Showing papers on "Sterol published in 1991"

Membrane cholesterol dynamics: cholesterol domains and kinetic pools.

Abstract: Nonreceptor mediated cholesterol uptake and reverse cholesterol transport in cells occur through cellular membranes. Thus, elucidation of cholesterol dynamics in membranes is essential to understanding cellular cholesterol accumulation and loss. To this end, it has become increasingly evident that cholesterol is not randomly distributed in either model or biologic membranes. Instead, membrane cholesterol appears to be organized into structural and kinetic domains or pools. Cholesterol-rich and poor domains can even be observed histochemically and physically isolated from epithelial cell surface membranes. The physiologic importance of these domains is 2-fold: (i) Select membrane proteins (receptors, transporters, etc.) are localized in either cholesterol-rich or cholesterol-poor domains. Consequently, the structure and properties of the domains rather than of the bulk lipid may selectively affect the function of proteins residing therein. (ii) Kinetic evidence suggests that cholesterol transport through and between membranes may occur through specific domains or pools. Regulation of the size and properties of such domains may be controlling factors of cholesterol transport or accumulation in cells. Recent technologic advances in the use of fluorescent sterols have allowed examination of cholesterol domain structure in model and biologic membranes. These techniques have been applied to examine the role of high-density lipoprotein, cholesterol lowering drugs, and intracellular lipid transfer proteins in membrane sterol domain structure and sterol movement between membranes.

Nutritional and biochemical aspects of the hypolipidemic action of rice bran oil: a review.

Abstract: In this paper, we review the effects of rice bran oil (RBO), an unconventional oil recently introduced onto the Indian market for human use. RBO contains oleic acid (38.4%), linoleic acid (34.4%), and linolenic acid (2.2%) as unsaturated fatty acids, and palmitic (21.5%) and stearic (2.9%) acids as saturated fatty acids. The unsaponifiable fraction (4.2%) has total tocopherols (81.3 mg%), gamma-oryzanol (1.6%), and squalene (320 mg%). Oryzanol is a mixture of ferulic acid esters of triterpene alcohols such as cycloartenol (CA) (106 mg%) and 24-methylene cycloartanol (494 mg%). Studies on experimental rats demonstrated a hypolipidemic effect of RBO. The unsaponifiable fraction of RBO lowers cholesterol levels. Feeding phytosterols, CA, and 24-methylene cycloartanol in amounts present in RBO to hypercholesterolemic rats for 8 weeks indicates that CA alone reduces cholesterol and triglyceride levels significantly. Endogenous sterol excretion increases in animals given CA. The accumulation of CA in the liver inhibits cholesterol esterase activity, which in turn leads to reduction in circulating cholesterol levels. CA is structurally similar to cholesterol and may compete with the binding sites of cholesterol and sequestrate cholesterol, which is metabolized to its derivatives. RBO, which is rich in tocopherols and tocotrienols, may improve oxidative stability. Tocotrienols inhibit HMG CoA reductase, resulting in hypocholesterolemia. The hypolipidemic effect of RBO has also been established in human subjects. Thus, RBO could be a suitable edible oil for patients with hyperlipidemia.

Differential effects of plant sterols on water permeability and on acyl chain ordering of soybean phosphatidylcholine bilayers.

Abstract: To gain some insight into the structural and functional roles of sterols in higher plant cells, various plant sterols have been incorporated into soybean phosphatidylcholine (PtdCho) bilayers and tested for their ability to regulate water permeability and acyl chain ordering. Sitosterol was the most efficient sterol in reducing the water permeability of these vesicles and stigmasterol appeared to have no significant effect. Vesicles containing 24zeta-methylcholesterol exhibited an intermediate behavior, similar to that of vesicles containing cholesterol. Cycloartenol, the first cyclic biosynthetic precursor of plant sterols, reduced the water permeability in a very effective way. Of two unusual plant sterols, 24-methylpollinastanol and 14alpha,24zeta-dimethylcholest-8-en-3beta-ol, the former was found to be functionally equivalent to sitosterol and the latter was found to be relatively inefficient. 2H NMR experiments have been performed with oriented bilayers consisting of soybean PtdCho with sitosterol, stigmasterol, or 24-methylpollinastanol. The results provided clear evidence that sitosterol and 24zeta-methylpollinastanol exhibit a high efficiency to order PtdCho acyl chains that closely parallels their ability to reduce water permeability. By contrast, stigmasterol shows a low efficiency for both functions. These results show that sitosterol and stigmasterol, two major 24-ethylsterols differing only by the absence or presence of the Delta22 double bond in the side chain, probably play different roles in regulating plant membrane properties; they also may explain why 9beta,19-cyclopropylsterols behave as good surrogates of sitosterol.

Sterol and triterpene derivatives from plants inhibit the effects of a tumor promoter, and sitosterol and betulinic acid inhibit tumor formation in mouse skin two-stage carcinogenesis.

Abstract: A single topical application of 1 microgram of 12-O-tetradecanoylphorbol- 13-acetate (TPA) to the ears of mice was shown to induce edema, and this TPA-induced inflammation was inhibited by 4-methylsterol and triterpene derivatives. The ED50 of these compounds against TPA-induced inflammation was 0.1-3 mumol. Phytosterols had only slight inhibitory effects. Furthermore, application of 5 micrograms TPA to mouse skin rapidly caused accumulation of ornithine decarboxylase (ODC). Similarly, sitosterol and lupane-type triterpene derivatives markedly inhibited this TPA-induced ODC accumulation. In addition, 5 mumol betulinic acid markedly inhibited the promoting effect of 2.5 micrograms TPA applied twice weekly on skin tumor formation in mice initiated with 50 micrograms of 7,12-dimethylbenz[a]anthracene, and 5 mumol of sitosterol caused slight suppression. Thus, the inhibitory effects of sterol and triterpene derivatives on TPA-induced inflammation roughly parallelled their inhibitory activities against tumor promotion.

High density lipoprotein apolipoproteins mediate removal of sterol from intracellular pools but not from plasma membranes of cholesterol-loaded fibroblasts.

Abstract: Cultured cells possess high-affinity binding sites (receptors) for high density lipoprotein (HDL) that appear to mediate removal of excess intracellular cholesterol from cells. To examine the role of intact HDL apoproteins in receptor-mediated cholesterol removal, HDL3 apoproteins were digested with the proteolytic enzymes trypsin and pronase, and the residual particles were used in sterol efflux experiments. Protease treatment abolished the interaction of HDL3 with the 110-kd cell membrane protein postulated to represent the HDL receptor molecule, indicating that this interaction is mediated by HDL apoproteins rather than lipids. Compared with native HDL3 protease-modified HDL3 had a markedly reduced ability to selectively remove sterol from intracellular pools, even though modified particles promoted greater cholesterol efflux from the plasma membrane than did native particles. These results indicate that whereas sterol efflux from plasma membranes is mediated by HDL lipids, removal of excess intracellular sterol from cells is mediated by HDL apoproteins. These findings are consistent with the hypothesis that receptor binding of HDL apoproteins stimulates translocation of excess intracellular sterol to the cell surface where it becomes accessible for removal by HDL or other lipid-rich acceptor particles.

The role of cholesterol in chronic blepharitis.

Abstract: Chronic blepharitis has been a difficult disease to define either microbiologically or biochemically. Sterols from meibomian secretions of normal subjects and patients were analyzed, and important differences were observed. Based on analyses of these secretions, two significantly different (P less than 0.001) types of normal subjects were found, those with and those without cholesterol esters [Norm(CP) and Norm(CA), respectively]. All patients' secretions contained cholesterol esters. Evidence was obtained which suggests that oxysterols may control the ester cholesterol accumulation. Furthermore, only when cholesterol esters were present did wax and sterol esters containing unsaturated fatty acids accumulate. Over 90% of these unsaturated fatty acids were normal (unbranched); the rest were iso-fatty acids. Preliminary results also suggest that the ester fatty alcohols are much more complex than previously reported; seven alcohols were common to all samples analyzed. Additionally, highly oxygenated alcohols were detected, especially in the meibomian keratoconjunctivitis (MKC) disease group. The MKC samples also contained an alcohol (mass, M/Z 378) not present in any of the other samples analyzed. Based on analysis of variance and linear-regression models, it was determined that the long-chain (C20-28) fatty acids were more important in determining disease signs. Furthermore, in the MKC group, the ratio of unsaturated C18 fatty acids to cholesterol in the wax and sterol esters was significantly different (P less than 0.05) from the Norm(CP) group. The authors discuss the fact that rabbit meibomian secretions are stable, despite containing a very high percentage of ester sterols, and relate this to their high percentage of branched-chain fatty acids and low percentage of unsaturated fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary cholesterol and the origin of cholesterol in the brain of developing rats.

Abstract: Milk substitutes containing cholesterol at concentrations lower, equal to or greater than the concentrations found in natural rat milk were fed to artificially reared rat pups from 5 d until 15 or 16 d after birth. Pups reared by their mother served as controls. In one experiment, D7-cholesterol was fed in the milk at four different concentrations. The purpose of the study was to determine whether cholesterol in milk influenced growth and the sterol composition of brain over the period of its most rapid accumulation in this organ. We found that body and brain weights were not different, irrespective of the concentration of cholesterol in the milk substitutes. High concentrations of cholesterol in milk caused a significant increase in cholesterol in liver and plasma, whereas the concentration of cholesterol in brain was not different from the concentration in the brain of controls. The amounts of D7-cholesterol in lung and liver, and in plasma and RBC that pass the brain, were consistent with the concentration fed in the milk and approached 70% of the total content of cholesterol in these organs at the highest concentration fed. Brain, by contrast, contained very small amounts of D7-cholesterol, which could readily be attributed to D7-cholesterol associated with the vascular system of the blood-brain barrier. We found that the sterol composition of brain is not influenced by the concentration of cholesterol in milk and that cholesterol exogenous to brain, even in a hypercholesterolemic condition, does not gain entry to the brain. We conclude that the brain biosynthesizes de novo all the cholesterol it requires.

Tellurium blocks cholesterol synthesis by inhibiting squalene metabolism: preferential vulnerability to this metabolic block leads to peripheral nervous system demyelination.

Abstract: Inclusion of 1.1% elemental tellurium in the diet of postweanling rats produces a peripheral neuropathy due to a highly synchronous primary demyelination of sciatic nerve; this demyelination is followed closely by remyelination. Sciatic nerves from animals fed tellurium for various times were removed and incubated ex vivo for 1 h with [14C]acetate, and radioactivity incorporated into individual lipid classes was determined. In nerves from rats exposed to tellurium, there was a profound and selective block in the conversion of radioactive acetate to cholesterol. Another radioactive precursor, [3H]water, gave similar results. We suggest that tellurium feeding inhibits squalene epoxidase activity and that the consequent lack of cholesterol destabilizes myelin, thereby causing destruction of the larger internodes. Ex vivo incubation experiments were also carried out with liver slices. As with nerve, tellurium feeding caused accumulation in squalene of label from radioactive acetate, whereas labeling of cholesterol was greatly inhibited. Unexpectedly, however, incorporation of label from [3H]water into both squalene and cholesterol was increased. Relevant is the demonstration that liver was the primary site of bulk accumulation of squalene, which accounted for 10% of liver dry weight at 5 days. Thus, accumulation of squalene (and other mechanisms, possibly including up-regulation of cholesterol biosynthetic pathways) drives squalene epoxidase activity at normal levels in liver even in the presence of inhibitors of this enzyme. This is reflected by continuing incorporation of [3H]water into cholesterol; incorporation of this precursor takes place at many of the postsqualene biosynthetic steps for sterol formation. [14C]Acetate entering the sterol pathway before squalene in liver is greatly diluted in specific activity when it reaches the large squalene pool, and thus increased squalene epoxidase activity does not transfer significant 14C label to sterols. In contrast to the situation with liver, synthesis of sterols is markedly depressed in sciatic nerve, and squalene does not accumulate to high levels.

Lipids of Plasma Membranes Prepared from Oat Root Cells: Effects of Induced Water-Deficit Tolerance

Abstract: Plasma membranes were isolated from oat (Avena sativa) roots by the phase-partitioning method. The membranes were exposed to repeated periods of moderate water-deficit stress, and a water-deficit tolerance was induced (acclimated plants). The plasma membranes of the controls (nonacclimated plants) were characterized by a high phospholipid content, 79% of total lipids, cerebrosides (9%) containing hydroxy fatty acids (>90% 24:1-OH) and free sterols, acylated sterylglucosides, sterylglucosides, and steryl esters, together amounting to 12%. Major phospholipids were phosphatidylcholine and phosphatidylethanolamine with lesser amounts of phosphatidylglycerol, phosphatidylinositol, and phosphatidic acid. After the membranes were acclimated to dehydration, the lipid to protein ratio decreased from 1.3 to 0.7 micromoles per milligram. Furthermore, the cerebrosides decreased to 5% and free sterols increased from 9% (nonacclimated plants) to 14%. Because the total phospholipids did not change significantly, the free sterol to phospholipid ratio increased from 0.12 to 0.19. There was no change in the relative distribution of sterols after acclimation. The ratio of phosphatidylcholine to phosphatidylethanolamine changed from 1.1 in the nonacclimated plants to 0.69 in the acclimated plants. The results show that acclimation to dehydration implies substantial alterations in the lipid composition of the plasma membrane.

Cloning and disruption of the yeast C-8 sterol isomerase gene.

Abstract: The yeastERG2 gene codes for the C-8 sterol isomerase, an enzyme required for the isomerization of the δ8 double bond to the δ7 position in ergosterol biosynthesis. TheERG2 gene was cloned by complementation of a C-8 sterol isomerase mutant strain (erg2). The complementing region of DNA required to restore ergosterol synthesis toerg2 was limited to a 1.0 kbStuI-BglII fragment. In order to determine whether theERG2 gene was essential for yeast viability, aLEU2 gene was inserted into theNdeI site (made blunt) of this 1.0 kb fragment. Transformation of a wild type diploid strain with theERG2 substituted DNA resulted in the generation of viable haploids containing theerg2 null allele (erg2–4∶∶Leu2). These results suggest that the C-8 sterol isomerase activity is not essential for yeast cell viability. This disruption represents the second ergosterol biosynthetic gene in the distal portion of the pathway to be disrupted without adversely affecting cell viability.

Sluggish sitosterol turnover and hepatic failure to excrete sitosterol into bile cause expansion of body pool of sitosterol in patients with sitosterolemia and xanthomatosis.

Abstract: Sitosterolemia and xanthomatosis are characterized by the development of tendon and tuberous xanthomas at an early age and premature atherosclerosis despite normal plasma cholesterol concentrations. The reason(s) for the xanthoma formation and premature atherosclerosis are not clearly understood. The accumulation of sitosterol in the tissues of these patients could be due to increased uptake of low density lipoprotein (LDL) via LDL receptors because of an expanded sitosterol pool caused by sluggish turnover and decreased excretion of sitosterol into bile and feces coupled with the hyperabsorption of sitosterol. We have studied sitosterol and cholesterol turnovers, the biliary and fecal excretion of neutral and acidic steroids, and the response of plasma sterol (sitosterol and cholesterol) levels to either a sterol-free formula or low plant sterol diet in three patients. The average half-life of the first exponential (tA1/2) for sitosterol was 9.2 +/- 3.3 (mean +/- SD) days, which was more than twice that in normal humans. The second exponential (tB1/2) was 156 +/- 108 days, which was nearly 10 times longer than that for normal humans. The average cholesterol production rate in pool A was 0.87 g/day, which is about 40% of that in normal humans. Cholesterol synthesis measured by the sterol balance technique was also found to be about 70% lower than that for normal humans. In two patients fed a sterol-free formula diet, by 25 days their plasma sitosterol and cholesterol levels had decreased by 42% and 36%, respectively. However, in one patient plasma sitosterol and cholesterol concentrations remained unchanged while on the low plant sterol-mixed food diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of plant sterols by nematodes

Abstract: Parasitic nematodes do not biosynthesize sterols de novo and therefore possess a nutritional requirement for sterol, which must be obtained from their hosts. Consequently, the metabolism of phytosterols by plant-parasitic nematodes is an important process with potential for selective exploitation. The sterol compositions of several species of plant-parasitic nematodes were determined by capillary gas chromatography-mass spectrometry and compared with the sterol compositions of their hosts. Saturation of the phytosterol nucleus was the major metabolic transformation performed by the root-knot nematodes Meloidogyne arenaria and M. incognita and the corn root lesion nematode, Pratylenchus agilis. In addition to saturation, the corn cyst nematode, Heterodera zeae, dealkylated its host sterols at C-24. Because free-living nematodes can be cultured in sterol-defined artificial medium, they have been successfully used as model organisms for investigation of sterol metabolism in plant-parasitic nematodes. Major pathways of phytosterol metabolism in Caenorhabditis elegans, Turbatrix aceti and Panagrellus redivivus included C-24 dealkylation and 4 alpha-methylation (a pathway unique to nematodes). C. elegans and T. aceti introduced double bonds at C-7, and T. aceti and P. redivivus saturated the sterol nucleus similarly to the plant-parasitic species examined. Several azasteroids and long-chain dimethylalkylamines inhibited growth and development of C. elegans and also the delta 24-sterol reductase enzyme system involved in the nematode C-24 dealkylation pathway.

Pigments, fatty acids, and sterols of the toxic dinoflagellate gymnodinium catenatum1

Abstract: Cultures and field samples of the toxic dinoflagellate Gymnodinium catenatum Graham from Tasmania, Australia, were analyzed for pigment, fatty acid, and sterol composition. Gymnodinium catenatum contained the characteristic pigments of photosynthetic dinoflagellates, including chlorophyll a, chlorophyll c2, and the carotenoids peridinin, dinoxanthin, diadinoxanthin, diatoxanthin, and β,β-carotene. In midlogarithmic and early stationary phase cultures, the chlorophyll a content ranged 50–72 pg · cell−1, total lipids 956–2084 pg · cell−1, total fatty acids 426–804 pg · cell−1, and total sterols 8–20 pg · cell−1. The major fatty acids (in order of decreasing abundance) were 16:0, 22:6(n-3), and 20:5(n-3) (collectively 65–70% of the total fatty acids), followed by 16:1(n-7), 18:2(n-6), and 14:0. This distribution is characteristic of most dinoflagellates, except for the low abundance (<3%) of the fatty acid 18:5(n-3), considered by some authors to be a marker for dinoflagellates. The three major sterols were 4α-methyl-5α-cholest-7-en-3β-ol, 4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol (the dinoflagellate sterol, dinosterol), and 4α,23,24-trimethyl-5α-cholest-7-en-3β-ol. These three sterols comprised about 75% of the total sterols in both logarithmic and early stationary phase cultures, and they were also found in high proportions (22–25%) in natural dinoflagellate bloom samples. 4-Desmethyl sterols, which are common in most microalgae, were only present in trace amounts in G. catenatum. The chemotaxonomic affinities of G. catenatum and the potential for using specific signature lipids for monitoring toxic dinoflagellate blooms are discussed.

Sterol biosynthesis inhibitors: their current status and modes of action.

Abstract: The mechanism of each of the reactions in the post-squalene segment of the fungal and higher plant sterol biosynthetic pathway is outlined. The inhibitors of the enzymes catalyzing the reactions are described and how inhibition is brought about is explained in the areas where it is known.