Bile acid biosynthesis in Smith-Lemli-Opitz syndrome bypassing cholesterol: Potential importance of pathway intermediates

Bile acids are the end products of cholesterol metabolism secreted into bile. They are essential for the absorption of lipids and lipid soluble compounds from the intestine. Here we have identified a series of unusual Δ5-unsaturated bile acids in plasma and urine of patients with Smith-Lemli-Opitz syndrome (SLOS), a defect in cholesterol biosynthesis resulting in elevated levels of 7-dehydrocholesterol (7-DHC), an immediate precursor of cholesterol. Using liquid chromatography - mass spectrometry (LC-MS) we have uncovered a pathway of bile acid biosynthesis in SLOS avoiding cholesterol starting with 7-DHC and proceeding through 7-oxo and 7β-hydroxy intermediates. This pathway also occurs to a minor extent in healthy humans, but elevated levels of pathway intermediates could be responsible for some of the features SLOS. The pathway is also active in SLOS affected pregnancies as revealed by analysis of amniotic fluid. Importantly, intermediates in the pathway, 25-hydroxy-7-oxocholesterol, (25R)26-hydroxy-7-oxocholesterol, 3β-hydroxy-7-oxocholest-5-en-(25R)26-oic acid and the analogous 7β-hydroxysterols are modulators of the activity of Smoothened (Smo), an oncoprotein that mediates Hedgehog (Hh) signalling across membranes during embryogenesis and in the regeneration of postembryonic tissue. Computational docking of the 7-oxo and 7β-hydroxy compounds to the extracellular cysteine rich domain of Smo reveals that they bind in the same groove as both 20S-hydroxycholesterol and cholesterol, known activators of the Hh pathway.

Performance Enhancement of Diffusion-Based Molecular Communication

Inter-Symbol Interference (ISI) is one of the challenges of bio-inspired diffusion-based molecular communication. The degradation of the remaining molecules from a previous transmission is the solution that biological systems use to mitigate this ISI. While most prior work has proposed the use of enzymes to catalyze the molecules degradation, enzymes also degrade the molecules carrying the information, which drastically decreases the signal strength. In this paper, we propose the use of photolysis reactions, which use the light to instantly transform the emitted molecules so they no longer be recognized after their detection. The light will be emitted in an optimal time, allowing the receiver to detect as many molecules as possible, which increases both the signal strength and ISI mitigation. A lower bound expression on the expectation of the observed molecules number at the receiver is derived. Bit error probability expression is also formulated, and both expressions are validated with simulation results, which show a visible enhancement when using photolysis reactions. The performance of the proposed method is evaluated using Interference-to-Total-Received molecules metric (ITR) and the derived bit error probability.

1α-Hydroxy derivatives of 7-dehydrocholesterol are selective liver X receptor modulators

The nuclear receptors liver X receptor (LXR) α and LXRβ are involved in the regulation of lipid metabolism, inflammation, immunity, cellular proliferation, and apoptosis. Oxysterols are endogenous LXR ligands, and also interact with other nuclear and membrane receptors. We previously reported that a phytosterol derivative with a 1α-hydroxy group acts as a potent LXR agonist with intestine-selective action and that 25-hydroxy and 26/27-hydroxy metabolites of 7-dehydrocholesterol (7-DHC) exhibit partial LXR agonism. In this study, we report that 1α-hydroxy derivatives of 7-DHC, 1α-OH-7-DHC and 1,25-(OH)₂-7-DHC, act as LXR modulators. Luciferase reporter gene assays showed that 1α-OH-7-DHC activates LXRα and LXRβ and that 1,25-(OH)₂-7-DHC activates both LXRs and vitamin D receptor. Examination of cofactor peptide association showed that the 1α-hydroxy derivatives, specifically 1,25-(OH)₂-7-DHC, induce association of coactivator/corepressor peptide in a different manner from the agonist T0901317. Docking modeling and alanine mutational analysis of LXRα demonstrated that 1,25-(OH)₂-7-DHC interacts with LXRα residues in a manner distinct from potent agonists, such as T0901317 and 24(S),25-epoxycholesterol. 1α-OH-7-DHC and 1,25-(OH)₂-7-DHC induced expression of LXR target genes in a cell type- and gene-selective manner. 1,25-(OH)₂-7-DHC effectively suppressed lipopolysaccharide-stimulated proinflammatory gene expression in an LXR-dependent manner. Therefore, 1α-hydroxy derivatives, such as 1,25-(OH)₂-7-DHC, are unique LXR modulators with selective agonistic activity and potent transrepression function. These oxysterols have potential as LXR-targeted therapeutics for inflammatory disease.

Steroid constituents from the soft coral Sinularia microspiculata

A methanol extract of the soft coral Sinularia microspiculata revealed five sterols, including two new compounds. Using combined chromatographic and spectroscopic experiments, the new compounds were found to be 7-oxogorgosterol (1) and 16α-hydroxysarcosterol (2). Their structures were determined on the basis of spectroscopic data ((1)H and (13)C NMR, HSQC, HMBC, (1)H-(1)H COSY, NOESY, and FT-ICR-MS) and by comparing obtained results to the values indicated in previous studies. Among the isolated compounds, 3 showed weak cytotoxic effects against HL-60 (IC50  =  89.02  ±  9.93 μM) cell line, whereas 5 was weakly active against HL-60 (IC50  =  82.80  ±  13.65 μM) and SK-Mel2 (IC50  =  72.32  ±  1.30 μM) cell lines.

Differential cytotoxic effects of 7-dehydrocholesterol-derived oxysterols on cultured retina-derived cells: Dependence on sterol structure, cell type, and density

Tissue accumulation of 7-dehydrocholesterol (7DHC) is a hallmark of Smith-Lemli-Opitz Syndrome (SLOS), a human inborn error of the cholesterol (CHOL) synthesis pathway. Retinal 7DHC-derived oxysterol formation occurs in the AY9944-induced rat model of SLOS, which exhibits a retinal degeneration characterized by selective loss of photoreceptors and associated functional deficits, Müller cell hypertrophy, and engorgement of the retinal pigment epithelium (RPE) with phagocytic inclusions. We evaluated the relative effects of four 7DHC-derived oxysterols on three retina-derived cell types in culture, with respect to changes in cellular morphology and viability. 661W (photoreceptor-derived) cells, rMC-1 (Müller glia-derived) cells, and normal diploid monkey RPE (mRPE) cells were incubated for 24 h with dose ranges of either 7-ketocholesterol (7kCHOL), 5,9-endoperoxy-cholest-7-en-3β,6α-diol (EPCD), 3β,5α-dihydroxycholest-7-en-6-one (DHCEO), or 4β-hydroxy-7-dehydrocholesterol (4HDHC); CHOL served as a negative control (same dose range), along with appropriate vehicle controls, while staurosporine (Stsp) was used as a positive cytotoxic control. For 661W cells, the rank order of oxysterol potency was: EPCD > 7kCHOL >> DHCEO > 4HDHC ≈ CHOL. EC50 values were higher for confluent vs. subconfluent cultures. 661W cells exhibited much higher sensitivity to EPCD and 7kCHOL than either rMC-1 or mRPE cells, with the latter being the most robust when challenged, either at confluence or in sub-confluent cultures. When tested on rMC-1 and mRPE cells, EPCD was again an order of magnitude more potent than 7kCHOL in compromising cellular viability. Hence, 7DHC-derived oxysterols elicit differential cytotoxicity that is dose-, cell type-, and cell density-dependent. These results are consistent with the observed progressive, photoreceptor-specific retinal degeneration in the rat SLOS model, and support the hypothesis that 7DHC-derived oxysterols are causally linked to that retinal degeneration as well as to SLOS.

Metabolic Precursor of Cholesterol Causes Formation of Chained Aggregates of Liquid-Ordered Domains

7-Dehydrocholesterol, an immediate metabolic predecessor of cholesterol, can accumulate in tissues due to some metabolic abnormalities, causing an array of symptoms known as Smith-Lemli-Opitz syndrome. Enrichment of cellular membranes with 7-dehydrocholesterol interferes with normal cell-signaling processes, which involve interaction between rafts and formation of the so-called signaling platforms. In model membranes, cholesterol-based ordered domains usually merge upon contact. According to our experimental data, ordered domains in the model systems where cholesterol is substituted for 7-dehydrocholesterol never merge on the time scale of the experiment, but clusterize into necklace-like aggregates. We attribute such different dynamical behavior to altered properties of the domain boundary. In the framework of thickness mismatch model, we analyzed changes of interaction energy profiles of two approaching domains caused by substitution of cholesterol by 7-dehydrocholesterol. The energy barrier for domain merger is shown to increase notably, with simultaneous appearance of another distinct local energy minimum. Such energy profile is in perfect qualitative agreement with the experimental observations. The observed change of domain dynamics can impair proper interaction between cellular rafts underlying pathologies associated with deviations in cholesterol metabolism.

Alterations in membrane caveolae and BKCa channel activity in skin fibroblasts in Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is an inherited disorder of cholesterol synthesis caused by mutations in DHCR7 which encodes the final enzyme in the cholesterol synthesis pathway. The immediate precursor to cholesterol synthesis, 7-dehydrocholesterol (7-DHC) accumulates in the plasma and cells of SLOS patients which has led to the idea that the accumulation of abnormal sterols and/or reduction in cholesterol underlies the phenotypic abnormalities of SLOS. We tested the hypothesis that 7-DHC accumulates in membrane caveolae where it disturbs caveolar bilayer structure-function. Membrane caveolae from skin fibroblasts obtained from SLOS patients were isolated and found to accumulate 7-DHC. In caveolar-like model membranes containing 7-DHC, subtle, but complex alterations in intermolecular packing, lipid order and membrane width were observed. In addition, the BK(Ca) K(+) channel, which co-migrates with caveolin-1 in a membrane fraction enriched with cholesterol, was impaired in SLOS cells as reflected by reduced single channel conductance and a 50 mV rightward shift in the channel activation voltage. In addition, a marked decrease in BK(Ca) protein but not mRNA expression levels was seen suggesting post-translational alterations. Accompanying these changes was a reduction in caveolin-1 protein and mRNA levels, but membrane caveolar structure was not altered. These results are consistent with the hypothesis that 7-DHC accumulation in the caveolar membrane results in defective caveolar signaling. However, additional cellular alterations beyond mere changes associated with abnormal sterols in the membrane likely contribute to the pathogenesis of SLOS.

Conversion of 7-dehydrocholesterol to 7-ketocholesterol is catalyzed by human cytochrome P450 7A1 and occurs by direct oxidation without an epoxide intermediate

7-Ketocholesterol is a bioactive sterol, a potent competitive inhibitor of cytochrome P450 7A1, and toxic in liver cells. Multiple origins of this compound have been identified, with cholesterol being the presumed precursor. Although routes for formation of the 7-keto compound from cholesterol have been established, we found that 7-dehydrocholesterol (the immediate precursor of cholesterol) is oxidized by P450 7A1 to 7-ketocholesterol (k(cat)/K(m) = 3 × 10(4) m(-1) s(-1)). P450 7A1 converted lathosterol (Δ(5)-dihydro-7-dehydrocholesterol) to a mixture of the 7-keto and 7α,8α-epoxide products (~1:2 ratio), with the epoxide not rearranging to the ketone. The oxidation of 7-dehydrocholesterol occured with predominant formation of 7-ketocholesterol and with the 7α,8α-epoxide as only a minor product; the synthesized epoxide was stable in the presence of P450 7A1. The mechanism of 7-dehydrocholesterol oxidation to 7-ketocholesterol is proposed to involve a Fe(III)-O-C-C(+) intermediate and a 7,8-hydride shift or an alternative closing to yield the epoxide (Liebler, D. C., and Guengerich, F. P. (1983) Biochemistry 22, 5482-5489). Accordingly, reaction of P450 7A1 with 7-[(2)H(1)]dehydrocholesterol yielded complete migration of deuterium in the product 7-ketocholesterol. The finding that 7-dehydrocholesterol is a precursor of 7-ketocholesterol has relevance to an inborn error of metabolism known as Smith-Lemli-Opitz syndrome (SLOS) caused by defective cholesterol biosynthesis. Mutations within the gene encoding 7-dehydrocholesterol reductase, the last enzyme in the pathway, lead to the accumulation of 7-dehydrocholesterol in tissues and fluids of SLOS patients. Our findings suggest that 7-ketocholesterol levels may also be elevated in SLOS tissue and fluids as a result of P450 7A1 oxidation of 7-dehydrocholesterol.

Products of vitamin D3 or 7-dehydrocholesterol metabolism by cytochrome P450scc show anti-leukemia effects, having low or absent calcemic activity

Background

Cytochrome P450scc metabolizes vitamin D3 to 20-hydroxyvitamin D3 (20(OH)D3) and 20,23(OH)₂D3, as well as 1-hydroxyvitamin D3 to 1α,20-dihydroxyvitamin D3 (1,20(OH)₂D3). It also cleaves the side chain of 7-dehydrocholesterol producing 7-dehydropregnenolone (7DHP), which can be transformed to 20(OH)7DHP. UVB induces transformation of the steroidal 5,7-dienes to pregnacalciferol (pD) and a lumisterol-like compounds (pL).

Methods and Findings

To define the biological significance of these P450scc-initiated pathways, we tested the effects of their 5,7-diene precursors and secosteroidal products on leukemia cell differentiation and proliferation in comparison to 1α,25-dihydroxyvitamin D3 (1,25(OH)₂D3). These secosteroids inhibited proliferation and induced erythroid differentiation of K562 human chronic myeloid and MEL mouse leukemia cells with 20(OH)D3 and 20,23(OH)₂D3 being either equipotent or slightly less potent than 1,25(OH)₂D3, while 1,20(OH)₂D3, pD and pL compounds were slightly or moderately less potent. The compounds also inhibited proliferation and induced monocytic differentiation of HL-60 promyelocytic and U937 promonocytic human leukemia cells. Among them 1,25(OH)₂D3 was the most potent, 20(OH)D3, 20,23(OH)₂D3 and 1,20(OH)₂D3 were less active, and pD and pL compounds were the least potent. Since it had been previously proven that secosteroids without the side chain (pD) have no effect on systemic calcium levels we performed additional testing in rats and found that 20(OH)D3 had no calcemic activity at concentration as high as 1 µg/kg, whereas, 1,20(OH)₂D3 was slightly to moderately calcemic and 1,25(OH)₂D3 had strong calcemic activity.

Conclusions

We identified novel secosteroids that are excellent candidates for anti-leukemia therapy with 20(OH)D3 deserving special attention because of its relatively high potency and lack of calcemic activity.

Studies on the Black Box: incorporation of 3-oxo-7-dehydrocholesterol into ecdysteroids by Drosophila melanogaster and Manduca sexta

It has long been hypothesized that the oxidation of 7-dehydrocholesterol (7dC), made from dietary cholesterol (C), to 3-oxo-7dC (3-oxo-Delta(5,7)C) immediately precedes the unknown "Black Box" oxidations that lead to the formation of the first up-stream intermediate exhibiting the highly characteristic ecdysteroid structure of the steroid molting hormone of insects, crustaceans and some other arthropods. Perhaps rate-limiting and under the control of the prothoracicotropic hormone (PTTH), the biosynthesis of 3-oxo-7dC and its subsequent oxidative modifications have been difficult to study because of their apparent instability, i.e. no intermediates between 7dC and the diketol (3-oxo-25,22,2-trideoxyecdysone) have ever been observed or identified in insect prothoracic gland incubations with radiolabelled precursors. However, we show that 3-oxo-7dC can be converted into lipophilic, photosensitive, ketone-blocked (PSKB) ketal derivatives which will release 3-oxo-7dC when and where desired following brief irradiation with innocuous long-wave (365 nm) UV-light both in vivo and in vitro. In this manner, 3-oxo-7dC is quickly and efficiently incorporated into ecdysteroids by adult male and female Drosophila raised on a diet containing the PSKB ketals and in prothoracic glands of Manduca sexta incubated with the ketals emulsified into media. The instability of 3-oxo-7dC and its spontaneous transformation into extensively electron-delocalized intermediates will be discussed in relation to a possible mechanism of the Black Box oxidations eventually leading to the production of the active molting hormone 20-hydroxyecdysone (20E).

Differential effects of cholesterol and 7-dehydrocholesterol on ligand binding of solubilized hippocampal serotonin1A receptors: implications in SLOS

The serotonin1A receptor is an important member of the G-protein coupled receptor family, and is involved in the generation and modulation of a variety of cognitive, behavioral, and developmental functions. Solubilization of the hippocampal serotonin1A receptor by 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS) is accompanied by loss of membrane cholesterol which results in a reduction in specific agonist binding activity. Replenishment of cholesterol to solubilized membranes restores the cholesterol content of the membrane and significantly enhances specific agonist binding activity. In order to test the stringency of the requirement of cholesterol in this process, we solubilized native hippocampal membranes followed by replenishment with 7-dehydrocholesterol (7-DHC). 7-DHC is an immediate biosynthetic precursor of cholesterol differing only in a double bond at the 7th position in its sterol ring. Our results show, for the first time, that replenishment of solubilized hippocampal membranes with 7-DHC does not restore ligand binding activity of the serotonin1A receptor, in spite of recovery of the overall membrane order. This observation shows that the requirement for restoration of ligand binding activity is more stringent than the requirement for the recovery of overall membrane order. These novel results have potential implications in understanding the interaction of membrane sterols with this important neuronal receptor under pathogenic conditions such as the Smith-Lemli-Opitz syndrome.

Role of sterol type on lateral pressure profiles of lipid membranes affecting membrane protein functionality: Comparison between cholesterol, desmosterol, 7-dehydrocholesterol and ketosterol

Lateral pressure profiles have been suggested to play a significant role in many cellular membrane processes by affecting, for example, the activation of membrane proteins through changes in their conformational state. This may be the case if the lateral pressure profile is altered due to changes in molecular composition surrounding the protein. In this work, we elucidate the effect of varying sterol type on the lateral pressure profile, an issue of topical interest due to lipid rafts and their putative role for membrane protein functionality. We find that the lateral pressure profile is altered when cholesterol is replaced by either desmosterol, 7-dehydrocholesterol, or ketosterol. The observed changes in the lateral pressure profile are notable and important since desmosterol and 7-dehydrocholesterol are the immediate precursors of cholesterol along its biosynthetic pathway. The results show that the lateral pressure profile and the resulting elastic behavior of lipid membranes are sensitive to the sterol type, and support a mechanism where changes in protein conformational state are facilitated by changes in the lateral pressure profile. From a structural point of view, the results provide compelling evidence that despite seemingly minor differences, sterols are characterized by structural specificity.

Nystatin-induced lipid vesicles permeabilization is strongly dependent on sterol structure

The selectivity of the antibiotic nystatin towards ergosterol compared to cholesterol is believed to be a crucial factor in its specificity for fungi. In order to define the structural features of sterols that control this effect, nystatin interaction with ergosterol-, cholesterol-, brassicasterol- and 7-dehydrocholesterol-containing palmitoyloleoylphosphocholine vesicles was studied by fluorescence spectroscopy. Variations in sterol structure were correlated with their effect on nystatin photophysical and activity properties. Substitution of cholesterol by either 7-dehydrocholesterol or brassicasterol enhance nystatin ability to dissipate a transmembrane K+ gradient, showing that the presence of additional double bonds in these sterols-carbon C7 and C22, plus an additional methyl group on C-24, respectively-as compared to cholesterol, is fundamental for nystatin-sterol interaction. However, both modifications of the cholesterol molecule, like in the fungal sterol ergosterol, are critical for the formation of very compact nystatin oligomers in the lipid bilayer that present a long mean fluorescence lifetime and induce a very fast transmembrane dissipation. These observations are relevant to the molecular mechanism underlying the high selectivity presented by nystatin towards fungal cells (with ergosterol) as compared to mammalian cells (with cholesterol).

C-24 Stereochemistry of Marine Sterols: (22E)-24-(Isopropenyl)-22-dehydrocholesterol and 24-Isopropenylcholesterol

The C-24 configuration of (22E,24xi)-24-isopropenyl-22-dehydrocholesterol (1), which was recently isolated from the Colombian Caribbean sponge, Topsentia ophiraphidites, was investigated. Synthesis of the stereodefined (24R)- and (24S)-(22E)-24-isopropenyl-22-dehydrocholesterols (1a, 1b) followed by (1)H- and (13)C-NMR data comparison of these sterols established the (24R)-configuration of 1. In addition, (24R)- and (24S)-24-isopropenylcholesterols (2a and 2b) were also synthesized and their NMR data are provided. The C-24 configurations of the samples of 24-isopropenylcholesterol reported previously are discussed.

Two new 19-oxygenated polyhydroxy steroids from the hainan soft coralSinulariasp

Two new 19-oxygenated polyhydroxy steroids, 24-methylcholesta-5, 24(28)-diene-3 beta, 7 beta, 19-triol-19-monoacetate (1), 24-methylcholesta-5, 24(28)-diene-3 beta, 7 beta, 19-triol-7 beta, 19-diacetate (3), together with a known steroid, 24-methylcholesta-5, 24(28)-diene-3 beta, 7 beta, 19-triol-7 beta-monoacetate (4), have been isolated from the soft coral Sinularia sp. collected from the South China Sea and characterized through interpretation of spectral data.

A comparison of the behavior of cholesterol and selected derivatives in mixed sterol-phospholipid Langmuir monolayers: a fluorescence microscopy study

Eukaryotic cells require sterols to achieve normal structure and function of their plasma membranes, and deviations from normal sterol composition can perturb these features and compromise cellular and organism viability. The Smith-Lemli-Opitz syndrome (SLOS) is a hereditary metabolic disease involving cholesterol (CHOL) deficiency and abnormal accumulation of the CHOL precursor, 7-dehydrocholesterol (7DHC). In this study, the interactions of CHOL and the related sterols desmosterol (DES) and 7DHC with l-alpha-dipalmitoylphosphatidylcholine (DPPC) monolayers were compared. Pressure-area isotherms and fluorescence microscopy were used to study DPPC monolayers containing 0, 10, 20, or 30 mol% sterol. Similar behavior was noted for CHOL- and DES-containing DPPC monolayers with both techniques. However, while 7DHC gave isotherms similar to those obtained with the other sterols, microscopy indicated limited domain formation with DPPC, indicating that 7DHC packs somewhat differently in DPPC membranes compared to CHOL and DES. These results are discussed in relation to SLOS pathobiology.

Effect of clothing varieties on solar photosynthesis of previtamin D3: an in vitro study

BACKGROUND/PURPOSE

Vitamin D3 plays important roles in the absorption of calcium and phosphorus from the gastrointestinal tract and in the treatment of rickets; in addition, it facilitates the deposition of minerals in bones, thus minimizing the possibility of developing osteomalacia. Sunlight naturally induces vitamin D3 photosynthesis. Such a process is affected by a number of factors such as age, geographical location, skin color, sunscreen application and clothing. It is intended in the present investigation to study in vitro the effect of clothing on the solar photoproduction of vitamin D3.

METHODS

Fifteen different fabric samples were tested for their effect on the efficiency of the in vitro solar conversion of 7-dehydrocholesterol (7-DHC) to vitamin D3. 7-DHC was dissolved in methanol to give a concentration of 2.6 x 10(-4) M. Solutions were exposed to sunlight in quartz containers for predetermined periods either uncovered or covered with the fabric sample under test. Changes in the concentrations of 7-DHC and the photoproducts were monitored by HPLC. Fabrics were graded as the number of threads per square inch (in(2)), and their sunlight attenuation was determined.

RESULTS

7-DHC is transformed to previtamin D3 upon exposure to sunlight, and the amount generated exhibited an almost linear relationship. When fabric-covered samples of 7-DHC were irradiated, photoproducts were also detected and their concentrations depended on the degree of sunlight attenuation imposed by the fabric. Generally, the higher the number of threads per in(2) the more the light attenuation produced.

CONCLUSION

Clothing plays an important role in attenuating sunlight, thus leading to diminished vitamin D3 production to an extent that would require dietary compensation.

Novel sterols synthesized via the CYP27A1 metabolic pathway

A major biologic role of the ubiquitous mitochondrial P450 enzyme CYP27A1 is the generation of ligands such as 27-hydroxycholesterol and 3 beta-hydroxy-5-cholestenoic acid, which regulate the expression of nuclear receptors that govern many aspects of cholesterol homeostasis. We now report that sterol intermediates in cholesterol synthesis, beginning with the initial post-cyclization sterol, lanosterol, continuing with zymosterol, and ending with desmosterol are also substrates for the enzyme. Using the human enzyme expressed in Escherichia coli, we characterized the retention times and major mass fragments of these novel metabolites. Although sequestration of the enzyme in the inner mitochondrial membrane and normal subcellular organization probably greatly restrict the proportion of these and other intermediates in cholesterol synthesis that undergo side chain oxidation, disruption of compartmentalization can bypass cholesterol as the end product and give rise to potent ligands that further modify gene expression.

Susceptibilities of phospholipid vesicles containing different sterols to amphotericin B-loaded lysophosphatidylcholine micelles

To investigate the susceptibilities of fungal and mammalian cells to amphotericin B (AmB), AmB-loaded lysophosphatidylcholine (LPC)micelles as drug delivery vehicles were incubated at 37 degrees C with phosphatidylcholine vesicles containing different sterols as model systems for fungal and mammalian cells. The binding and kinetics of AmB to sterols in the membranes were judged by UV-visible spectroscopy. In the 91% monomeric form, AmB interacted rapidly with ergosterol and slowly with 7-dehydrocholesterol (7-DHC), while it did not interact with cholesterol. In the 50% monomeric form, AmB formed complexes more rapidly with ergosterol or 7-DHC than in the monomeric form, whereas it did not still interact with cholesterol. The interaction was also characterized by resonance energy transfer between the fluorescent probe trimethylammonium diphenylhexatriene (TMA-DPH) and AmB. In the 91% monomeric form, AmB caused initial fluorescence quenching in bilayer membranes containing any sterol as well as sterol-free bilayer membranes due to the release of AmB and its incorporation within the membranes. However, a second phase of increasing fluorescence was found in the case of ergosterol alone. On the other hand, in the 47% monomeric form, AmB gave a biphasic intensity profile in membranes containing any sterol as well as sterol-free membranes. However, the extent of the second phase of increasing fluorescence intensity was markedly dependent upon sterol composition. Studies using sterol-containing vesicles provide important insights into the role of the aggregation state of AmB in its effects on cells.

Chemical and photolytical transformation of biomedically significant compounds in the presence of deuterated solvents

The effect of the nature of solvent on the properties of biomedically important compounds is of particular importance. The conversion of certain biomedical compounds with deuterated solvents is an area of research that has not been accorded adequate recognition in the literature. We explored this area in the interest of shedding some light on the possible effects of solvent on the nature of the solute. The transformation of specific medically important compounds such as bilirubin, thymine, uracil, dehydrocholesterol (7-DHC) and vitamin D(3) was observed in the presence of deuterated solvents such as heavy water and deuterated chloroform. The products of the relevant reactions were confirmed spectrophotometrically. An additional feature to our investigation involved the photolysis of the aforementioned compounds by solar irradiation. The pure samples were dissolved in solutions of the deuterated solvents, corresponding to concentrations of typically 10(-2) mM, and exposed to sunlight for about 15-30 min. The deuterated solvents caused chemical transformation in all chemical compounds tested, and produced intense characteristic absorbance maxima between 200 and 700 nm. Sunlight exposure was also effective in either augmenting the effects of deuterated solvent as in bilirubin and 7-DHC or reducing it as with thymine or having no effect as with uracil or completely changing it as in vitamin D(3). It has been shown that the use of deuterated solvents produces unique chemical and photochemical conversions of bilirubin, 7-DHC, thymine, uracil and vitamin D(3). This was attributed to the fact that deuterated compounds display a somewhat different chemistry to their ordinary counterparts and that possibly thermodynamic considerations could be responsible for the novel transformations.

Cholesterol induce oligomerization of Vibrio vulnificus cytolysin specifically

Vibrio vulnificus cytolysin (VVC) has been implicated as one of the important virulence determinants of V. vulnificus that causes serious septicemia and wound infection. An attempt was made to investigate that VVC could act as a ligand which stimulates intracellular signaling systems. Cholesterol dose-dependently blocked VVC hemolytic activity through oli-gomerization of cytolysin. Among cholesterol derivatives including 7-dehydrocholesterol, cholesteryl esters, deoxycholate, and cholestane tested, only 7-dehydrocholesterol induced oligomerization as well as inactivation of VVC. These results show that oligomerization of VVC is completely dependent on three-dimensional structure of cholesterol where specific interaction of cholesterol at oligomerization sites of VVC is very selective. These findings support the idea that cholesterol which constitute many of cellular plasma membrane could be a receptor of VVC on plasma membrane of target cells.

A colorimetric assay for 7-dehydrocholesterol with potential application to screening for Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS; MIM 270400) is a genetic disorder characterized by hypocholesterolemia and elevated 7-dehydrocholesterol (7DHC) levels resulting from mutations affecting 7-dehydrocholesterol reductase. We describe a colorimetric assay for 7DHC with potential application to large-scale screening for SLOS. Reaction of 7DHC and its esters with the Liebermann-Burchard reagent resulted in a brief initial absorbance at 510 nm (pink color) followed by an absorbance at 620 nm (blue color) after 2 min, while cholesterol samples were essentially colorless. The assay could identify typical SLOS blood samples by their pink color and increased absorbance at 620 nm after 2 min. Colorimetric identification of mild SLOS cases requires monitoring of the transient absorbance at 510 nm, which must be detected immediately after rapid, consistent mixing of the reagents. The need for special mixing devices and rigorous validation precludes sporadic use of the assay for diagnosing suspected SLOS cases. We also studied the stability of 7DHC in dried SLOS blood spots on Guthrie cards, which are widely used for archiving neonatal blood. Decomposition of 7DHC was effectively retarded by storage at low temperature and by precoating of the cards with antioxidants. The combined results provide a foundation for development of a simple, automated test for SLOS screening.

A rapid screening procedure for cholesterol and dehydrocholesterol by electrospray ionization tandem mass spectrometry

The mono-(dimethylaminoethyl) succinyl (MDMAES) ester is a new derivative for rapid, mild, and sensitive electrospray ionization tandem mass spectrometry (ESI-MS/MS) analysis of cholesterol and dehydrocholesterol. It is an order of magnitude more sensitive than the previous most practical alternative, the N-methylpyridyl ether derivative. The MDMAES derivative was used to develop a rapid screening procedure for the biochemical diagnosis of Smith-Lemli-Opitz syndrome (SLOS) by measuring the dehydrocholesterol/cholesterol ratio in plasma (5 microl) and plasma spotted onto filter paper. Details of the synthesis of [25,26,26,26,27,27,27-(2)H7]-7-dehydrocholesterol, used as a standard for quantitation, are included. The measurement of total sterols as MDMAES esters, after base hydrolysis of plasma, afforded a dehydrocholesterol/cholesterol ratio of 0.05-2.95 for SLOS patient samples (n = 5) compared with 0.001-0.003 for normal adult controls (n = 20). Direct hexane extraction of plasma without base hydrolysis enabled the measurement of free sterols with a total sample analysis time of <1 h. The free dehydrocholesterol/cholesterol ratio was 0.10-4.47 for SLOS patient samples (n = 5) and 0.003-0.011 for normal adult controls (n = 20).

Comparative behavior of sterols in phosphatidylcholine-sterol monolayer films

The ability of sterols other than cholesterol (CHOL) to support membrane functions in membranes that normally contain CHOL as the primary, if not sole, sterol may be due, in part, to how well such sterols can mimic CHOL's behavior and physical properties in membranes. We compared the mixing properties of CHOL, 7-dehydrocholesterol (7DHC), and desmosterol (DES) in egg phosphatidylcholine-sterol monolayer films containing 10, 20, and 30 mol percent sterol, measuring pressure-area isotherms on a Langmuir-Blodgett trough with the aqueous, buffered subphase maintained at 37 degrees C. Under the conditions employed, the pressure-area isotherms for all three sterols were similar, with 7DHC exhibiting slightly larger molecular areas on the water surface at all compositions. These results are discussed in the context of the ability of sterols such as 7DHC and DES to substitute structurally and functionally for CHOL in biological membranes.

Deuterated delta 7-cholestenol analogues as mechanistic probes for wild-type and mutated delta 7-sterol-C5(6)-desaturase

Deuterium-labeled 5alpha-cholest-7-en-3beta-ol (1) bearing one or two deuteriums at the C-5alpha and (or) C-6alpha positions was synthesized in high isotopic and chiral purity. These compounds were used as substrates with the microsomal wild-type Zea mays and recombinant Arabidopsis thaliana Delta(7)-sterol-C5(6)-desaturases (5-DES) to probe directly the stereochemistry and the mechanism of the enzymatic reaction. Clearly, in the conversion of 1 by both 5-DESs, the 6alpha-hydrogen is removed. [6alpha-(2)H]-5alpha-Cholest-7-en-3beta-ol shows an intermolecular deuterium kinetic isotope effect (DKIE) on V and V/K, (D6)V = 2.6+/-0.3, (D6)V/K = 2.4+/-0.1; and (D6)V = 2.3 +/-0.3, (D6)V/K = 2.3+/-0.2 for the Zea mays and A. thaliana wild-type 5-DES, respectively. In contrast, negligible or minor isotope effects, (D5)V = 0.99+/-0.04, (D5)V/K = 0.91+/-0.08; and (D5)V = 0.93 +/-0.06, (D5)V/K = 0.96+/-0.04, respectively, were observed with [5alpha-(2)H]-cholest-7-en-3beta-ol. The observed pattern of isotope effects strongly suggests that the plant 5-DES initiates oxidation by cleavage of the chemically activated C6alpha-H bond, a step which appears to be partially rate-limiting in the desaturation process. Cleavage of the C5-H bond has a negligible isotope effect, indicating that the desaturation involves asynchronous scission of the two C-H bonds at C5 and C6. We showed previously [Taton, M., et al. (2000) Biochemistry 39, 701] that threonine 114 was not essential to maintaining desaturase activity, although V/K values for mutant T114I and T114S were respectively 10-fold lower and 4-fold higher than that of the native 5-DES. In this study, we combined variation in enzyme structure and DKIE studies and showed that (D6)V and (D6)V/K increased respectively to 3.8+/-0.3 and 3.8+/-0.4 in mutant T114I and decreased respectively to 1.6+/-0.4 and 1.7+/- 0.1 in mutant T114S. The data suggest that the conserved hydroxyl function at position 114 in the ERG3 family makes the abstraction of the 6alpha-hydrogen atom substantially less rate-limiting during the 5-DES reaction. Based on the data, a tentative mechanism for the desaturation of cholest-7-en-3beta-ol is proposed.

Modulation of ribonuclease P activity by calcipotriol

The effects of cholesterol, 7-dehydrocholesterol, vitamin D3 and several synthetic vitamin D3 analogs on ribonuclease P (RNase P) were investigated using a cell-free system from the slime mold Dictyostelium discoideum. RNase P is an ubiquitous and essential enzyme that endonucleolytically cleaves all tRNA precursors to produce the mature 5' end. Among the compounds tested, only calcipotriol was capable of affecting RNase P activity, and revealed a bimodal action at the kinetic phase of the reaction. Depending on the concentration of the drug, both activation and inhibition of tRNA maturation were observed, indicating that calcipotriol may have a direct effect on tRNA biogenesis, possibly associated with the presence of a highly reactive small ring on the side chain of its molecule.

Identification of 8-dehydrocholesterol (cholesta-5,8-dien-3 beta-ol) in patients with Smith-Lemli-Opitz syndrome

Cholesta-5,8-dien-3 beta-ol (8-dehydrocholesterol) and cholesta-5,7-dien-3 beta-ol (7-dehydrocholesterol) were isolated from the fecal neutral sterol fraction from homozygotes with Smith-Lemli-Opitz syndrome. The structures of the sterols were conclusively established from their mass spectra and 1H and 13C nuclear magnetic resonance spectra. It is probable that 8-dehydrocholesterol arises from 7-dehydrocholesterol and is not a direct precursor of cholesterol.

Doubly allylic hydroperoxide formed in the reaction between sterol 5,7-dienes and singlet oxygen

Ergosterol and 7-dehydrocholesterol, common 5,7-conjugated diene sterols, react with photochemically produced singlet oxygen very efficiently to yield, in parallel pathways, the corresponding 5,8-endoperoxides and the 7 beta-hydroperoxy-5,8(9),22-trienol or -5,8(9)-dienol, respectively. The hydroperoxides decompose in an acid-catalyzed reaction to generate hydrogen peroxide and the 5,7,9(11),22-tetraenol or 5,7,9(11) trienol, respectively, with 1:1 stochiometry. The molar ratio of endoperoxide to hydroperoxide was constant (16:5) with two different reaction solvents, two different photosensitizers, and at all time points between 5 min and 3 h from the start of irradiation. Ergosterol did not react with either hydrogen peroxide or superoxide ion under our reaction conditions. Inhibition studies with nitrogen, 2,5-dimethylfuran, beta-carotene, and tert-butanol confirmed the involvement of singlet oxygen in these reactions. The unstable hydroperoxide would be expected to have undesirable biological consequences if formed in vivo.

Occurrence of isomeric dehydrocholesterols in human plasma

Three isomeric dehydrocholesterols were found in plasma from healthy subjects and patients with abnormal production or metabolism of cholesterol. These chemically labile steroids were isolated by a mild liquid-solid extraction procedure using octadecylsilane-bonded silica as sorbent. Sterol-protein interactions were minimized by diluting plasma with aqueous isopropanol. The dehydrocholesterols were identified by high-performance liquid chromatography-ultraviolet spectroscopy and gas chromatography-mass spectrometry as cholesta-5,7-dien-3 beta-ol (7-dehydrocholesterol), 5 alpha-cholesta-6,8(9)-dien-3 beta-ol (isodehydrocholesterol), and tentatively as cholesta-5,8(9)-dien-3 beta-ol. There was a strong positive correlation between plasma levels of the two former compounds, isodehydrocholesterol levels usually being about 1.4 times higher than those of 7-dehydrocholesterol. The median concentration of 7-dehydrocholesterol in plasma from healthy subjects was 52 ng/ml. Similar concentrations were found in colectomized patients (median concentration 47 ng/ml) and patients with extrahepatic cholestasis and alcoholic liver cirrhosis (median concentrations 79 and 67 ng/ml, respectively). Patients with ileal resection or under treatment with cholestyramine had elevated levels (median concentrations 142 and 160 ng/ml, respectively) whereas patients with primary biliary cirrhosis had subnormal levels (median concentration 26 ng/ml). The results are consistent with a positive correlation between levels of the dehydrocholesterols in plasma and the rate of cholesterol synthesis. The sterols were also analyzed in human skin and bile and the results indicate that the liver may be an important source of isodehydrocholesterol.

Separation of 24- and 25-dehydrocholesterols; and the impure state of commercial desmosterol preparations

Chromatographic methods for the separation of 24- and 25-dehydrocholesterols are described. The purities of three commercial samples of 24-dehydrocholesterol examined by thin-layer and gas-liquid chromatography were only 42, 79, and 80%, respectively; a commercial sample of radioactive 24-dehydrocholesterol was shown to contain 40% 25-dehydrocholesterol.