Conditions for quantitation of dolichyl phosphate, dolichol, ubiquinone and cholesterol by HPLC

Quantification of Dolichyl Phosphates Using Phosphate Methylation and Reverse-Phase Liquid Chromatography-High Resolution Mass Spectrometry

Dolichyl monophosphates (DolPs) are essential lipids in glycosylation pathways that are highly conserved across almost all domains of life. The availability of DolP is critical for all glycosylation processes, as these lipids serve as membrane-anchored building blocks used by various types of glycosyltransferases to generate complex post-translational modifications of proteins and lipids. The analysis of DolP species by reverse-phase liquid chromatography-mass spectrometry (RPLC-MS) remains a challenge due to their very low abundance and wide range of lipophilicities. Until now, a method for the simultaneous qualitative and quantitative assessment of DolP species from biological membranes has been lacking. Here, we describe a novel approach based on simple sample preparation, rapid and efficient trimethylsilyl diazomethane-dependent phosphate methylation, and RPLC-MS analysis for quantification of DolP species with different isoprene chain lengths. We used this workflow to selectively quantify DolP species from lipid extracts derived of Saccharomyces cerevisiae, HeLa, and human skin fibroblasts from steroid 5-α-reductase 3- congenital disorders of glycosylation (SRD5A3-CDG) patients and healthy controls. Integration of this workflow with global lipidomics analyses will be a powerful tool to expand our understanding of the role of DolPs in pathophysiological alterations of metabolic pathways downstream of HMG-CoA reductase, associated with CDGs, hypercholesterolemia, neurodegeneration, and cancer.

Ethanol-induced impairment in the biosynthesis of N-linked glycosylation

Deficiency in N-linked protein glycosylation is a long-known characteristic of alcoholic liver disease and congenital disorders of glycosylation. Previous investigations of ethanol-induced glycosylation deficiency demonstrated perturbations in the early steps of substrate synthesis and in the final steps of capping N-linked glycans in the Golgi. The significance of the biosynthesis of N-glycan precursors in the endoplasmic reticulum, however, has not yet been addressed in alcoholic liver disease. Ethanol-metabolizing hepatoma cells were treated with increasing concentrations of ethanol. Transcript analysis of genes involved in the biosynthesis of N-glycans, activity assays of related enzymes, dolichol-phosphate quantification, and analysis of dolichol-linked oligosaccharides were performed. Upon treatment of cells with ethanol, we found a decrease in the final N-glycan precursor Dol-PP-GlcNAc(2) Man(9) Glc(3) and in C95- and C100-dolichol-phosphate levels. Transcript analysis of genes involved in N-glycosylation showed a 17% decrease in expression levels of DPM1, a subunit of the dolichol-phosphate-mannose synthase, and an 8% increase in RPN2, a subunit of the oligosaccharyl transferase. Ethanol treatment decreases the biosynthesis of dolichol-phosphate. Consequently, the formation of N-glycan precursors is affected, resulting in an aberrant precursor assembly. Messenger RNA levels of genes involved in N-glycan biosynthesis are slightly affected by ethanol treatment, indicating that the assembly of N-glycan precursors is not regulated at the transcriptional level. This study confirms that ethanol impairs N-linked glycosylation by affecting dolichol biosynthesis leading to impaired dolichol-linked oligosaccharide assembly. Together our data help to explain the underglycosylation phenotype observed in alcoholic liver disease and congenital disorders of glycosylation.

Glycoprotein maturation and the UPR

Glycosylation is a complex form of protein modification occurring in the secretory pathway. The addition of N- and O-glycans affects intracellular processes like the folding and trafficking of most glycoproteins. To better understand the impact of glycosylation in protein folding and maturation, parameters like glycosylation site occupancy and oligosaccharide structure must be measured quantitatively. In this chapter, we describe current methods enabling the determination of N-glycosylation by assessment of cellular dolichol phosphate levels, dolichol-linked oligosaccharides, and the occupancy of N-glycosylation sites. We also provide detailed methods for the analysis of O-glycosylation, whose role in intracellular protein maturation is often overlooked.

Improvement of dolichol-linked oligosaccharide biosynthesis by the squalene synthase inhibitor zaragozic acid

The majority of congenital disorders of glycosylation (CDG) are caused by defects of dolichol (Dol)-linked oligosaccharide assembly, which lead to under-occupancy of N-glycosylation sites. Most mutations encountered in CDG are hypomorphic, thus leaving residual activity to the affected biosynthetic enzymes. We hypothesized that increased cellular levels of Dol-linked substrates might compensate for the low biosynthetic activity and thereby improve the output of protein N-glycosylation in CDG. To this end, we investigated the potential of the squalene synthase inhibitor zaragozic acid A to redirect the flow of the polyisoprene pathway toward Dol by lowering cholesterol biosynthesis. The addition of zaragozic acid A to CDG fibroblasts with a Dol-P-Man synthase defect led to the formation of longer Dol-P species and to increased Dol-P-Man levels. This treatment was shown to decrease the pathologic accumulation of incomplete Dol pyrophosphate-GlcNAc(2)Man(5) in Dol-P-Man synthase-deficient fibroblasts. Zaragozic acid A treatment also decreased the amount of truncated protein N-linked oligosaccharides in these CDG fibroblasts. The increased cellular levels of Dol-P-Man and possibly the decreased cholesterol levels in zaragozic acid A-treated cells also led to increased availability of the glycosylphosphatidylinositol anchor as shown by the elevated cell-surface expression of the CD59 protein. This study shows that manipulation of the cellular Dol pool, as achieved by zaragozic acid A addition, may represent a valuable approach to improve N-linked glycosylation in CDG cells.

HPLC and mass spectrometry analysis of dolichol-phosphates at the cell culture scale

Dolichols (Dol) are polyprenol lipids that are essential structural components of eukaryotic membranes. In addition, the phosphorylated derivatives of Dol function as lipid anchors of mono- and oligosaccharide precursors involved in protein glycosylation. The biological importance of Dol phosphates (Dol-P) is illustrated by the severe outcome of human disorders linked to Dol biosynthetic defects, such as Dol-kinase deficiency. For characterization of inherited human diseases and evaluation of therapeutic trials, cultured cells often serve as a sole possible source for experimentation. Limited amounts of cell culture material render the quantitative analysis of Dol a challenging task. Here, we present HPLC- and mass spectrometry-based approaches to analyze and quantitate Dol-P from cultured human cells. The composition of naturally occurring Dol-P and the saturation state of the alpha-isoprene units was identified by negative-ion electrospray ionization mass spectrometry. Furthermore, fluorescently labeled Dol-P were separated by HPLC and quantified by comparison to known amounts of the internal standard polyprenol-P. The effect of pravastatin, a 3-hydroxy-3-methyl-glutaryl coenzyme-A reductase inhibitor, on the formation of Dol-P in HeLa cells was investigated. As expected, this treatment led to a decrease of Dol-P down to 35% of normal levels.

Analysis of ubiquinones, dolichols, and dolichol diphosphate-oligosaccharides by liquid chromatography-electrospray ionization-mass spectrometry

Prenols, a class of lipids formed by the condensation of five carbon isoprenoids, have important roles in numerous metabolic pathways of the eukaryotic cell. Prenols are found in the cell as free alcohols, such as dolichol, or can be attached to vitamins, as with the fat soluble vitamins. In addition, prenols such as farnesyl- and geranylgeranyl-diphosphate are substrates for the transfer of farnesyl and geranylgeranyl units to proteins with important implications for signal transduction within the cell. Dolichol phosphate- and dolichol diphosphate-linked sugars are central to the formation of the lipid-linked branched oligosaccharide, Dol-PP-(GlcNAc)2(Man)9(Glc)3, used for co-translational en bloc protein N-glycosylation in the lumen of the endoplasmic reticulum. Toward furthering our understanding of the role of prenol lipids in the cell, we have developed a method for the detection and quantification of dolichol and coenzyme Q by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). These methods, developed using the mouse macrophage RAW 264.7 tumor cells, are broadly applicable to other cell lines, tissues, bacteria, and yeast. We also present a new MS-based method for the detection and structural characterization of the intact dolichol diphosphate oligosaccharide Dol-PP-(GlcNAc)2 (Man)9(Glc)3 from porcine pancreas.

MPDU1 mutations underlie a novel human congenital disorder of glycosylation, designated type If

Deficiencies in the pathway of N-glycan biosynthesis lead to severe multisystem diseases, known as congenital disorders of glycosylation (CDG). The clinical appearance of CDG is variable, and different types can be distinguished according to the gene that is altered. In this report, we describe the molecular basis of a novel type of the disease in three unrelated patients diagnosed with CDG-I. Serum transferrin was hypoglycosylated and patients' fibroblasts accumulated incomplete lipid-linked oligosaccharide precursors for N-linked protein glycosylation. Transfer of incomplete oligosaccharides to protein was detected. Sequence analysis of the Lec35/MPDU1 gene, known to be involved in the use of dolichylphosphomannose and dolichylphosphoglucose, revealed mutations in all three patients. Retroviral-based expression of the normal Lec35 cDNA in primary fibroblasts of patients restored normal lipid-linked oligosaccharide biosynthesis. We concluded that mutations in the Lec35/MPDU1 gene cause CDG. This novel type was termed CDG-If.

An alternative cis-isoprenyltransferase activity in yeast that produces polyisoprenols with chain lengths similar to mammalian dolichols

Dolichyl monophosphate (Dol-P) is a polyisoprenoid glycosyl carrier lipid essential for the assembly of a variety of glycoconjugates in the endoplasmic reticulum of eukaryotic cells. In yeast, dolichols with chain lengths of 14--17 isoprene units are predominant, whereas in mammalian cells they contain 19--22 isoprene units. In this biosynthetic pathway, t,t-farnesyl pyrophosphate is elongated to the appropriate long chain polyprenyl pyrophosphate by the sequential addition of cis-isoprene units donated by isopentenyl pyrophosphate with t,t,c-geranylgeranyl pyrophosphate being the initial intermediate formed. The condensation steps are catalyzed by cis-isoprenyltransferase (cis-IPTase). Genes encoding cis-IPTase activity have been identified in Micrococcus luteus, Escherichia coli, Arabidopsis thaliana, and Saccharomyces cerevisiae (RER2). Yeast cells deleted for the RER2 locus display a severe growth defect, but are still viable, possibly due to the activity of an homologous locus, SRT1. The dolichol and Dol-P content of exponentially growing revertants of RER2 deleted cells (Delta rer2) and of cells overexpressing SRT1 have been determined by HPLC analysis. Dolichols and Dol-Ps with 19--22 isoprene units, unusually long for yeast, were found, and shown to be utilized for the biosynthesis of lipid intermediates involved in protein N-glycosylation. In addition, cis-IPTase activity in microsomes from Delta rer2 cells overexpressing SRT1 was 7- to 17-fold higher than in microsomes from Delta rer2 cells. These results establish that yeast contains at least two cis-IPTases, and indicate that the chain length of dolichols is determined primarily by the enzyme catalyzing the chain elongation stage of the biosynthetic process.

Effect of long term fluoride exposure on lipid composition in rat liver

Chronic fluorosis can severely damage many systems of human body, but its pathogenesis is unclear. Normal composition and structure of cellular membrane lipids are a basic factor to maintain cell function. In this investigation, cellular membrane lipids of the liver were analysed after a long term fluoride treatment for rats and the results are discussed in order to give an explanation for the pathogenesis of this disease. Wistar rats were supplied with drinking water containing either 30 or 100 ppm fluoride (NaF) for seven months. Contents of phospholipid and neutral lipid in rat liver were analyzed by high-performance liquid chromatography, and fatty acid composition from individual phospholipids was measured by gas chromatography. Results showed that the total liver phospholipid content decreased in the rats treated with high dose of fluoride due to a lower content of phosphatidylethanolamine (PE), phosphatidylcholine (PC) and phosphatidylserine (PS). Among the fatty acid compositions of PE and PC in the livers of fluoride poisoned animals, the proportion of polyunsaturated fatty acids (20:4 and 22:6) decreased, whereas saturated fatty acids (16:0 and 18:0) increased. No changes could be detected in the amounts of liver cholesterol and dolichol. Total ubiquinone contents in rat liver were reduced by 11% in the group treated with 30 ppm fluoride and by 42% in the group treated with 100 ppm fluoride. In the subclasses of ubiquinone, both ubiquinone-9 and ubiquinoine-10 amounts decreased after fluoride treatment. These modifications of membrane lipids might be induced by oxidative stress, which might be an important factor in the pathogenesis of chronic fluorosis.

The Saccharomyces cerevisiae CWH8 gene is required for full levels of dolichol-linked oligosaccharides in the endoplasmic reticulum and for efficient N-glycosylation

The Saccharomyces cerevisiae mutant cwh8 was previously found to have an anomalous cell wall. Here we show that the cwh8 mutant has an N -glycosylation defect. We found that cwh8 cells were resistant to vanadate and sensitive to hygromycin B, and produced glycoforms of invertase and carboxypeptidase Y with a reduced number of N -chains. We have cloned the CWH8 gene. We found that it was nonessential and encoded a putative transmembrane protein of 239 amino acids. Comparison of the in vitro oligosaccharyl transferase activities of membrane preparations from wild type or cwh8 Delta cells revealed no differences in enzyme kinetic properties indicating that the oligosaccharyl transferase complex of mutant cells was not affected. cwh8 Delta cells also produced normal dolichols and dolichol-linked oligosaccharide intermediates including the full-length form Glc3Man9GlcNAc2. The level of dolichol-linked oligosaccharides in cwh8 Delta cells was, however, reduced to about 20% of the wild type. We propose that inefficient N -glycosylation of secretory proteins in cwh8 Delta cells is caused by an insufficient supply of dolichol-linked oligosaccharide substrate.

Influence of chronic fluorosis on membrane lipids in rat brain

Brain membrane lipid in rats were analyzed after being fed either 30 or 100 ppm fluoride for 3, 5, and 7 months. The protein content of brain with fluorosis decreased, whereas the DNA content remained stable during the entire period of investigation. After 7 months of fluoride treatment, the total brain phospholipid content decreased by 10% and 20% in the 30 and 100 ppm fluoride groups, respectively. The main species of phospholipid influenced by fluorosis were phosphatidylethanolamine, phosphatidylcholine, and phosphatidylserine. The fatty acid and aldehyde compositions of individual phospholipid classes were unchanged. No modifications could be detected in the amounts of cholesterol and dolichol. After 3 months of fluoride treatment, ubiquinone contents in brain were lower; however, at 7 months they were obviously increased in both groups of fluoride treatment. The results demonstrate that the contents of phospholipid and ubiquinone are modified in brains affected by chronic fluorosis and these changes of membrane lipids could be involved in the pathogenesis of this disease.

Simultaneous reversed-phase high-performance liquid chromatographic separation of non-polar isoprenoid lipids and their determination

A procedure for the rapid identification and determination of non-polar isoprenoid lipids from animal tissues was developed. The complete determination can be carried out by reversed-phase HPLC of just two samples. The first, extracted from unaltered tissues and suitably processed by column chromatography, provides information about free cholesterol, cholesteryl esters, coenzymes Q, free dolichols and dolichyl esters. The second, obtained from saponified tissues, can be used to detect both total cholesterol and total dolichols. Specific calibration graphs were constructed for the determination of the different constituents.

The relative essentiality of the antioxidative function of coenzyme Q--the interactive role of DT-diaphorase

This paper will address two aspects regarding the antioxidative role of coenzyme Q (CoQ): (1) Is the antioxidant function of CoQ primary or secondary (coincidental), i.e. was this molecule selected during evolution to function primarily as an essential functional component of the mitochondrial electron transfer chain and oxidative phosphorylation processes, is its antioxidative capability merely a coincidence of its hydroquinone structure, or was its synthetic enzyme sequence selected on the basis of the advantage to the evolving organism of both functions of CoQ? (2) What is the mechanism whereby the hydroquinone (antioxidant) form of CoQ (CoQH2) is maintained in high proportion in the various and many membranes in which it resides, and in which an obvious electron transfer mechanism to reduce it is not present? The essentiality of the antioxidative role of CoQH2 will be explored and compared to other primary and secondary antioxidants. Recent evidence implicating the two-electron quinone reductase, DT-diaphorase, in the maintenance of the reduced, antioxidant state of CoQ during the oxidative stress of exhaustive exercise will be presented, and a hypothesis concerning the evolutionary significance of DT-diaphorase will be offered.

Regulation of isoprenoid metabolism in rat liver: near constant chain lengths of dolichyl phosphate and ubiquinone are maintained during greatly altered rates of cholesterogenesis

When rat liver slices were incubated with varying concentrations of [3H]mevalonolactone, the chain lengths of radiolabeled dolichyl phosphate and ubiquinone varied according to the initial mevalonolactone concentration, indicating that product chain length is dependent on the level of isoprenoid diphosphate intermediates. However, when livers were analyzed from rats which had been maintained on diets of either colestipol (which induces cholesterogenesis 3-fold), or normal chow, or cholesterol (which suppresses cholesterogenesis to 5% of normal) there were only minor changes in the isoprene distribution of either dolichyl phosphate or ubiquinone. In contrast, when rats were maintained on 2% cholesterol plus mevalonolactone (conditions prone to increase the levels of intermediates), the isoprene distributions of both of these compounds were greatly shifted to the higher homologs. However, under none of these conditions were the hepatic levels of these compounds changed significantly. It is concluded that under conditions of greatly altered cholesterogenesis, regulatory mechanisms exist which stabilize the levels of isoprenoid diphosphate intermediates, and that even when levels are increased (e.g., by dietary manipulation), the effect is only to alter isoprene distribution and not the rate of synthesis of dolichyl phosphate and ubiquinone.

Characterization of the in vitro conversion of dolichol to dolichoate in bovine thyroid

The enzymic conversion of dolichol into dolichoic acid has been studied in bovine thyroid subcellular fractions using [1-3H]dolichol as a substrate. The presence of conversion activity could be demonstrated in both the mitochondrial- and supernatant fractions. Investigation of cofactor requirements revealed that NAD+ was essential for reaching optimal activity. From kinetic studies Km-values of 3.5-4 microM and 0.29 mM could be calculated for, respectively, dolichol and NAD+ using the mitochondrial fraction as an enzyme source. No inhibitory effects from ethanol or pyrazole were detected suggesting that alcohol dehydrogenase is not involved in the dolichol-->dolichoate conversion as observed in a bovine thyroid mitochondrial fraction. From inhibitor studies the conversion system behaves distinctly differently from the NADP(+)-depending microsomal oxidoreductase as well as from catalase. The conversion activity in the supernatant on the other hand must be ascribed, at least partially, to a side activity of alcohol dehydrogenase.

Effects of mevinolin treatment on tissue dolichol and ubiquinone levels in the rat

Rats were treated with mevinolin by intraperitoneal injection (15 days) or dietary administration (30 days). The cholesterol, dolichol, dolichyl phosphate and ubiquinone contents of the liver, brain, heart, muscle and blood were then investigated. The cholesterol contents of these organs did not change significantly, with the exception of muscle. Intraperitoneal administration of the drug increases the amount of dolichol in liver, muscle and blood and decreases the dolichyl-P amount in muscle. The same treatment increases the level of ubiquinone in muscle and blood and decreases this value in liver and heart. Oral administration decreases dolichol, dolichyl-P and ubiquinone levels in heart and muscle, while in liver the dolichol level is elevated and ubiquinone level lowered. In brain the amount of dolichyl-P is increased. Intraperitoneal injection of mevinolin also modifies the liver dolichol and dolichyl-P isoprenoid pattern, with an increase in shorter chain polyisoprenes. The levels of dolichol and ubiquinone in the blood do not follow the changes observed in other tissues. Incorporation of [3H]acetate into cholesterol by liver slices prepared from mevinolin-treated rats exhibited an increase, whereas in brain no change was seen. Labeling of dolichol and ubiquinone was increased in both liver and brain, but incorporation into dolichyl phosphate remained relatively stable. The results indicate that mevinolin affects not only HMG-CoA reductase but, to some extent, also affects certain of the peripheral enzymes, resulting in considerable effects on the various mevalonate pathway lipids.

Lipid composition in different regions of the brain in Alzheimer's disease/senile dementia of Alzheimer's type

The lipid compositions of 10 different brain regions from patients affected by Alzheimer's disease/senile dementia of Alzheimer's type were analyzed. The total phospholipid amount decreased somewhat in nucleus caudatus and in white matter. The cortical areas that are morphologically affected by Alzheimer's disease, i.e., frontal and temporal cortex and the hippocampus, showed elevated contents of lipid solvent-extractable phosphatidylinositol. Sphingomyelin content was decreased in regions rich in myelin. There was a 20-50% decrease in dolichol amount in all investigated parts of the brain, but no change was seen in the polyisoprenoid pattern. Levels of alpha-unsaturated polyprenes were decreased in Alzheimer brains. Dolichyl-phosphate content increased in most regions, up to 100%. In both control and Alzheimer tissue almost all of the dolichyl-phosphate was covalently bound, apparently through glycosylation. Cholesterol amounts were highly variable but mostly unchanged, whereas ubiquinone concentrations increased by 30-100% in most regions in brains affected by Alzheimer's disease. These results demonstrate that both phospholipids and neutral lipids are modified in brains affected by Alzheimer's disease/senile dementia of Alzheimer's type.

Distribution and redox state of ubiquinones in rat and human tissues

The distribution and redox state of ubiquinone in rat and human tissues have been investigated. A rapid extraction procedure and direct injection onto HPLC were employed. It was found in model experiments that in postmortem tissue neither oxidation nor reduction of ubiquinone occurs. In rat the highest concentrations of ubiquinone-9 were found in the heart, kidney, and liver (130-200 micrograms/g). In brain, spleen, and intestine one-third and in other tissues 10-20% of the total ubiquinone contained 10 isoprene units. In human tissues ubiquinone-10 was also present at highest concentrations in heart, kidney, and liver (60-110 micrograms/g), and in all tissues 2-5% of the total ubiquinone contained 9 isoprene units. High levels of reduction, 70-100%, could be observed in human tissues, with the exception of brain and lung. The extent of reduction displayed a similar pattern in rat, but was generally lower.

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.

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)

Inhibition of lipid peroxidation by ubiquinol in submitochondrial particles in the absence of vitamin E

The relationship between the antioxidant effects of reduced coenzyme Q10 (ubiquinol, UQH2) and vitamin E (alpha-tocopherol) was investigated in beef heart submitochondrial particles in which lipid peroxidation was initiated by incubation with ascorbate + ADP-Fe3+. These effects were examined after extraction of coenzyme Q10 (UQ-10) and vitamin E from the particles and reincorporation of the same components alone or in combination. The results show that UQH2 efficiently inhibits lipid peroxidation even when vitamin E is absent. It is concluded that UQH2 can inhibit lipid peroxidation directly, without the mediation of vitamin E.

Covalent binding of dolichyl phosphate to proteins in rat liver

Rats were injected via the portal vein with (RS)-[5-3H]-mevalonolactone and the lipids were extracted. From fractions of liver homogenate, all labeled dolichol, cholesterol and ubiquinone could be extracted, but about 40% of microsomal and lysosomal dolichyl phosphate was only released after alkaline hydrolysis. Only a small amount of the non-extractable radioactivity was found to be associated with alpha-unsaturated polyprenyl phosphate. There was no difference in the polyisoprenoid pattern when the two pools of dolichyl phosphate were compared. On the other hand, the specific activity of the bound lipid was only half that of the extractable form. After phenyl-Sepharose chromatography, a peak of protein was isolated exhibiting a 25-fold enrichment in bound radioactive dolichyl phosphate. Treatment with a non-specific protease, followed by chromatography, gave polypeptide fragments associated with bound lipids. On SDS/PAGE a major protein band at 23 kDa and some minor bands with higher molecular masses were found to be associated with this lipid. The results indicate the presence of covalently bound dolichyl phosphate in rat liver.

Rates of cholesterol, ubiquinone, dolichol and dolichyl-P biosynthesis in rat brain slices

Slices from the brain and liver of rats were prepared and upon incubation exhibited a continuous and high capacity for incorporation of radioactive precursors into proteins and lipids. Using [3H]mevalonate as precursor, the rates of biosynthesis of cholesterol, ubiquinone, dolichol and dolichyl-P in brain slices were determined and found to be 5.5, 0.25, 0.0093 and 0.0091 nmol/h/g, respectively. Dolichol and dolichyl-P accumulate to a limited extent, but almost all of these lipids in the brain originate from de novo synthesis. The calculated half-lives for cholesterol, ubiquinone, dolichol and dolichyl-P were 4076, 90, 1006 and 171 h, respectively. The results indicate that lipids formed via the mevalonate pathway in the brain have an active and independently regulated biosynthesis.

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.

Uptake and metabolism of dolichol and cholesterol in perfused rat liver

The uptake of dolichol and cholesterol by perfused rat liver was studied. When these radioactive lipids were incorporated into egg phosphatidylcholine liposomes, both dolichol and cholesterol appeared initially in the supernatant and in the microsomal fraction and, later on, in the mitochondrial-lysosomal fraction. The lipids taken up were esterified to some extent, but no phosphorylation of dolichol occurred. Incorporation of dolichol and cholesterol into lipoproteins increased the efficiency of uptake, which was receptor-mediated in this case. Accumulation of these lipids occurred in lysosomes followed by a transport to the endoplasmic reticulum (ER). Both labeled dolichol and cholesterol appeared in the bile. In the case of dolichol, the majority of this radioactivity was not associated with the original substance itself, and probably represented lipid-soluble catabolites. In the case of cholesterol, most of the radioactivity was associated with bile acids. It appears that, in contrast to the receptor-mediated uptake of lipoproteins from the perfusate, the uptake of liposomal lipids involves a different mechanism. After association with the plasma membrane, the lipids enter into the cytoplasm and are transported to the ER and later to the lysosomes.

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.

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.