2H NMR investigation of the organization and dynamics of polyisoprenols in membranes

A carotenoid-deficient mutant of the plant-associated microbe Pantoea sp. YR343 displays an altered membrane proteome

Membrane organization plays an important role in signaling, transport, and defense. In eukaryotes, the stability, organization, and function of membrane proteins are influenced by certain lipids and sterols, such as cholesterol. Bacteria lack cholesterol, but carotenoids and hopanoids are predicted to play a similar role in modulating membrane properties. We have previously shown that the loss of carotenoids in the plant-associated bacteria Pantoea sp. YR343 results in changes to membrane biophysical properties and leads to physiological changes, including increased sensitivity to reactive oxygen species, reduced indole-3-acetic acid secretion, reduced biofilm and pellicle formation, and reduced plant colonization. Here, using whole cell and membrane proteomics, we show that the deletion of carotenoid production in Pantoea sp. YR343 results in altered membrane protein distribution and abundance. Moreover, we observe significant differences in the protein composition of detergent-resistant membrane fractions from wildtype and mutant cells, consistent with the prediction that carotenoids play a role in organizing membrane microdomains. These data provide new insights into the function of carotenoids in bacterial membrane organization and identify cellular functions that are affected by the loss of carotenoids.

In Search for the Membrane Regulators of Archaea

Membrane regulators such as sterols and hopanoids play a major role in the physiological and physicochemical adaptation of the different plasmic membranes in Eukarya and Bacteria. They are key to the functionalization and the spatialization of the membrane, and therefore indispensable for the cell cycle. No archaeon has been found to be able to synthesize sterols or hopanoids to date. They also lack homologs of the genes responsible for the synthesis of these membrane regulators. Due to their divergent membrane lipid composition, the question whether archaea require membrane regulators, and if so, what is their nature, remains open. In this review, we review evidence for the existence of membrane regulators in Archaea, and propose tentative location and biological functions. It is likely that no membrane regulator is shared by all archaea, but that they may use different polyterpenes, such as carotenoids, polyprenols, quinones and apolar polyisoprenoids, in response to specific stressors or physiological needs.

A Convenient Method for the Synthesis of (S)-Dolichol and (S)-Nordolichol

A procedure for the preparation of (S)-dolichol and (S)-nor-dolichol starting from the polyprenyl fraction extracted from Gingko Biloba integer or extracted leaves is described. Two chiral isoprenoid compounds in good yields and high degree of enantiomeric excess were obtained. The (S)-nordolichol appears to be a good chiral precursor for the preparation of 14C-labeled (S)-dolichol which is to be used in investigations aimed at gaining further information with respect to the role of dolichol in the function of living cells.

Dolichol: A Component of the Cellular Antioxidant Machinery

Dolichol, an end product of the mevalonate pathway, has been proposed as a biomarker of aging, but its biological role, not to mention its catabolism, has not been fully understood. UV-B radiation was used to induce oxidative stress in isolated rat hepatocytes by the collagenase method. Effects on dolichol, phospholipid-bound polyunsaturated fatty acids (PL-PUFA) and known lipid soluble antioxidants [coenzyme Q (CoQ) and α-tocopherol] were studied. The increase in oxidative stress was detected by a probe sensitive to reactive oxygen species (ROS). Peroxidation of lipids was assessed by measuring the release of thiobarbituric acid reactive substances (TBARS). Dolichol, CoQ, and α-tocopherol were assessed by high-pressure liquid chromatography (HPLC), PL-PUFA by gas-liquid chromatography (GC). UV-B radiation caused an immediate increase in ROS as well as lipid peroxidation and a simultaneous decrease in the levels of dolichol and lipid soluble antioxidants. Decrease in dolichol paralleled changes in CoQ levels and was smaller to that in α-tocopherol. The addition of mevinolin, a competitive inhibitor of the enzyme 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoAR), magnified the loss of dolichol and was associated with an increase in TBARS production. Changes in PL-PUFA were minor. These findings highlight that oxidative stress has very early and similar effects on dolichol and lipid soluble antioxidants. Lower levels of dolichol are associated with enhanced peroxidation of lipids, which suggest that dolichol may have a protective role in the antioxidant machinery of cell membranes and perhaps be a key to understanding some adverse effects of statin therapy.

Lipid-linked oligosaccharides in membranes sample conformations that facilitate binding to oligosaccharyltransferase

Lipid-linked oligosaccharides (LLOs) are the substrates of oligosaccharyltransferase (OST), the enzyme that catalyzes the en bloc transfer of the oligosaccharide onto the acceptor asparagine of nascent proteins during the process of N-glycosylation. To explore LLOs' preferred location, orientation, structure, and dynamics in membrane bilayers of three different lipid types (dilauroylphosphatidylcholine, dimyristoylphosphatidylcholine, and dioleoylphosphatidylcholine), we have modeled and simulated both eukaryotic (Glc3-Man9-GlcNAc2-PP-Dolichol) and bacterial (Glc1-GalNAc5-Bac1-PP-Undecaprenol) LLOs, which are composed of an isoprenoid moiety and an oligosaccharide, linked by pyrophosphate. The simulations show no strong impact of different bilayer hydrophobic thicknesses on the overall orientation, structure, and dynamics of the isoprenoid moiety and the oligosaccharide. The pyrophosphate group stays in the bilayer head group region. The isoprenoid moiety shows high flexibility inside the bilayer hydrophobic core, suggesting its potential role as a tentacle to search for OST. The oligosaccharide conformation and dynamics are similar to those in solution, but there are preferred interactions between the oligosaccharide and the bilayer interface, which leads to LLO sugar orientations parallel to the bilayer surface. Molecular docking of the bacterial LLO to a bacterial OST suggests that such orientations can enhance binding of LLOs to OST.

Lipid bilayer nanodisc platform for investigating polyprenol-dependent enzyme interactions and activities

Membrane-bound polyprenol-dependent pathways are important for the assembly of essential glycoconjugates in all domains of life. However, despite their prevalence, the functional significance of the extended linear polyprenyl groups in the interactions of the glycan substrates, the biosynthetic enzymes that act upon them, and the membrane bilayer in which they are embedded remains a mystery. These interactions are investigated simultaneously and uniquely through application of the nanodisc membrane technology. The Campylobacter jejuni N-linked glycosylation pathway has been chosen as a model pathway in which all of the enzymes and substrates are biochemically accessible. We present the functional reconstitution of two enzymes responsible for the early membrane-committed steps in glycan assembly. Protein stoichiometry analysis, fluorescence-based approaches, and biochemical activity assays are used to demonstrate the colocalization of the two enzymes in nanodiscs. Isotopic labeling of the substrates reveals that undecaprenyl-phosphate is coincorporated into discs with the two enzymes, and furthermore, that both enzymes are functionally reconstituted and can sequentially convert the coembedded undecaprenyl-phosphate into undecaprenyl-diphosphate-linked disaccharide. These studies provide a proof-of-concept demonstrating that the nanodisc model membrane system represents a promising experimental platform for analyzing the multifaceted interactions among the enzymes involved in polyprenol-dependent glycan assembly pathways, the membrane-associated substrates, and the lipid bilayer. The stage is now set for exploration of the roles of the conserved polyprenols in promoting protein-protein interactions among pathway enzymes and processing of substrates through sequential steps in membrane-associated glycan assembly.

At the membrane frontier: a prospectus on the remarkable evolutionary conservation of polyprenols and polyprenyl-phosphates

Long-chain polyprenols and polyprenyl-phosphates are ubiquitous and essential components of cellular membranes throughout all domains of life. Polyprenyl-phosphates, which include undecaprenyl-phosphate in bacteria and the dolichyl-phosphates in archaea and eukaryotes, serve as specific membrane-bound carriers in glycan biosynthetic pathways responsible for the production of cellular structures such as N-linked protein glycans and bacterial peptidoglycan. Polyprenyl-phosphates are the only form of polyprenols with a biochemically-defined role; however, unmodified or esterified polyprenols often comprise significant percentages of the cellular polyprenol pool. The strong evolutionary conservation of unmodified polyprenols as membrane constituents and polyprenyl-phosphates as preferred glycan carriers in biosynthetic pathways is poorly understood. This review surveys the available research to explore why unmodified polyprenols have been conserved in evolution and why polyprenyl-phosphates are universally and specifically utilized for membrane-bound glycan assembly.

Configuration of polyisoprenoids affects the permeability and thermotropic properties of phospholipid/polyisoprenoid model membranes

The influence of α-cis- and α-trans-polyprenols on the structure and properties of model membranes was analyzed. The interaction of Ficaprenol-12 (α-cis-Prenol-12, α-Z-Prenol-12) and Alloprenol-12 (α-trans-Prenol-12, α-E-Prenol-12) with model membranes was compared using high performance liquid chromatography (HPLC), differential scanning calorimetry (DSC) and fluorescent methods. l-α-Phosphatidylcholine from egg yolk (EYPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as the main lipid components of unilamellar (SUVs) and multilamellar (MLVs) vesicles were used. The two-step extraction procedure (n-pentane and hexane, respectively) allowed to separately analyze the fractions of polyprenol as non-incorporated (Prenol(NonInc)) and incorporated (Prenol(Inc)) into liposomes. Consequently, distribution coefficients, P', describing the equilibrium of prenol content between phospholipid (EYPC) membrane and the aqueous phase gave different logP' for α-cis- and α-trans-Prenol-12, indicating that the configuration of the α-terminal residue significantly alters the hydrophobicity of the polyisoprenoid molecule and consequently the affinity of polyprenols for EYPC membrane. In fluorescence experiments α-trans-Pren-12 increased up to 1.7-fold the permeability of EYPC bilayer for glucose while the effect of α-cis-Pren-12 was almost negligible. Considerable changes of thermotropic behavior of DPPC membranes in the presence of both prenol isomers were observed. α-trans-Pren-12 completely abolished the pretransition while in the case of α-cis-Pren-12 it was noticeably reduced. Furthermore, for both prenol isomers, the temperature of the main phase transition (T(m)) was shifted by about 1°C to lower values and the height of the peak was significantly reduced. The DSC analysis profiles also showed a new peak at 38.7°C, which may suggest the concomitant presence of more that one phase within the membrane. Results of these experiments and the concomitant occurrence of alloprenols and ficaprenols in plant tissues suggest that cis/trans isomerization of the α-residue of polyisoprenoid molecule might comprise a putative mechanism responsible for modulation of the permeability of cellular membranes.

Substrate specificity and membrane topology of Escherichia coli PgpB, an undecaprenyl pyrophosphate phosphatase

The synthesis of the lipid carrier undecaprenyl phosphate (C(55)-P) requires the dephosphorylation of its precursor, undecaprenyl pyrophosphate (C(55)-PP). The latter lipid is synthesized de novo in the cytosol and is also regenerated after its release from the C(55)-PP-linked glycans in the periplasm. In Escherichia coli the dephosphorylation of C(55)-PP was shown to involve four integral membrane proteins, BacA, and three members of the type 2 phosphatidic acid phosphatase family, PgpB, YbjG, and YeiU. Here, the PgpB protein was purified to homogeneity, and its phosphatase activity was examined. This enzyme was shown to catalyze the dephosphorylation of C(55)-PP with a relatively low efficiency compared with diacylglycerol pyrophosphate and farnesyl pyrophosphate (C(15)-PP) lipid substrates. However, the in vitro C(55)-PP phosphatase activity of PgpB was specifically enhanced by different phospholipids. We hypothesize that the phospholipids are important determinants to ensure proper conformation of the atypical long axis C(55) carrier lipid in membranes. Furthermore, a topological analysis demonstrated that PgpB contains six transmembrane segments, a large periplasmic loop, and the type 2 phosphatidic acid phosphatase signature residues at a periplasmic location.

Investigation of the lipid component of neuromelanin

OBJECTIVE

Neuromelanin (NM) is different to other melanins in that its ultrastructure includes a lipid component. The objectives of this study were to identify and quantify lipids associated with NM.

RESULTS

Quantification of the lipid component associated with the pigment on electron micrographs demonstrated that this component comprises 35% of the NM granule volume in the normal brain. The irregular ultrastructural appearance of the NM granules was quite different to the round regular boundary of melanin granules. Using reversed phase high performance liquid chromatography (HPLC) coupled with atmospheric pressure chemical ionization (APCI) mass spectrometry we demonstrated that the isoprenoid dolichol accounted for approximately 12% of total NM pigment mass. Low levels of other lipids were detectable (cholesterol, ubiquinone-10 and alpha-tocopherol) and account for <0.05% of NM lipid, in contrast to cholesterol accounting for 35% of total brain lipids.

CONCLUSION

Unlike other melanins, a substantial proportion of NM volume is comprised of lipid and the major type of lipid associated with NM granules is the isoprenoid dolichol.

Effects of farnesol on the physical properties of DMPC membranes

Farnesol interacts with membranes in a wide variety of biological contexts, yet our understanding of how it affects lipid bilayers is not yet complete. This study investigates how the 15-carbon isoprenoid, farnesol, influences the phase behaviour, lateral organization, and mechanical stability of dimyristol phosphatidylcholine (DMPC) model membranes. Differential scanning calorimetry (DSC) of multilamellar DMPC-farnesol mixtures (up to 26 mol% farnesol) demonstrates how this isoprenoid lowers and broadens the gel-fluid phase transition. A gel-fluid coexistence region becomes progressively more dominant with increasing farnesol concentration and at concentrations of and greater than 10.8 mol%, an upper transition emerges at about 35 degrees C. Atomic force microscopy images of supported farnesol-DMPC bilayers containing 10 and 20 mol% farnesol provide structural evidence of gel-fluid coexistence around the main transition. Above this coexistence region, membranes exhibit homogeneous lateral organization but at temperatures below the main gel-fluid coexistence region, another form of phase coexistence is observed. The solid nature of the gel phase is confirmed using micropipette aspiration. The combined thermodynamic, structural, and mechanical data allow us to construct a phase diagram. Our results show that farnesol preferentially partitions into the fluid phase and induces phase coexistence in membranes below the main transition of the pure lipid.

Farnesol-DMPC phase behaviour: a 2H-NMR study

Involved in a number of diverse metabolic and functional contexts, farnesol is a central component of the mevalonate pathway, post-translationally attaches to proteins, and affects a number of other membrane-associated events. Despite farnesol's biological implications, a detailed analysis of how farnesol affects the physical properties and phase behaviour of lipid membranes is lacking. As (2)H-NMR spectra are sensitive to molecular motions and acyl chain orientation, they can be used to measure the degree of molecular order present in the system. Also, since the (2)H-NMR spectra of fluid and gel phase lipids are very different, they are sensitive probes of membrane phase equilibrium and can be used to determine fluid-gel phase boundaries. In this study, dimyristoyl phosphatidylcholine-d(54) (DMPC-d(54)) bilayers containing varying concentrations of trans-trans farnesol (2.5-20.0 mol%) are investigated over a range of temperatures (8-30 degrees C). Analysis of these spectra has led to the construction of a farnesol-DMPC-d(54) temperature-composition plot. We show that increasing concentrations of farnesol induce a decrease in the fluid-gel phase transition temperature and promote fluid-gel coexistence. Interestingly, farnesol does not seem to affect the quadrupolar splittings (Delta v(Q)) in the fluid phase, i.e., the organization of farnesol within the bilayer and its interaction with phospholipids does not appreciably influence acyl chain order in the fluid phase.

Lipid II induces a transmembrane orientation of the pore-forming peptide lantibiotic nisin

Nisin is an antimicrobial peptide produced by Lactococcus lactis and used as a food preservative in dairy products. The peptide kills Gram-positive bacteria via the permeabilization of the membrane, most probably via pore formation using the cell wall precursor Lipid II as its docking molecule. In this study, site-directed tryptophan spectroscopy was used to determine the topology of nisin in the Lipid II containing membrane, as a start to elucidate the mechanism of targeted pore formation. Three single tryptophan mutants were used, which are viable representatives of the wild-type peptide. The emission spectra of tryptophans located at the N-terminus, the center, and the C-terminus as well as quenching by acrylamide and spin-labeled lipids were investigated using model membrane vesicles composed of DOPC containing 1 mol % Lipid II. Nisin was shown to adopt an orientation where the most probable position of the N-terminus was found to be near the Lipid II headgroup at the bilayer surface, the position of the center of nisin was in the middle of the phospholipid bilayer, and the C-terminus was located near the interface between the headgroups and acyl chain region. These results were used to propose a model for the orientation of nisin in Lipid II containing membranes. Our findings demonstrated that Lipid II changes the overall orientation of nisin in membranes from parallel to perpendicular with respect to the membrane surface. The stable transmembrane orientation of nisin in the presence of Lipid II might allow us to determine the structure of the nisin-Lipid II pores in the lipid bilayer.

Content of dolichol and retinol in isolated rat non-parenchymal liver cells

The liver sinusoids, that are considered as a functional unit, harbour four types of sinusoidal cells (Ito, Kupffer, endothelial and pit cells). Dolichol content has been determined in many tissues and subcellular compartments, alteration has been reported in many types of liver injury, but until now no data are available on its content in every type of sinusoidal non-parenchymal liver cells. Dolichol and retinol metabolism might intersect in their traffic in biological membranes. Intercellular as well as intracellular exchange of retinoids is an essential element of important processes occurring in liver cells. It has been suggested that the role of dolichol, besides being a carrier of oligosaccharides in the biosynthesis of N-linked glycoproteins, may be to modify membrane fluidity and permeability, and facilitate fusion of membranes. Dolichol in the membrane is intercalated between the two-halves of the phospholipid bilayer, but its exact disposition is not known and the movement and distribution of retinoid in membranes may vary with the geometry of the membranes. Therefore the aim of this study is to obtain a global understanding of the sinusoidal system regarding dolichol and retinol content in each type of isolated rat liver sinusoidal cell, in normal conditions and after vitamin A administration. The information that can be drawn from the present results is that with normal vitamin A status of the animal, the dolichol content is almost uniform in all liver cells. After vitamin A supplementation, a great increase of dolichol, together with the known increase of retinol, can be measured only in a subpopulation of the Ito cells, the Ito-1 subfraction. Therefore in the cells that are present in the hepatic sinusoid, different pools of dolichol may have separate functions. Because retinol traffic among cells, membranes and plasma still remains to be fully understood, roles of dolichol in the exchange of vitamin A among sinusoidal liver cells are discussed.

Role of apolipoprotein A-IV in hepatic lipase-catalyzed dolichol acylation and phospholipid hydrolysis

Hepatic lipase catalyzes the hydrolysis of phospholipids and neutral glycerides as well as transacylation reactions between several of these lipids. We have previously reported that this enzyme also transacylates the sn-I fatty acid of phosphatidylethanolamine to dolichol and that this reaction requires a plasma cofactor. In this study, we have purified the cofactor from the lipoprotein-free fraction of human plasma and present evidence demonstrating that it is identical to apolipoprotein A-IV. The effect of apolipoprotein A-IV on hepatic lipase-catalyzed dolichol acylation and phospholipid hydrolysis was studied in model membranes and compared with the effects of apolipoprotein A-I and E. Apolipoprotein A-IV strongly stimulated dolichol acylation and phosphatidylethanolamine hydrolysis but partly inhibited phosphatidylcholine hydrolysis. Apolipoprotein A-I had only a minor influence on the various activities studied and could not replace apolipoprotein A-IV. Apolipoprotein E stimulated the hydrolysis of both phospholipids but had no effect on dolichol acylation. The effect of apolipoprotein A-IV on hepatic lipase activity was then studied with the gum arabic-stabilized triglyceride emulsion. The apolipoprotein neither stimulated nor inhibited triglyceride hydrolysis in the emulsion. Finally, human high-density lipoprotein-2 and very low-density lipoprotein were also used as substrates. Apolipoprotein A-IV strongly stimulated the hydrolysis of phosphatidylcholine and phosphatidylethanolamine in both lipoproteins, while the hydrolysis of triglycerides was completely inhibited. These results demonstrate that apolipoprotein A-IV is an important cofactor to hepatic lipase affecting both catalytic rates and the substrate specificity of the enzyme. We therefore suggest that apolipoprotein A-IV-rich high-density lipoprotein is the preferred substrate for hepatic lipase.

The lipid compositions of different regions of rat brain during development and aging

The lipid contents in different regions of the rat brain were analyzed from birth to the age of 2 years. The total brain phospholipid content increased threefold during the first 20 postnatal days. The cholesterol content elevated extensively during the first 2 months of life and, after this period, remained unchanged. The level of dolichol increased almost 100-fold during the first 10 months of life and continued to increase thereafter. Some modifications in the dolichol isoprenoid pattern were also observed. An increase in the brain ubiquinone level occurred during the first few months of life, but no further change was observed after this period. Ubiquinone-9 and -10 constituted 70 and 30%, respectively, of the total ubiquinone in all regions and all subcellular fractions. The alpha-tocopherol content increased during the first 3 weeks of life and was unchanged thereafter. These results demonstrate characteristic changes in the lipid contents of various regions of rat brain during development and aging.

Dolichol and vitamin A content in rat liver Ito (perisinusoidal) cells

Dolichol has been determined in many tissues but to date no data are available on liver Ito (fat storing) cells. In this note dolichol was determined in two subpopulations of liver Ito cells isolated from rats pretreated with vitamin A: Ito-1, vitamin A enriched and Ito-2, relatively poor of vitamin A. Differences were observed in the behaviour of the two fractions after vitamin A pretreatment of rats. In fact, in Ito-1 fraction dolichol increases with the increase of vitamin A, while in Ito-2 fraction it does not change significantly with the increase of vitamin A. These results, while confirming the heterogeneity of fat storing cells, are discussed as to the possible role of dolichol and vitamin A metabolism.

Age-dependent modifications in the metabolism of mevalonate pathway lipids in rat brain

The levels and rates of biosynthesis of mevalonate pathway lipids in rat brain were investigated during development and aging. Between birth and 18 months of age there are only moderate decreases in the phospholipid and cholesterol contents but an increase in the levels of dolichyl-P and, particularly of dolichol. The amount of ubiquinone is unchanged. The rate of incorporation of [3H]leucine into protein decreases by 10% during the first year, while the incorporation of [3H]glycerol into phospholipids decreases by 20%. The high rates of [3H]mevalonate incorporation into cholesterol and dolichol after birth decreases rapidly. In contrast, the rate of incorporation into ubiquinone is constant. Squalene synthase activity decreases rapidly in the early postnatal period and at 18 months of age this activity is 10-fold lower than immediately after birth. cis-Prenyltransferase activity is also high during the first postnatal month and reaches a constant level at 4 months of age. Significantly, nonaprenyl 4-hydroxybenzoate transferase activity is high during the entire period investigated. The rate of lipid peroxidation does not change during aging. These results demonstrate that brain cholesterol and dolichol exhibit a low rate of turnover during aging, whereas ubiquinone is synthesized at a high rate and exhibits rapid turnover throughout the entire lifespan.

Age-dependent changes in rat liver prenyltransferases

Mevalonate pathway lipids including cholesterol, ubiquinone and dolichol, are of great importance for cellular function. Many of the enzymes of this pathway are thus strictly regulated. During development of the rat, the cellular levels of certain of these lipids vary. Prenyltransferases have been investigated and it is reported here that farnesyl pyrophosphate synthase activity in rat liver cytosol decreases after birth to a lower, steady level. This decrease is not paralleled by the level of synthase protein, which shows two maxima, one immediately after birth and the other 30 days later. cis-Prenyltransferase activity is low after birth, increases continuously up to day-54 and then decreases to a low level which was maintained throughout the remainder of the study (365 days). Squalene synthase exhibits high activity after birth, but decreases during the first 100 days thereafter, and subsequently remains at the low level thus reached. In contrast to these changes in the activities of the prenyltransferases, the level of cholesterol is constant and the dolichol concentration increases continuously throughout the entire period studied.

Lipid peroxidation of microsomal and mitochondrial membranes extracted with n-pentane and reconstituted with ubiquinol, dolichol and cholesterol

Microsomes and mitochondria prepared from rat liver were extracted with n-pentane, a procedure which does not denature enzyme proteins. Protein and phospholipid were not extracted, but 75-80% of the total dolichol, 80-100% of the ubiquinone and 85-95% of the cholesterol were removed from both organelles by this procedure. Enzymatic and non-enzymatic lipid peroxidation in microsomes and non-enzymatic peroxidation in mitochondria were strongly inhibited when ubiquinol was reinserted into n-pentane-extracted membranes. When reconstitution with dolichol was performed, lipid peroxidation was increased or unchanged, while cholesterol decreased this activity in a concentration-dependent manner. In reconstitution experiments ubiquinol and dolichol together were less inhibitory than ubiquinol alone, whereas cholesterol accentuated the inhibitory effect of ubiquinol. Reconstitution with dolichols of different lengths, dolichyl esters or with alpha-unsaturated polyprenols further demonstrated that dolichol is not an antioxidant. It appears that mevalonate pathway lipids influence lipid peroxidation in membranes by modifying the properties of the bilayer.

Dolichol delays G1-arrest for one cell cycle in human fibroblasts subjected to depletion of serum or mevalonate

It is well-established that some product(s) or metabolite(s) of mevalonate is (are) critical for growth of mammalian cells. In the search for this (these) compound(s) it seems meaningful to distinguish between compounds needed for cell cycle progression in proliferating cells and compounds needed for growth activation of arrested cells. By using time-lapse video recording we have studied the possible regulatory role of cholesterol, dolichol and mevalonate in the cell cycle of human diploid fibroblasts (HDF). HDF, which are serum-dependent, were rapidly growth-arrested in the first part of G1 upon removal of serum factors. They also responded to mevinolin (an HMG CoA reductase inhibitor) by a similar G1-block, indicating that a mevalonate-derived product is involved in the G1-located cell cycle control of HDF. Interestingly, dolichol counteracted the G1-block caused by both types of treatment. Hence, the early G1-cells could traverse the remainder of the cell cycle and divide despite depletion of serum or mevalonate. We also demonstrated that addition of dolichol resulted in a significant decrease in the rate of protein degradation. This protein stabilizing effect may constitute the mechanism by which dolichol delays the G1-arrest of HDF.

Transfer of liposomes containing dolichol into isolated hepatocytes

Isolated rat hepatocytes were preincubated with egg lecithin liposomes containing [3H]dolichol and [3H]dolichyl ester, and the intracellular levels and distributions of these lipids were subsequently determined after incubation in a liposome-free medium. [3H]Dolichol was recovered initially mainly in microsomes, and no increase with time in the low level of this compound in the mitochondrial/lysosomal fraction could be observed. A small portion of the labeled dolichol was esterified in the endoplasmic reticulum and transferred to the lysosome-containing fraction. [3H]Dolichyl linoleate was initially localized in microsomes and supernatant, but later accumulated in the mitochondria/lysosomes. Dolichyl linoleate was found in the membrane of microsomes, in the membrane and lumen of lysosomes, and in the soluble cytoplasm. Exogenous dolichol recovered in microsomes was not phosphorylated to any significant extent. Liposomal phosphatidylcholine also showed preferential accumulation in microsomes after incubation with hepatocytes. These results indicate that exogenous or endogenously formed dolichyl esters are transferred from the endoplasmic reticulum to lysosomes, probably through the cytoplasm. It appears that fatty acids play a role in targeting these lipids to their intracellular locations.

Nonmembrane associated dolichol in rat liver

The distribution of dolichol in rat liver was studied. Upon high-speed centrifugation, 9% of the total tissue dolichol was recovered in the supernatant. Dolichol was enclosed in vesicles and in lipidic particles which were isolated by gel filtration and density gradient centrifugation. The particles had a diameter of 20 nm and contained dolichol, ubiquinone, cholesterol, phospholipid and some protein. Similar particles were recovered upon incubation of isolated hepatocytes with liposomes containing dolichol. From the lysosomal lumen, lipid particles containing dolichol, ubiquinone, cholesterol and phospholipid, but no protein, were isolated. The diameter of the particles was 20-40 nm with a molecular weight of 130 kDa. Puromycin treatment inhibited protein synthesis, but did not affect dolichol transfer from the endoplasmic reticulum to lysosomes, suggesting that the transfer is not mediated by newly synthesized apoprotein. The results indicate that a sizeable portion of the total cellular dolichol is present in cytoplasm and in lysosomal lumen. Furthermore, dolichol probably participates in the translocation process.

Membrane biochemistry and chemical hepatocarcinogenesis

Biochemical membrane alterations appearing during the process of chemical carcinogenesis are described. Emphasis is put on membrane composition, structure, and biogenesis. In this presentation the knowledge gained from experimental studies of liver and skin in the process of cancer development is acknowledged. Important biochemical changes have been reported in lipid composition, fatty acid saturation, constitutional enzyme expression, receptor turnover and oligomerization. Functional consequences of the altered membrane structure is discussed within the concepts of regulation of cell proliferation, regulation of membrane receptor expression, redox control, signal transduction, drug metabolism, and multidrug resistance. Data from malignant tumours and normal tissue are addressed to evaluate the importance of the alterations for the process and for the eventual malignant transformation.

Effect of dietary fat on rat liver microsomal and mitochondrial/lysosomal dolichol, phospholipid and cholesterol

The influence of different fat diets on liver phospholipid, cholesterol and dolichol was studied. Rats were separated into four groups and fed standard laboratory chow (control), a diet containing linolenic acid, a coconut oil diet, or a corn oil-containing diet. After five weeks, microsomes and mitochondrial/lysosomal fractions were prepared from the liver, and lipid compositions were analyzed. No changes in phospholipid content were observed. In control animals, the fatty acid compositions of phosphatidylcholine and phosphatidylethanolamine in the two subfractions were similar. However, these two phospholipids showed different fatty acid patterns, which were altered independently upon dietary treatment. The dietary treatments resulted, in most cases, in decreased cholesterol and dolichol contents and, especially in microsomes, in a decreased level of esterification of both lipids. The fatty acid compositions of cholesteryl esters in the two subfractions showed significant differences and cholesterol was esterified to a large extent with linolenic acid when this fatty acid was supplied in the diet. The same dietary treatment exerted different effects on the cholesterol localized in the two different intracellular compartments. This difference was most pronounced in rats fed the corn oil-containing diet; microsomal cholesteryl esters exhibited increased saturation, whereas cholesteryl esters exhibited increased saturation, whereas cholesteryl esters in the mitochondrial/lysosomal fraction displayed decreased saturation. Dolichyl esters in the two cellular compartments had different fatty acyl compositions, with a considerably higher degree of saturation in microsomes. The various diets influenced the nature of the fatty acid moieties present in the isolated fractions and the effects on the two subfractions were opposite.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid compositions of different regions of the human brain during aging

The neutral and phospholipid compositions of various regions of the human brain were analyzed using autopsy material covering the life period between 33 and 92 years of age. The protein content was also measured and, on a weight basis, this content is unchanged in the cerebellum, pons, and medulla oblongata, whereas in the 90-year-old group it decreases in the hippocampus, gray matter, and nucleus caudatus. In white matter, the protein content decreases continuously with age. The phospholipid composition is characteristic of the region investigated, but remains unchanged during aging. The total phospholipid content exhibits only a 5-10% decrease in the oldest age group. The content of dolichol and its polyisoprenoid pattern are also characteristic of the region analyzed. Between 33 and 92 years of age, the amount of dolichol in all portions of the brain increases three- to fourfold, but the isoprenoid pattern remains constant. The level of dolichyl-P varies between different regions, but only a moderate increase is seen with age. Ubiquinone content is highest in the nucleus caudatus, gray matter, and hippocampus, and in all areas this content is decreased to a great extent in the oldest age groups. All regions of the human brain are rich in cholesterol, but alterations in the amount of this lipid are highly variable during aging, ranging from no change to a 40% decrease.

The effects of inducers of the endoplasmic reticulum, peroxisomes and mitochondria on the amounts and synthesis of ubiquinone in rat liver subcellular membranes

Rats were treated with inducers of peroxisomes, mitochondria and the endoplasmic reticulum, as well as receiving diets and drug known to influence the mevalonate pathway. Treatment with clofibrate and 2-diethylhexylphthalate (DEHP) increased microsomal and mitochondrial ubiquinone contents, but a decrease was observed in lysosomes. In vivo labeling of this lipid with [3H]mevalonate was also elevated. The amount of cholesterol did not change upon exposure to these inducers of peroxisomes and mitochondria, but its rate of labeling was decreased. The concentration of dolichol increased only after treatment with DEHP and only in lysosomes. The inducers of the endoplasmic reticulum phenobarbital, 3-methylcholanthrene and N-nitrosodiethylamine enhanced the rate of ubiquinone synthesis and exposure to the latter two substances also elevated the amount of this lipid in microsomes. A cholesterol-rich diet increased the labeling of ubiquinone and decreased cholesterol labeling, while cholestyramine treatment had opposite effects on lipid labeling in both microsomes and mitochondria. The results demonstrate that the ubiquinone contents of the various membranes of hepatocytes change in a characteristic manner under the influence of inducers and dietary factors. Clearly, the level of ubiquinone and its biosynthesis are regulated separately from those of the other products of the mevalonate pathway, cholesterol and dolichol.

Biosynthesis of dolichol by rat liver peroxisomes

The ability of peroxisomes and microsomes to synthesize dolichol from [3H]mevalonate, [3H]isopentenyl-P2 or [3H]farnesyl-P2 in vitro was investigated. It was found that isoprenoid biosynthesis also occurs in peroxisomes and that this process demonstrates properties differing from those of isoprenoid biosynthesis by microsomes. The pH optimum in peroxisomes was 8.0 and, in contrast to microsomes, the peroxisomal biosynthesis was largely insensitive to detergents. After treatment with proteolytic enzymes, microsomes lost their capacity to incorporate [3H]mevalonate into dolichol, whereas proteolysis of intact peroxisomes did not influence their corresponding rate of incorporation. The soluble content of peroxisomes was separated from the membranes and found to demonstrate half of the biosynthetic capacity of the intact organelle. Fasting and cholestyramine treatment decreased only the microsomal incorporation of [3H]mevalonate into dolichol, while treatment with clofibrate, di-2-ethylhexyl phthalate or phenobarbital increased microsomal, but decreased peroxisomal labeling. After injection of [3H]mevalonate into the portal vein of rats, high initial labeling of dolichol was recovered both in isolated microsomes and peroxisomes, whereas when [3H]glycerol was administered, peroxisomal phospholipids became labeled later than the corresponding microsomal constituents. These results support the conclusion that dolichol is synthesized both in peroxisomes and the endoplasmic reticulum, but that the biosynthetic processes at these two locations have different properties.

Discharge of newly-synthesized dolichol and ubiquinone with lipoproteins to rat liver perfusate and to the bile

An effective system for perfusing rat liver using complete tissue culture medium and washed calf erythrocytes as oxygen carriers was devised. Infusion of taurocholate and glucose proved necessary to maintain stable metabolic activity and bile secretion during a 6-hr period. Perfusate pO2, pCO2 and pH values were monitored continuously and found to be stable. Electron microscopic examination revealed the maintenance of normal hepatic structure, even after 6 hr. Normal rates of protein and urea synthesis, no leakage of cytoplasmic enzymes, and continuous bile acid production demonstrated the functional integrity of this system. Using [3H]mevalonic acid as precursor, dolichol, dolichyl phosphate, ubiquinone and cholesterol were found to be continuously synthesized in this perfused liver system. All these lipids appeared in the perfusate, indicating discharge through the ER-Golgi system. The lipoproteins of the perfusate were isolated by ultracentrifugation and characterized with respect to size distribution and lipid composition. Dolichol was found in VLDL, LDL and HDL fractions, with the highest concentration present in the latter. In rat and human blood plasma this lipid was mainly associated with HDL. The ubiquinone in the perfusate was primarily associated with the VLDL fraction, while in rat plasma it was found more evenly distributed among all the three lipoprotein fractions studied. Dolichol, ubiquinone and cholesterol were also discharged to the bile, whereas dolichyl phosphate was not. Thus, newly-synthesized dolichol and ubiquinone are transported out of the hepatocyte to the blood and to the bile.

Nuclear magnetic resonance studies of polyisoprenols in model membranes

2H-NMR investigation of polyisoprenols (PIs) in model membranes has revealed information about their motions, relative order, and locale within the membrane. Initial 2H-NMR studies of the organization of the shorter chain homologues geraniol (C10), farnesol (C15), and solanesol (C45) were carried out by incorporating 2H-acetyl esters of the alcohol or the di-perdeuterome-thylated derivatives of the omega-labeled prenols into multilamellar phosphatidylcholine (PC) vesicles. 2H-NMR powder patterns interpretable in terms of quadrupole splittings and spin-lattice relaxation times were obtained. Similar experiments have now been carried out with the labeled free alcohol, acetyl ester, and phosphate ester of dolichol (C95) and undecaprenol (C55). 2H-NMR results show that the head and tail 2H-labeled sites of C55 and C95 exhibit a fast motion isotropic signal only; no slower motion anisotropy, as exhibited by the short chain PIs, was observed. These data suggest that C55 and C95 either have substantially different (faster) motions and/or conformations relative to the shorter chain PIs within the membrane, and that the longer PIs alter the membrane host packing matrix. This conclusion was supported by 31P-NMR studies of C55 and C95 derivatives in PC and PE/PC membranes, which showed new pronounced spectral changes relative to the results obtained with the shorter chain PIs. These spectral changes indicate that undecaprenol and dolichol derivatives appear to induce a non-bilayer (isotropic) organization of phospholipid molecules in PE/PC (2:1) vesicles. The possible physiological consequences of this perturbation remains to be determined.

Age-related changes in the lipid compositions of rat and human tissues

The levels of cholesterol, ubiquinone, dolichol, dolichyl-P, and total phospholipids in human lung, heart, spleen, liver, kidney, pancreas, and adrenal from individuals from one-day-old to 81 years of age were investigated and compared with the corresponding organs from 2 to 300 day-old rats. The amount of cholesterol in human tissues did not change significantly during aging, but the level of this lipid in the rat was moderately elevated in the organs of the oldest animals. In human pancreas and adrenal the ubiquinone content was highest at one year of age, whereas in other organs the corresponding peak value was at 20 years of age, and was followed by a continuous decrease upon further aging. A similar pattern was observed in the rats, with the highest concentration of ubiquinone being observed at 30 days of age. Dolichol levels in human tissues increase with aging, but they increase to very different extents. In the lungs this increase is seven-fold, and in the pancreas it is 150-fold. The elevation in the dolichol contents of rat tissues ranges from 20 to 30-fold in our material. In contrast, the levels of the phosphorylated derivative of dolichol increased to a more limited extent, i.e., 2 to 6-fold in human tissues and even less in the rat. These results demonstrate that the levels of a number of lipids in human and rat organs are modified in a characteristic manner during the life-span. This is in contrast to phospholipids, which constitute the bulk of the cellular lipid mass.

The half-lives of dolichol and dolichyl phosphate in rat liver

Rat liver dolichol and dolichyl-P were labeled by injection of [3H]mevalonate into the portal vein and their rates of synthesis and breakdown determined. In the initial phase the radioactivity appeared in alpha-unsaturated polyprenols. Subsequent saturation required 90 min. The half-lives of dolichols in microsomes were between 80 and 118 h, and shorter dolichols had shorter values of T1/2. The half-lives of dolichols in lysosomes were between 115 and 137 h, while microsomal dolichyl-P exhibited a T1/2 of 32 h. Injected dolichol was recovered in the lysosomes of hepatocytes and exhibited a rate of breakdown which was slower than that of the endogenous compound. These results indicate differences in the catabolism of dolichol at different subcellular locations, as well as differences between the catabolism of dolichol and dolichyl-P.

Separation and quantitation of dolichyl esters by high-performance liquid chromatography

An HPLC procedure for the isolation and quantitation of total and individual dolichyl esters in tissues has been developed. The purified lipid extracts are subjected to sequential reversed-phase, straight-phase, and reversed-phase HPLC, which yield complete resolution and high recovery of the individual dolichyl esters. The isoprenoid distribution in the esterified fraction was similar to that of the free alcohol fraction in liver, kidney, and spleen. All fatty acids present in the total fraction were also recovered in all the individual polyisoprenoids. Dolichyl esters thus appear to differ from other lipid esters in tissues in containing a broader range of fatty acids.

Ubiquinone biosynthesis by the microsomal fraction from rat liver

The distribution and biosynthesis of ubiquinone were investigated in vivo in rats and using liver slices. In addition to mitochondria, Golgi vesicles and lysosomes also contain large amounts of this lipid, and even the plasma membrane, peroxisomes and microsomes demonstrate easily measurable amounts. The spectral and chromatographic properties of microsomal ubiquinone were identical to those of its mitochondrial counterpart. When pentane was used to deplete beef heart submitochondrial particles of ubiquinone, NADH and succinate oxidase activities could be restored by reincorporation of microsomal ubiquinone. Injection of [3H]mevalonate into the portal vein of rats and incubation of liver slices with [3H]mevalonate and [3H]- and [14C]tyrosine demonstrated that labeling of mitochondrial ubiquinone was initially much lower than labeling of the microsomal lipid. Furthermore, intraportal injection of [3H]mevalonate resulted in the rapid appearance of labeled ubiquinone in the blood. These results indicate that ubiquinone is synthesized not only in mitochondria, but also on the endoplasmic reticulum of rat liver.

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.

Total synthesis of chain-length-uniform dolichyl phosphates and their fitness to accept hexoses in the enzymatic formation of lipoglycans

Dolichols of defined uniform chain length (C35, C45, and C55) and geometry were prepared by total synthesis according to the following principle: (E,E)-Farnesol, activated as its 4-tolyl sulfone, is condensed with 8-chloroneryl benzyl ether, the sulfonyl group removed and the ether linkage cleaved by lithium/triethylamine. The resulting elongated prenol is converted again to its corresponding 4-toly/sulfone; at this stage isomers are removed by chromatography. After several cycles of this C10-elongation sequence the synthesis is completed in the same way but using 8-chlorocitronellyl benzyl ether as building block to introduce the saturated alpha-isoprene unit. The dolichols obtained were chemically phosphorylated (POCl3/Et3N). Both, the alcohols and their phosphate esters, are characterized spectroscopically. 1H- and 13C-NMR data are recorded for qualitative and stereochemical comparison with natural dolichols. The authentic dolichyl phosphates (Dol-7-P, Dol-9-P, and Dol-11-P) were assayed relative to the natural dolichyl phosphate mixture from pig liver as acceptors for transglycosylation from nucleoside diphosphate sugars (glucose, mannose) by standardized membrane vesicle preparations from plants (Volvox) and animals (liver). Even the shortest chain dolichyl 7-phosphate has full activity in this lipoglycan-forming reaction.