[Biosynthesis of dolichol phosphate mannose in the mitochondrial outer membrane (author's transl)]

Divergence to apoptosis from ROS induced cell cycle arrest: effect of cadmium

Recently, the role of cadmium (Cd) in immunosupression has gained importance. Nevertheless, the signaling pathways underlying cadmium-induced immune cell death remains largely unclear. In accordance to our previous in vivo report, and to evaluate the further details of the mechanism, we have investigated the effects of cadmium (CdCl(2), H(2)O) on cell cycle regulation and apoptosis in splenocytes in vitro. Our results have revealed that reactive oxygen species (ROS) and p21 are involved in cell cycle arrest in a p53 independent manner but late hour apoptotic response was accompanied by the p53 up-regulation, loss of mitochondrial transmembrane potential (MTP), down-regulation of Bcl-xl, activation of caspase-3 and release of cytochrome c (Cyt c). However, pifithrin alfa (PFT-alpha), an inhibitor of p53, fails to rescue the cells from the cadmium-induced cell cycle arrest but prevents Bcl-xl down-regulation and loss of Deltapsi(m), which indicates that there is an involvement of p53 in apoptosis. In contrast, treatment with N-acetyl cysteine (NAC) can prevent cell cycle arrest and p21 up-regulation at early hours. Although it is clear that, NAC has no effect on apoptosis, p53 expression and MPT changes at late stage events. Taken together, we have demonstrated that cadmium promotes ROS generation, which potently initiates the cell cycle arrest at early hours and finally induces p53-dependent apoptosis at later part of the event.

Omi/HtrA2 Promotes Cell Death by Binding and Degrading the Anti-apoptotic Protein ped/pea-15

ped/pea-15 is a ubiquitously expressed 15-kDa protein featuring a broad anti-apoptotic function. In a yeast two-hybrid screen, the pro-apoptotic Omi/HtrA2 mitochondrial serine protease was identified as a specific interactor of the ped/pea-15 death effector domain. Omi/HtrA2 also bound recombinant ped/pea-15 in vitro and co-precipitated with ped/pea-15 in 293 and HeLa cell extracts. In these cells, the binding of Omi/HtrA2 to ped/pea-15 was induced by UVC exposure and followed the mitochondrial release of Omi/HtrA2 into the cytoplasm. Upon UVC exposure, cellular ped/pea-15 protein expression levels decreased. This effect was prevented by the ucf-101 specific inhibitor of the Omi/HtrA2 proteolytic activity, in a dose-dependent fashion. In vitro incubation of ped/pea-15 with Omi/HtrA2 resulted in ped/pea-15 degradation. In intact cells, the inhibitory action of ped/pea-15 on UVC-induced apoptosis progressively declined at increasing Omi/HtrA2 expression. This further effect of Omi/HtrA2 was also inhibited by ucf-101. In addition, ped/pea-15 expression blocked Omi/HtrA2 co-precipitation with the caspase inhibitor protein XIAP and caspase 3 activation. Thus, in part, apoptosis following Omi/HtrA2 mitochondrial release is mediated by reduction in ped/pea-15 cellular levels. The ability of Omi/HtrA2 to relieve XIAP inhibition on caspases is modulated by the relative levels of Omi/HtrA2 and ped/pea-15.

Regulation of HAX-1 Anti-apoptotic Protein by Omi/HtrA2 Protease during Cell Death

Omi/HtrA2 is a nuclear-encoded mitochondrial serine protease that has a pro-apoptotic function in mammalian cells. Upon induction of apoptosis, Omi translocates to the cytoplasm and participates in caspase-dependent apoptosis by binding and degrading inhibitor of apoptosis proteins. Omi can also initiate caspase-independent apoptosis in a process that relies entirely on its ability to function as an active protease. To investigate the mechanism of Omi-induced apoptosis, we set out to isolate novel substrates that are cleaved by this protease. We identified HS1-associated protein X-1 (HAX-1), a mitochondrial anti-apoptotic protein, as a specific Omi interactor that is cleaved by Omi both in vitro and in vivo. HAX-1 degradation follows Omi activation in cells treated with various apoptotic stimuli. Using a specific inhibitor of Omi, HAX-1 degradation is prevented and cell death is reduced. Cleavage of HAX-1 was not observed in a cell line derived from motor neuron degeneration 2 mice that carry a mutated form of Omi that affects its proteolytic activity. Degradation of HAX-1 is an early event in the apoptotic process and occurs while Omi is still confined in the mitochondria. Our results suggest that Omi has a unique pro-apoptotic function in mitochondria that involves removal of the HAX-1 anti-apoptotic protein. This function is distinct from its ability to activate caspase-dependent apoptosis in the cytoplasm by degrading inhibitor of apoptosis proteins.

Activation and mitochondrial translocation of protein kinase Cdelta are necessary for insulin stimulation of pyruvate dehydrogenase complex activity in muscle and liver cells

In L6 skeletal muscle cells and immortalized hepatocytes, insulin induced a 2-fold increase in the activity of the pyruvate dehydrogenase (PDH) complex. This effect was almost completely blocked by the protein kinase C (PKC) delta inhibitor Rottlerin and by PKCdelta antisense oligonucleotides. At variance, overexpression of wild-type PKCdelta or of an active PKCdelta mutant induced PDH complex activity in both L6 and liver cells. Insulin stimulation of the activity of the PDH complex was accompanied by a 2.5-fold increase in PDH phosphatases 1 and 2 (PDP1/2) activity with no change in the activity of PDH kinase. PKCdelta antisense blocked insulin activation of PDP1/2, the same as with PDH. In insulin-exposed cells, PDP1/2 activation was paralleled by activation and mitochondrial translocation of PKCdelta, as revealed by cell subfractionation and confocal microscopy studies. The mitochondrial translocation of PKCdelta, like its activation, was prevented by Rottlerin. In extracts from insulin-stimulated cells, PKCdelta co-precipitated with PDP1/2. PKCdelta also bound to PDP1/2 in overlay blots, suggesting that direct PKCdelta-PDP interaction may occur in vivo as well. In intact cells, insulin exposure determined PDP1/2 phosphorylation, which was specifically prevented by PKCdelta antisense. PKCdelta also phosphorylated PDP in vitro, followed by PDP1/2 activation. Thus, in muscle and liver cells, insulin causes activation and mitochondrial translocation of PKCdelta, accompanied by PDP phosphorylation and activation. These events are necessary for insulin activation of the PDH complex in these cells.

Occurrence of ceramides and neutral glycolipids with unusual long-chain base composition in purified rat liver mitochondria

The free ceramide content of rat liver mitochondria was found to be 1.7 nmol/mg protein and outer membranes contained a three-fold higher concentration than inner membranes. The mitochondrial content in neutral glycolipids was 0.6 nmol/mg protein. The long-chain bases found in free ceramides were d18:1 sphingosine, d18:0 3-ketosphinganine and t21:1 phytosphingosine in increasing order. In contrast, 3-ketosphinganine was the only base of glucosylceramide and lactosylceramide of inner membranes, whereas d18:1 sphingosine was the major long-chain base of glucosylceramide of outer membranes.

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

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

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

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

Effects of glycosylation enzymes from membrane fractions, induced by chronic ethanol administration

To study the effect of chronic ethanol administration on the enzyme activities involved in the glycosylation processes (glycosidases and glycosyltransferases), we used 6-week ethanol-treated female Wistar rats and pair-control rats. Biological material was membrane fractions of microsomes and plasma membrane obtained by subcellular fractionation technique. Ethanol treatment increased with statistical significance the Vmax of N-acetyl-beta-D-glucosaminidase (P less than 0.10), beta-D-glucuronidase and alpha-D-mannosidase (P less than 0.001) activities from microsomal fractions and likewise it increased the Km value of beta-D-glucuronidase (P less than 0.001). In vitro doses of ethanol (0.1-1.7 M) were added to delucidate hypothetical membrane tolerance responses. However in vitro addition of ethanol provoked no differential effects in treated and untreated rats. Glycosyltransferase assays were carried out using [14C]sugar derivatives. We detected several glycosyltransferase activities in both microsome and plasma membrane fractions. Chronic ethanol exposure appeared to affect N-acetyl-neuraminyltransferase activity from both membrane fractions, producing a greatly increased incorporation in the presence of asialofetuin. Glucosyl and mannosyltransferase activities from plasma membrane fractions were also altered by ethanol treatment, producing an increased enzyme activity when reaction was performed in the presence of phosphatidylcholine + dolichol-phosphate liposomes.

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.

Triggering of mannosyltransferase activity in inner mitochondrial membranes by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids

The activity of GDPmannose:dolichyl monophosphate mannosyltransferase in inner mitochondrial membranes can be triggered by dolichyl-monophosphate incorporation mediated through phospholipids or fatty acids. The efficiency of this incorporation and the efficiency of the enzyme activity are not equivalent. Among a variety of amphiphiles which were tested, the highest mannosyltransferase activity was obtained with the mixture of lipids extracted from the outer mitochondrial membranes. The results presented here appear consistent only with a mechanism involving collisional contacts of the phospholipid vesicles and fusion with the membranes. ESR spectroscopy confirms that (a) the incorporation process is followed by solubilization of dolichyl monophosphate molecules in the lipid phase and (b) the general organization of the inner mitochondrial membranes is not perturbed by the addition of dolichyl monophosphate.

Effect of desipramine on a glycoprotein sialyltransferase activity in C6 cultured glioma cells

The tricyclic antidepressant desipramine, when added to culture medium, gave rise in C6 rat glioma cells to a decrease of the activity of the enzyme asialofetuin sialyltransferase. The inhibition was dose and time dependent and was observed in both multiplying cells and cells blocked with 2 mM thymidine or depletion of amino acids. This inhibition was rather specific to the sialyltransferase, as under the conditions where this enzyme was inhibited up to 70%, other enzymes such as dolichol phosphate mannose synthetase, glutamine synthetase, and glycerol phosphate dehydrogenase remained unaffected. This inhibition was not reversed after removal of desipramine from the medium and was not observed by direct addition of desipramine to the sialyltransferase incubation assay. Under the same conditions, W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], which is known to be a potent calmodulin antagonist and an inhibitor of calmodulin-dependent kinases, gave the same concentration-dependent inhibition profile of sialyltransferase as desipramine, whereas H-7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine], which is an inhibitor of protein kinase C and cyclic nucleotide-dependent kinases, had no effect. So, it is suggested that desipramine inhibits the sialyltransferase activity in C6 glioma cells through a calmodulin-dependent system.

Comparative study of the N-glycoprotein synthesis through dolichol intermediates in mitochondria, Golgi apparatus-rich fraction and endoplasmic reticulum-rich fraction

1. Glycosylation of endogenous dolichol acceptors was higher in mitochondria than in C 30,000 g (Golgi apparatus-rich fraction) and C 100,000 g (endoplasmic reticulum-rich fraction). 2. In mitochondria, N-glycoprotein biosynthesized were composed of high mannose type and non-fucosylated biantennary complex type while in C 30,000 g and C 100,000 g preparations, they contained biantennary complex type as tri and tetraantennary complex type oligosaccharides in both fucosylated and non-fucosylated forms.

Effects of chronic ethanol administration on plasma-membrane-bound glycosyltransferase activities

Physiological and membranous modifications induced by a 4-week ethanol administration in mouse liver plasma membrane were studied. Galactosyl- and glucosyltransferase activities were stimulated in the presence of dolichylphosphate alone or with phosphatidyl-choline. The galactosyltransferase activity was inhibited by chronic ethanol administration. These enzymes were modulated by different phospholipids. The phosphatidic acid was the most efficient activator. Ethanol provoked an inhibition of the galactosyltransferase activity whatever the phospholipid used, as well as an inhibition of the glucosyltransferase activity, chiefly in presence of phosphatidyl-inositol. The preincubation of control or treated mouse liver plasma membranes with liposomes loaded by dolichylphosphate and cholesterol greatly enhanced the enzymatic activities without removing the inhibition by ethanol treatment.

Mitochondrial biogenesis: do liver mitochondria contain glycoproteins and glycosyltransferases?

1. Subcellular fractions isolated from livers of 19-day-old chicken embryos were analyzed in order to assess whether liver mitochondria contained glycosylated proteins or had mannosyl- or sialyl-transferases that could transfer sugars to mitochondrial macromolecules. 2. Proteins in liver mitochondrial membranes and matrix fractions were screened for their affinities for concanavalin A (Con A). 3. After separation by gel electrophoresis under denaturing conditions, a significant number of the proteins bound [125I]Con A, and the binding of the lectin was substantially inhibited by alpha-methyl-D-mannoside. 4. In addition, radio-iodinated matrix proteins were screened for lectin-binding properties by chromatography on Con A covalently linked to agarose. 5. A number of proteins, representing 14% of those loaded onto the column, became tightly bound to the agarose-linked lectin, and the molecular weights of several of those proteins are reported. 6. Mannosyltransferase activities were measured in fractions highly enriched for mitochondria. 7. In the reactions, mannose was transferred from guanosine diphosphomannose to materials insoluble in 0.3% trichloroacetic acid or in chloroform:methanol (2:1). 8. The fractions also catalyzed the transfer of mannose to materials extractable in chloroform:methanol and which migrated with the Rf of dolichol phosphate on Silica Gel H. 9. Dolichol phosphate stimulated the transfer of mannose to those materials extractable in the organic solvents. 10. Marker enzyme analyses indicated that the mannosyl transferase activity in the mitochondrial fraction could not be accounted for entirely by contaminating microsomal membranes. 11. Although sialyltransferase activity was detected also in the mitochondrial fractions, the levels of the activity and the kinetics of the reactions indicated that Golgi membranes were most likely the sources of the enzyme.

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

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

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

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

Glycosyltransferase activities in normal and leukaemic monocytic cells

Glycosyltransferase activities were measured in normal monocytes and in leukaemic monoblasts. Biosynthesis of glycosylated derivatives of dolichyl-monophosphate, which act as intermediates in glycosylation, was measured. Transfer of mannose from GDP-mannose was greatly increased in leukaemic monoblasts. Galactosyltransferase activities, using endogenous protein acceptors, were increased in leukaemic cells of the monocytic lineage compared to normal cells. No significant difference was observed on specific exogenous glycoprotein acceptors. These selective increases of some glycosyltransferase activities in normal and leukaemic monocytic cells can be correlated either with different expression of specific carbohydrate structures or with changes in glycosylation regulation.

Purification of liver plasma membranes and localization of a protein galactosyltransferase

Plasma membranes were isolated from mouse liver. The purification was achieved by a modified method of Ray (1970). The validity of this fractionation procedure is controlled by measurements of specific enzymatic activities and by electron microscopy. The purified plasma membranes were found to contain galactosyltransferase which transferred galactose from UDP-galactose onto endogenous acceptors. This activity requires Mn2+ for its catalytic activity.

Topological orientation of mitochondrial GDPmannose: dolichyl-monophosphate mannosyltransferase in the outer membrane

Previous studies have shown the existence of an autonomous mitochondrial GDPmannose:dolichylmonophosphate mannosyltransferase, located in mitochondrial outer membrane of liver cells. As nothing is known about glycosylation sites in mitochondria, we have investigated the topological orientation of this enzyme in intact mitochondria, using controlled proteolysis with trypsin. Mitochondria were purified sequentially by mild ultrasonic treatment and sucrose density gradient. Purity and homogeneity of mitochondrial fraction were assessed by electron microscopy and specific marker enzymes measures. Our data provide evidence for a mitochondrial GDPmannose:dolichylmonophosphate mannosyltransferase facing the cytoplasmic side of the outer membrane. However, the exposure of this enzyme to the water phase has been shown to be dependent on the ionic strength of the environment.

Glucosyltransferase activity in mitochondria. Biosynthesis of glucosyl-phosphoryl-dolichol in inner mitochondrial membranes

1. Inner mitochondrial membranes are able to transfer [14C]glucose from UDP-[14C]glucose onto dolichylmonophosphate. 2. Synthesis of dolichyl-phosphoryl-glucose takes place only in the presence of exogenous dolichyl-monophosphate loaded into phospholipid vesicles. 3. Neutral phospholipids interact preferentially with the membrane-bound enzyme. The effect of phospholipids is not related to the length of fatty acid chains but a correlation between the activation and the degree of unsaturation of fatty acid chains has been found. 4. This enzyme required divalent cations for activity. Such a requirement might be related to lipid-protein interactions which favour a suitable conformation of glycosyltransferase.

Galactosyltransferase activities in mitochondria outer membrane: biosynthesis of galactosylated proteins

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

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

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

Galactosyltransferase activities in mitochondrial outer membrane: biosynthesis of dolichylmonophosphate-galactose

Mitochondrial outer membranes were prepared from mouse liver homogenates by swelling purified mitochondria in phosphate buffer and were purified on a discontinuous sucrose gradient. Assays for marker enzymes and controls in electron microscopy confirmed the purity and homogeneity of this subfraction. Mitochondrial outer membranes had significant galactosyltransferase activity when incubated with UDP-[14C]galactose: 14C-labelling was found in products extractable with organic solvents and in a residual precipitate. Addition of exogenous dolichylmonophosphate loaded into phosphatidylcholine liposomes strongly enhanced the incorporation of [14C]galactose into chloroform/methanol (2:1, v/v) -extractable products. Thin-layer chromatography of these 2:1 extracts showed that the increase of [14C]galactose incorporation was attributable to the synthesis of a new galactosylated lipid, 'lipid L'. This 'lipid L' has been purified on silicic acid columns by elution with chloroform/methanol (1:1, v/v). The purified 'lipid L' was labile in acid and released [14C]galactose. It had the same chromatographic behaviour as dolichylmonophosphate-mannose in neutral, acid and alkaline solvent systems. Upon incubation in presence of [3H]dolichylmonophosphate and UDP-[14C]galactose, purified 'lipid L' contained both 3H- and 14C-labelling. 'Lipid L', synthesized by mitochondrial outer membranes, was therefore characterized as dolichylmonophosphate-galactose.

Characterization of a glucosyltransferase activity in liver plasma membrane: modulation by cations and lipidic effectors

1. Glucosyltransferase activity incorporating [14C]glucose from UDP-[14C]glucose onto endogenous lipidic acceptors was localized primarily in the plasma membrane of liver. 2. Incubation of plasma membrane by phosphatidyl-choline liposomes loaded with dolichyl-phosphate stimulated the enzymatic activity. 3. This enzyme required Mg2+ for maximal catalitic activity. Ca2+ could substitute Mg2+. 4. Mn2+ acted as a partial non-competitive inhibitor of the Mg2+-activated glucosyltransferase. 5. This enzyme can be modulated by neutral and acidic phospholipids; the most efficient were phosphatidyl-serine and phosphatidyl-inositol. 6. The enzymatic activity was not significantly changed by cholesterol alone but it is greatly enhanced by liposomes loaded with dolichyl-phosphate and cholesterol.

Purification and properties of a mannosyltransferase solubilized from mitochondrial outer membranes

The enzyme GDPmannose: dolichyl monophosphate mannosyltransferase has been solubilized and purified from mice liver mitochondrial outer membranes. The purification combines detergent extraction of purified outer membranes using Nonidet P-40, with subsequent ion-exchange chromatography on DEAE-cellulose. At this stage, a 400-fold purification is obtained. The partially purified mannosyltransferase is activated by choline-containing lipids such as phosphatidylcholine, lysophosphatidylcholine and sphingomyelin. The reaction is dependent upon the addition of exogenous dolichyl monophosphate. The sole reaction product has been identified as dolichyl phosphate-mannose. The partially purified mannosyltransferase exhibits a Km of 1.33 microM for GDPmannose. Enzyme activity, eluted from DEAE-cellulose, could be further purified after incorporation into sphingomyelin vesicles containing dolichyl monophosphate followed by a sucrose density gradient centrifugation. The mannosyltransferase activity is completely associated with the liposomes at the top of the gradient. Significant stabilization and purification (approx. 1600-fold) of enzyme activity associated with these liposomes is obtained. Furthermore, the reconstitution of this purified enzyme within specific liposomes provides a good model membrane to investigate the molecular requirement of this mitochondrial mannosyltransferase.

Glycosyltransferase activities in liver mitochondria. Phospholipid-dependence of inner membrane mannosyltransferase

The role of phospholipids in the activity of inner mitochondrial mannosyltransferase was investigated. This enzyme catalyzes the direct transfer from GDP-mannose to lipidic acceptor. Inner mitochondrial membranes from purified mice liver mitochondria are prepared by digitonin treatment. Swelling of mitoplasts leads to the formation of inner membrane vesicles, which are then purified on a discontinuous sucrose gradient. The validity of this fractionation procedure is controlled by measurements of specific enzymatic activities and by electron microscopy. Measurement of mannosyltransferase activity in native inner mitochondrial membranes is unsuccessful, even in the presence of exogenous dolichyl monophosphate. Treatment of inner membranes with specific phospholipid liposomes in the presence of exogenous dolichyl monophosphate is essential in order to measure this enzymatic activity. Addition of phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and cardiolipin in the presence of Mg2+ results in a high degree of activation of the mannosyltransferase system. Maximal enzymatic activity is obtained with an approximate 3:7 mass ratio of exogenous phospholipid to inner membrane proteins. These experiments establish that sensitivity to activation by phospholipids is an inherent property of inner membrane mannosyltransferase. Another approach to this problem was to reconstitute an in vivo lipidic environment of the inner membrane. The results of this procedure suggest that the activity of inner mitochondrial mannosyltransferase may be subject to modulation by outer membrane lipidic extract treatment.

Subcellular localisation of cerebral fucosyltransferase

GDP-fucose: asialofetuin fucosyltransferase from sheep brain was fractionated on a sucrose gradient into two activity peaks. Using purification on Ficoll adapted from the proposed method [(1980) J. Neurochem. 35, 281-296], double localisation of cerebral fucosyltransferase was confirmed and the subcellular active fractions identified as light microsomes and mitochondria.

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.

Effects of defined synthetic phospholipids on glycosylation processes. Modifications of membrane-bound N-acetylglucosaminyl-transferase and solubilized sialyl-transferase activities

In this paper, we study effects of phosphatidylcholine synthetic derivatives on two different glycosyltransferasic systems, a membranous mitochondrial N-acetylglucosaminyl-transferase and a solubilized microsomal sialyl-transferase. From all our results, we proved the strong inhibitory effect of lysophosphatidylcholines in these glycosylation processes. We investigated then the relationship between this inhibitory effect and the chemical structure of the studied phospholipids. From this work, it became obvious that the most important inhibitory effect is obtained with a molecule containing a long aliphatic side chain in sn-1 of glycerol, a carbon atom devoid of any large group in sn-2 and an hydrophilic part in sn-3 (specially choline).

Characterization of a mannosyl-lipid compound of microsomal fractions of rat pancreas and influence of diet

1. High-starch diet induces an activation of rat pancreatic microsomal mannosyl-transferase activity as compared with a standard diet or a high-fat diet. 2. This increase is found in a mannose-containing lipid which is identified as a dolichyl-phosphoryl-mannose on the criteria of DEAE-cellulose chromatography, alkaline and acid hydrolysis, thin-layer chromatography and identity of this endogenous product with the [14C]mannose-containing product synthesized in presence of exogenous dolichyl-monophosphate added to the incubation medium. 3. Kinetic parameters (Km and Vmax) of the enzyme versus polyprenic acceptor are not modified by the diet. 4. The results indicate that the activation of the mannose transfer is principally due to an increase of the polyprenic endogenous acceptor by the high-starch diet.

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

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

Dolichyl phosphate phosphatase from Tetrahymena pyriformis

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

Isolation and characterization of dolichols from Tetrahymena pyriformis

Dolichols of Tetrahymena pyriformis were isolated and characterized by TLC, HPLC and mass spectrometry. Four strains of Tetrahymena were studied and found to have relatively small amounts of dolichol, from 0.26 to 2.60 mg dolichol/kg wet weight. All four strains had approximately the same relative proportions of isoprenologs, dolichol-13 (2%), dolichol-14 (74%), dolichol-15 (23%), and dolichol-16 (less than 1%). Tetrahymena dolichols were found mainly in the mitochondrial subcellular fraction (86%). The pellicle fraction contained 9% and the microsomal fraction, 5% of the remaining dolichol. Free dolichol has also been found in the mitochondrial fraction of four other organisms. We were not able to demonstrate dolichyl esters in these organisms, but their presence is inferred, because reduced yields of dolichol were obtained if the lipid extracts were not saponified prior to HPLC assay.

Glucosyltransferase activities in liver mitochondria. I. Biosynthesis of dolichyl[14C]glucosyl phosphate and [14C]glucosylceramide

Mitochondria, and specially outer mitochondrial membranes, incorporate D-[14C]glucose from UDP-D-[14C]glucose into products extracted with organic solvents and into a residual precipitate, with a pH optimum of about 6.5 in (2-N-morpholino-ethane)-sulfonic acid (MES) buffer. The chloroform/methanol (2:1, v/v) extract contains two products. The major [14C]glucolipid is stable to mild alkali, but releases [14C]glucose upon mild acid hydrolysis. It is retained on DEAE-cellulose (acetate form) and is eluted with the same ionic strength as an hexosyldolichyl monophosphate diester. This [14C] glucolipid has the same chromatographic behaviour as dolichyl-mannosylphosphate in neutral, acidic and basic solvent systems; and its biosynthesis is greatly increased by exogenous dolichylmonophosphate. The other [14C]glucolipid is stable upon mild acid hydrolysis and is not retained on DEAE-cellulose. On silicic acid it is eluted with acetone. The biosynthesis of this compound is stimulated by exogenous ceramide. This glucolipid has the same chromatographic mobility in different solvent systems as glucosylceramide isolated from the liver of a patient with Gaucher's disease. Biosynthesis of these two glucolipids is inhibited by UDP, but only biosynthesis of dolichylglucosyl monophosphate is reversible with this nucleotide. The biosynthesis of these different glucosylated derivatives is stimulated by the addition of divalent cations (Mn2+, Mg2+). the effect of these two metal ions on dolichylglucosyl monophosphate and glucosylceramide formation is studied in different conditions.

[Mitochondrial N-acetylglucosaminyl transferase connected with viral infection]

A higher level of N-acetylglucosamine incorporation by proteinic and polyprenic endogenous acceptors is observed after infection by Myxovirus. This phenomenon occurs in whole mitochondria as in outer mitochondrial membranes, where it is particularly obvious with proteinic endogenous acceptors. Under viral infection, no new N-acetylglucosaminylated polyprenol is detected. In the case of infected animals as in the case of control animals, compounds P1 (extracted by chloroform/methanol 2:1) are identified by thin layer chromatography as a N-acetylglucosamyl-pyrophosphoryl-dolichol and a N, N'-diacetylchitobiosyl-pyrophosphoryl-dolichol. In the case of infected animals, biosynthesis of proteinic acceptors and dolichol is not modified; therefore the increase of N-acetylglucosamine incorporation is not due to a modification of the endogenous acceptors level. By the use of exogenous dolichol-monophosphate we demonstrate that the increased transfer of [14C] N-acetylglucosamine into polyprenic acceptors is the result of a higher activity of the mitochondrial N-acetylglucosaminyltransferase after viral infection.

[Mitochondrial sialyltransferase (author's transl)]

A sialyl transferase activity is found in purified mitochondria. It is not due to residual contamination and this enzymatic system is located in the outer mitochondrial membrane. This proves mitochondrial autonomy in regard to glyconconjugate sialylation.