent metals. Dol-P phosphatase is inhibited by inorganic phosphate but not by other phosphate-containing compounds. Dol-P-P phosphatase is inhibited by bacitracin but not by phosphate-containing compounds including the methylene analogue of pyrophosphate. These reactions are similar to those previously found in the cycle of bacterial wall peptidoglycan biosynthesis. A chemical synthesis of [32P]Dol-P and [32P]Dol-P-P is reported.

A dolichyl phosphate-cleaving acid phosphatase from Tetrahymena pyriformis

Tetrahymena pyriformis contains an enzyme which hydrolyzed dolichyl phosphate. This activity was solubilized from lyophilized samples of this organism and was relatively stable when stored frozen. The soluble enzyme preparation had an acid pH optimum and hydrolyzed both dolichyl and phytanyl phosphates at equivalent rates. The polyprenylphosphate phosphatase activity was compared with the acid phosphatases which hydrolyzed p-nitrophenyl phosphate and marked differences were found. Dolichyl phosphate hydrolysis required Mg2+ for maximum activity while the bulk of the phosphatase activity was not effected by the absence of this ion. Other differences were that the polyprenylphosphate phosphatase was relatively insensitive to inhibitors such as tartrate and vanadium oxide sulfate which had a pronounced effect on the rate of p-nitrophenyl phosphate hydrolysis. The two activities also appeared to have different subcellular distributions. The polyprenylphosphate phosphatase was markedly inhibited by ethoxy formic anhydride, a reagent which is active against enzymes containing a histidine residue at their active site, while p-nitrophenyl phosphate hydrolysis was unaffected. The polyprenylphosphate phosphatase may be important in regulating the level of dolichyl phosphate in T. pyriformis and thus the rate of glycoprotein synthesis. It is also a useful tool which is capable of liberating dolichol from dolichyl phosphate under mild conditions which will permit the further characterization of the polyprenols.

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

Triton X-100 and exogenous dolichol monophosphate have been used to investigate the nature of enzymes responsible for the transfer of mannose, glucose and N-acetylglucosamine phosphate from nucleotide donors to dolichol monophosphate in vesicles derived from rough and smooth endoplasmic reticulum and mitochondria. Mitochondria were shown to contain the highest specific activities of these enzymes. The responses of the glycosyltransferases to increasing concentrations of Triton X-100 and the effect on these responses of exogenous dolichol monophosphate suggest that the enzymes for mannose and glucose transfer are less hydrophobic, and therefore less intrinsic, in the membrane than the enzyme for N-acetylglucosamine phosphate transfer. In smooth vesicles the results are consistent with mannosyl- and glucosyl-transferases being located at both inner and outer faces of the membrane. In rough vesicles and in mitochondria mannosyl- and glucosyl-transferases were confirmed at the outer face. There is, however, only one site of N-acetylglucosamine phosphate transfer, this being more hydrophobically located in the membrane than the other sites of glycosyl transfer. Mitochondrial enzyme activity closely resembled that of rough endoplasmic reticulum in response to Triton X-100 and exogenous dolichol monophosphate, and is probably associated with the outer membrane.

Biosynthesis of yeast glycoproteins. Processing of the oligosaccharides transferred from dolichol derivatives

The oligosaccharides previously bound to dolichol diphosphate were isolated from Saccharomyces cerevisiae cells incubated with [U-14C]glucose. Five compounds were obtained that migrated with RGlucose of 0.100, 0.120, 0.145, 0.180, and 0.215 on paper chromatography. All of them contained mannose and 2 N-acetylhexosamine residues. The substances that migrated with the three lower RGlucose values had, in addition, glucose units. The structure of the oligosacchardies was very similar if not identical with that of the oligosaccharides isolated from the dolichol diphosphate derivatives synthesized "in vitro" by yeast or rat liver particulate preparations or "in vivo" by dog thyroid or rat liver slices as judged by their migration on paper chromatography, monosaccharide composition, and degradation compounds produced by alpha-mannosidase treatment or acetolysis. The oligosaccharides previously bound to asparagine residues in proteins were isolated from yeast cells which had been pulsed with [U-14C]glucose and chased with medium containing the unlabeled monosaccharide. The samples taken after very short pulses contained four oligosaccharides that migrated with RGlucose of 0.100, 0.120, 0.145, and 0.180 on paper chromatography. The first three compounds contained glucose, mannose, and 2 N-acetylhexosamine residues whereas the one that migrated with a RGlucose of 0.180 was devoid of the former monosaccharide. Samples taken after short chase periods revealed that the compounds that migrated with the lower RGlucose values gradually disappeared and were converted to the oligosaccharide with the higher RGlucose value was they lost their glucose residues. Similar analysis as those mentioned above showed that the structures of these compounds were similar to those of the dolichol diphosphate-bound oligosaccharides. Samples taken after longer chase periods revealed that the oligosaccharide that migrated with a RGlucose of 0.180 was subsequently either enlarged by the addition of more mannose residues or trimmed to smaller sizes.

Squalene synthetase

In the first part of the review the background to the discovery of the asymmetric synthesis of squalene from two molecules of farnesyl pyrophosphate and NADPH is described, then the stereochemistry of the overall reaction is summarized. The complexity of the biosynthesis of squalene by microsomal squalene synthetase demanded the existence of some intermediate(s) between farnesyl pyrophosphate and squalene. This demand was satisfied by the discovery of presqualene pyrophosphate, an optically active C30 substituted cyclopropylcarbinyl pyrophosphate, the absolute configuration of which at all three asymmetric centers of the cyclopropane ring was deduced to be R. Possible mechanisms for the biosynthesis of presqualene pyrophosphate and its reductive transformation into squalene are presented. In the second part of the review the nature of the enzyme is discussed. The question whether presqualene pyrophosphate is an obligate intermediate in the biosynthesis of squalene is examined, with the firm conclusion that it is. It is as yet uncertain whether the two half reactions of squalene synthesis, i.e. (i) 2 x farnesyl pyrophosphate leads to presqualene pyrophosphate; (ii) presqualene pyrophosphate + NADPH (NADH) leads to squalene, are catalyzed by one or two enzymes or by a large complex with two catalytic sites. Evidence is cited for the existence on the enzyme of two distinct binding sites with different affinities for the two farnesyl pyrophosphate molecules. The types of enzyme preparations available at present are described and types of experiments carried out with these are critically examined. The implications of the properties of a low molecular weight squalene synthetase solubilized with deoxycholate from microsomal membranes is discussed and a model for the enzyme in an organized membrane structure is presented.

Properties of the dolichol phosphate: GDPmannose mannosyltransferase of liver microsomes

The reaction of GDP[14C]-mannose with dolichol phosphate (Dol-P) in hepatic microsomes is characterized by an initial brief period of relatively rapid Dol-P-[14C]-mannose synthesis. The time course of this 1--3 min period of rapid synthesis follows approximate first order kinetics. However, the rate of reaction does not decrease to zero as predicted by the kinetics of the initial period of synthesis, but continues instead at a slow, steadily decreasing, rate. Examination of the time course of Dol-P-mannose synthesis for different concentrations of GDP[14C]-mannose revealed that the extrapolated final level of Dol-P-mannose synthesized is increased when the concentration of GDPmannose is raised. These data, plus those derived from studies of the reverse reaction, suggest that the non-linear time course for the synthesis of Dol-P-mannose is due in part to the reaction approaching equilibrium between the forward and reverse reactions. The effects of Mn++ on the time course of the forward and reverse reaction are complex and suggest that the Mn++ complexes of both GDPmannose and GDP are poorer substrates for the enzyme than the free nucleotides. Perturbations of the lipid environment of the microsomal membrane by treatment with phospholipase A, detergent, sonication, or alkaline pH lead to a decrease in the final level of Dol-P-mannose synthesized, but do not affect the time required for half maximal labeling. When the reverse reaction was investigated in phospholipase A-treated microsomes, the final extent of the reaction was also reduced. These data suggest that perturbation of the membrane lipid environment decreases in some undefined way the availability of Dol-P and Dol-P-mannose to enzyme.

Subcellular localization of the enzyme that forms mannosyl retinyl phosphate from guanosine diphosphate [14C]mannose and retinyl phosphate

The subcellular distribution of the enzyme catalysing the conversion of retinyl phosphate and GDP-[14C]mannose into [14C]mannosyl retinyl phosphate was determined by using subcellular fractions of rat liver. Purity of fractions, as determined by marker enzymes, was 80% or better. The amount of mannosyl retinyl phosphate formed (pmol/min per mg of protein) for each fraction was: rough endoplasmic reticulum 0.48 +/- 0.09 (mean +/- S.D.); smooth membranes (consisting of 60% smooth endoplasmic reticulum and 40% Golgi apparatus), 0.18 +/- 0.03; Golgi apparatus, 0.13 +/- 0.03; and plasma membrane 0.02.

Formation of steroids by the pregnant mare. VI. Metabolism of [14C]farnesyl pyrophosphate and [3H]dehydroepiandrosterone injected into the fetus

A mixture of [4,8,12-14C]farnesyl pyrophosphate and [3H]dehydroepiandrosterone was injected into a horse fetus im during laparotomy, after which maternal urine was collected for 6 days. Steroid conjugates in the urine were extracted with Amberlite XAD-2 resin, hydrolyzed, and separated into phenolic and neutral fractions. Estrone, 17 alpha-estradiol, equilin [3-hydroxy-1,3,5(10),7-estratetraen-17-one], and 17 alpha-dihydroequilin [1,3,4(10),7-estratetraene-3,17 alpha-diol] were isolated from the phenolic fraction and their radiochemical purities were established. Only estrone and 17 alpha-estradiol contained both 3H and 14C, while the B ring unsaturated estrogens, equilin and 17 alpha-dihydroequilin, contained only 14C. From the neutral fraction, 14C-labeled 3 beta-hydroxy-5 alpha-pregnan-20-one, 5 alpha-pregnane-3 beta-20 beta-diol, and 5 alpha-pregnane-3 beta, 20 alpha-diol were isolated. These results together with our previous findings demonstrate that the route of biosynthesis of both the ring B saturated and unsaturated estrogens is the same up to the stage of farnesyl pyrophosphate. Thus, the bifurcation in the classical pathway of steroid biosynthesis is occurring at a point after the formation of farnesyl pyrophosphate and before the formation of squalene and cholesterol.

Inhibition of liver prenyltransferase by alkyl phosphonates and phosphonophosphates

n-Pentyl and n-decyl phosphonate and the corresponding phosphonophosphates were found to inhibit cholesterol synthesis from mevalonate in the 10000 X g supernatants of liver homogenates and the synthesis of farnesyl pyrophosphate from geranyl and isopentenyl pyrophosphate by purified liver prenyltransferase. Kinetic analysis of the inhibition of prenyltransferase showed that the phosphonates and the phosphonophosphates interacted with two forms, or two sites, of the enzyme. The order of increasing potency was C5-phosphonate less than C10-phosphonate less than C5-phosphonophosphate less than C10-phosphonophosphate. The phosphonophosphates were at least ten times stronger inhibitors than the phosphonates.

[Synthesis of retinyl phosphate, its physico-chemical properties and biological activity]

Retinyl phosphate (RP) has been synthesized from retinol (R), using phosphodimorpholine chloride (PDMC) or POCl3. A method for isolating and purifying RP from the reaction mixture has been developed. The RP yield varies from 3 to 6% depending on the quantities of R and POCl3. The RP yield can be increased by 10--15% as a result of RP separation and rerun of the reaction. The resultant RP has an absorption maximum at 325 nm, is described by the formula C20H27PO4(NH4)2 and distributed between equal volumes of diethyl ester and 20% aqueous methanol at a ratio of 1:1. RP is alkaline-resistant and rapidly transforms into anhydroretinol (AR) in the acid medium. Eighteen hours after RP administration into the caudal vein or parenterally retinyl palmitate and R are accumulated in the liver of A-avitaminous rats.

Glycosyl transfer from nucleotide sugars to C85- and C55-polyprenyl and retinyl phosphates by microsomal subfractions and Golgi membranes of rat liver

The capacity of isolated membrane fractions to catalyse transfer of sugars from sugar nucleotides to alpha-saturated and non-saturated forms of phosphorylated C85 and C55 polyprenols and retinyl phosphate was examined. The amount of endogenous lipid acceptor present for various sugars was also measured. It appears that the types and amounts of polyprenyl phosphates present in rough- and smooth-microsomal fractions and Golgi membranes are different and the individual polyprenyl phosphates exhibit specificity as sugar acceptors.

Lipid intermediates in the synthesis of the inner core of yeast mannan

The synthesis by yeast microsomes of compounds that are probably dolichol-pyrophosphate derivatives containing N,N'-diacetylchitobiose and several mannose residues is described. However, the presence of monosaccharide residues other than N-acetylglucosamine and mannose has not been ruled out. The amount of the lipid derivatives synthesized was enhanced by the addition to the incubation mixture of an organic-solvent-soluble extract from rat liver known to contain dolicholpyrophosphate oligosaccharides. Incubation of these derivatives with yeast microsomes led to the transfer of about fifteen percent of their saccharide moieties to endogenous proteins. The oligosaccharides released from the dolichol derivatives by mild acid hydrolysis could serve as primers for the synthesis of a polysaccharide having the characteristics of mannan outer-chain. It is suggested that the dolichol-pyrophosphate derivatives described in this paper are intermediates in the synthesis of mannan inner core.

Factors affecting glucosyl and mannosyl transfer to dolichyl monophosphate by liver cell-free preparations

GDP-mannose and UDP-mannose (each at less than 1 micrometer) markedly inhibit glucosyl transfer from UDP-glucose (1.6 micrometer( to dolichyl phosphate in liver microsomal preparations. The biphasic response suggests the presence of two glucosyl transferases only one of which is inhibited. The inhibition appears to be a property of the intact nucleotide phosphate sugars and not due to competition for a limited pool of dolichyl phosphate. UDP-galactose and UDP-xylose cause a less marked inhibition of the same enzyme. The failure of UDP-glucose to inhibit mannosyl transfer suggests that the pool of dolichol monophosphate used by mannosyl transferase is not available to the glucosyl transferase. The relationship between the degree to which an exogenous prenol phosphate acts as an acceptor of mannose and the degree to which it inhibits mannosylation of endogenous dolichyl monophosphate varies among different prenyl phosphates. Mannosyl transferase exhibits two pH optima.