Reactivities and products of free radical oxidation of cholestadienols

The Chemical Reactivity of Membrane Lipids

It is well-known that aqueous dispersions of phospholipids spontaneously assemble into bilayer structures. These structures have numerous applications across chemistry and materials science and form the fundamental structural unit of the biological membrane. The particular environment of the lipid bilayer, with a water-poor low dielectric core surrounded by a more polar and better hydrated interfacial region, gives the membrane particular biophysical and physicochemical properties and presents a unique environment for chemical reactions to occur. Many different types of molecule spanning a range of sizes, from dissolved gases through small organics to proteins, are able to interact with membranes and promote chemical changes to lipids that subsequently affect the physicochemical properties of the bilayer. This Review describes the chemical reactivity exhibited by lipids in their membrane form, with an emphasis on conditions where the lipids are well hydrated in the form of bilayers. Key topics include the following: lytic reactions of glyceryl esters, including hydrolysis, aminolysis, and transesterification; oxidation reactions of alkenes in unsaturated fatty acids and sterols, including autoxidation and oxidation by singlet oxygen; reactivity of headgroups, particularly with reactive carbonyl species; and E/Z isomerization of alkenes. The consequences of reactivity for biological activity and biophysical properties are also discussed.

Effects of Neonatal Hypoxic-Ischemic Injury on Brain Sterol Synthesis and Metabolism

BACKGROUND

 Neonatal hypoxic-ischemic brain injury (HIBI) results from disruptions to blood supply and oxygen in the perinatal brain. The goal of this study was to measure brain sterol metabolites and plasma oxysterols after injury in a neonatal HIBI mouse model to assess for potential therapeutic targets in the brain biochemistry as well as potential circulating diagnostic biomarkers.

METHODS

 Postnatal day 9 CD1-IGS mouse pups were randomized to HIBI induced by carotid artery ligation followed by 30 minutes at 8% oxygen or to sham surgery and normoxia. Brain tissue was collected for sterol analysis by liquid chromatography with tandem mass spectrometry (LC-MS/MS). Plasma was collected for oxysterol analysis by LC-MS/MS.

RESULTS

 There were minimal changes in brain sterol concentrations in the first 72 hours after HIBI. In severely injured brains, there was a significant increase in desmosterol, 7-DHC, 8-DHC, and cholesterol 24 hours after injury in the ipsilateral tissue. Lanosterol, 24-dehydrolathosterol, and 14-dehydrozymostenol decreased in plasma 24 hours after injury. Severe neonatal HIBI was associated with increased cholesterol and sterol precursors in the cortex at 24 hours after injury.

CONCLUSIONS

 Differences in plasma oxysterols were seen at 24 hours but were not present at 30 minutes after injury, suggesting that these sterol intermediates would be of little value as early diagnostic biomarkers.

7-Dehydrocholesterol is an endogenous suppressor of ferroptosis

Ferroptosis is a form of cell death that has received considerable attention not only as a means to eradicate defined tumour entities but also because it provides unforeseen insights into the metabolic adaptation that tumours exploit to counteract phospholipid oxidation1,2. Here, we identify proferroptotic activity of 7-dehydrocholesterol reductase (DHCR7) and an unexpected prosurvival function of its substrate, 7-dehydrocholesterol (7-DHC). Although previous studies suggested that high concentrations of 7-DHC are cytotoxic to developing neurons by favouring lipid peroxidation3, we now show that 7-DHC accumulation confers a robust prosurvival function in cancer cells. Because of its far superior reactivity towards peroxyl radicals, 7-DHC effectively shields (phospho)lipids from autoxidation and subsequent fragmentation. We provide validation in neuroblastoma and Burkitt's lymphoma xenografts where we demonstrate that the accumulation of 7-DHC is capable of inducing a shift towards a ferroptosis-resistant state in these tumours ultimately resulting in a more aggressive phenotype. Conclusively, our findings provide compelling evidence of a yet-unrecognized antiferroptotic activity of 7-DHC as a cell-intrinsic mechanism that could be exploited by cancer cells to escape ferroptosis.

The complex photochemistry of coumarin-3-carboxylic acid in acetonitrile and methanol

Irradiation of coumarin-3-carboxylic acid in acetonitrile and methanol solutions at 355 nm results in complex multistep photochemical transformations, strongly dependent on the solvent properties and oxygen content. A number of reaction intermediates, which themselves undergo further (photo)chemical reactions, were identified by steady-state and transient absorption spectroscopy, mass spectrometry, and NMR and product analyses. The triplet excited compound in acetonitrile undergoes decarboxylation to give a 3-coumarinyl radical that traps molecular oxygen to form 3-hydroxycoumarin as the major but chemically reactive intermediate. This compound is oxygenated by singlet oxygen, produced by coumarin-3-carboxylic acid sensitization, followed by a pyrone ring-opening reaction to give an oxalic acid derivative. The subsequent steps lead to the production of salicylaldehyde, carbon monoxide, and carbon dioxide as the final products. When 3-coumarinyl radical is not trapped by oxygen in degassed acetonitrile, it abstracts hydrogen from the solvent and undergoes triplet-sensitized [2 + 2] cycloaddition. The reaction of 3-coumarinyl radical with oxygen is largely suppressed in aerated methanol as a better H-atom donor, and coumarin is obtained as the primary product in good yields. Because coumarin derivatives are used in many photophysical and photochemical applications, this work provides detailed and sometimes surprising insights into their complex phototransformations.

7-Dehydrocholesterol Encapsulated Polymeric Nanoparticles As a Radiation-Responsive Sensitizer for Enhancing Radiation Therapy

Therapeutics that can be activated by radiation in situ to enhance the efficacy of radiotherapy are highly desirable. Herein, 7-Dehydrocholesterol (7-DHC), a biosynthetic precursor of cholesterol, as a radiosensitizer, exploiting its ability to propagate the free radical chain reaction is explored. The studies show that 7-DHC can react with radiation-induced reactive oxygen species and in turn promote lipid peroxidation, double-strand breaks, and mitochondrial damage in cancer cells. For efficient delivery, 7-DHC is encapsulated into poly(lactic-co-glycolic acid) nanoparticles, forming 7-DHC@PLGA NPs. When tested in CT26 tumor bearing mice, 7-DHC@PLGA NPs significantly enhanced the efficacy of radiotherapy, causing complete tumor eradication in 30% of the treated animals. After treatment, 7-DHC is converted to cholesterol, causing no detectable side effects or hypercalcemia. 7-DHC@PLGA NPs represent a radiation-responsive sensitizer with great potential in clinical translation.

Prescription Medications Alter Neuronal and Glial Cholesterol Synthesis

Mouse brain contains over 100 million neuronal, glial, and other support cells. Developing neurons and astrocytes synthesize their own cholesterol, and disruption of this process can occur by both genetic and chemical mechanisms. In this study we have exposed cultured murine neurons and astrocytes to six different prescription medications that cross the placenta and blood-brain barriers and analyzed the effects of these drugs on cholesterol biosynthesis by an LC-MS/MS protocol that assays 14 sterols and 7 oxysterols in a single run. Three antipsychotics (haloperidol, cariprazine, aripiprazole), two antidepressants (trazodone and sertraline), and an antiarhythmic (amiodarone) inhibited one or more sterol synthesis enzymes. The result of the exposures was a dose-dependent increase in levels of various sterol intermediates and a decreased level of cholesterol in the cultured cells. Four prescription medications (haloperidol, aripiprazole, cariprazine, and trazodone) acted primarily on the DHCR7 enzyme. The result of this exposure was an increase in 7-dehydrocholesterol in neurons and astrocytes to levels that were comparable to those found in cultured neurons and astrocytes from transgenic mice that carried a Dhcr7 pathogenic mutation modeling the neurodevelopmental disorder Smith-Lemli-Opitz syndrome.

Development and Application of a Peroxyl Radical Clock Approach for Measuring Both Hydrogen-Atom Transfer and Peroxyl Radical Addition Rate Constants

The rate-determining step in free radical lipid peroxidation is the propagation of the peroxyl radical, where generally two types of reactions occur: (a) hydrogen-atom transfer (HAT) from a donor to the peroxyl radical; (b) peroxyl radical addition (PRA) to a "C═C" double bond. Peroxyl radical clocks have been used to determine the rate constants of HAT reactions (k_H), but no radical clock is available to measure the rate constants of PRA reactions (k_add). In this work, we modified the analytical approach on the linoleate-based peroxyl radical clock to enable the simultaneous measurement of both k_H and k_add. Compared to the original approach, this new approach involves the use of a strong reducing agent, LiAlH₄, to completely reduce both HAT and PRA-derived products and the relative quantitation of total linoleate oxidation products with or without reduction. The new approach was then applied to measuring the k_H and k_add values for several series of organic substrates, including para- and meta-substituted styrenes, substituted conjugated dienes, and cyclic alkenes. Furthermore, the k_H and k_add values for a variety of biologically important lipids were determined for the first time, including conjugated fatty acids, sterols, coenzyme Q10, and lipophilic vitamins, such as vitamins D₃ and A.

Reactive Sterol Electrophiles: Mechanisms of Formation and Reactions with Proteins and Amino Acid Nucleophiles

Radical-mediated lipid oxidation and the formation of lipid hydroperoxides has been a focal point in the investigation of a number of human pathologies. Lipid peroxidation has long been linked to the inflammatory response and more recently, has been identified as the central tenet of the oxidative cell death mechanism known as ferroptosis. The formation of lipid electrophile-protein adducts has been associated with many of the disorders that involve perturbations of the cellular redox status, but the identities of adducted proteins and the effects of adduction on protein function are mostly unknown. Both cholesterol and 7-dehydrocholesterol (7-DHC), which is the immediate biosynthetic precursor to cholesterol, are oxidizable by species such as ozone and oxygen-centered free radicals. Product mixtures from radical chain processes are particularly complex, with recent studies having expanded the sets of electrophilic compounds formed. Here, we describe recent developments related to the formation of sterol-derived electrophiles and the adduction of these electrophiles to proteins. A framework for understanding sterol peroxidation mechanisms, which has significantly advanced in recent years, as well as the methods for the study of sterol electrophile-protein adduction, are presented in this review.

Recent development on liquid chromatography-mass spectrometry analysis of oxidized lipids

Polyunsaturated fatty acids (PUFAs) in the cellular membrane can be oxidized by various enzymes or reactive oxygen species (ROS) to form many oxidized lipids. These metabolites are highly bioactive, participating in a variety of physiological and pathophysiological processes. Mass spectrometry (MS), coupled with Liquid Chromatography, has been increasingly recognized as an indispensable tool for the analysis of oxidized lipids due to its excellent sensitivity and selectivity. We will give an update on the understanding of the molecular mechanisms related to generation of various oxidized lipids and recent progress on the development of LC-MS in the detection of these bioactive lipids derived from fatty acids, cholesterol esters, and phospholipids. The purpose of this review is to provide an overview of the formation mechanisms and technological advances in LC-MS for the study of oxidized lipids in human diseases, and to shed new light on the potential of using oxidized lipids as biomarkers and mechanistic clues of pathogenesis related to lipid metabolism. The key technical problems associated with analysis of oxidized lipids and challenges in the field will also discussed.

Calibration of Squalene, p-Cymene, and Sunflower Oil as Standard Oxidizable Substrates for Quantitative Antioxidant Testing

The autoxidation kinetics of stripped sunflower oil (SSO), squalene (SQ), and p-cymene ( p-C) initiated by 2,2'-azobis(isobutyronitrile) at 303 K were investigated under controlled conditions by differential oximetry in order to build reference model systems that are representative of the natural variability of oxidizable materials, for quantitative antioxidant testing. Rate constants for oxidative chain propagation ( k_p) and chain termination (2 k_t) and the oxidizability ( k_p/√2 k_t) were measured using 2,6-di- tert-butyl-4-methoxyphenol, 2,2,5,7,8-pentamethyl-6-chromanol, BHT, and 4-methoxyphenol as reference antioxidants. Measured values of k_p (M^-1 s^-1)/2 k_t (M^-1 s^-1)/oxidizability (M^-1/2 s^-1/2) at 303 K in chlorobenzene were 66.9/3.45 × 10⁶/3.6 × 10^-2, 68.0/7.40 × 10⁶/2.5 × 10^-2, and 0.83/2.87 × 10⁶/4.9 × 10^-4, respectively, for SSO, SQ, and p-C. Quercetin, magnolol, caffeic acid phenethyl ester, and 2,4,6-trimethylphenol were investigated to validate calibrations. The distinctive usefulness of the three substrates in testing antioxidants is discussed.

Graphene Oxide Quantum Dots Reduce Oxidative Stress and Inhibit Neurotoxicity In Vitro and In Vivo through Catalase-Like Activity and Metabolic Regulation

Both oxidative stress and neurotoxicity are huge challenges to human health, and effective methods and agents for resisting these adverse effects are limited, especially in vivo. It is shown here that, compared to large graphene oxide (GO) nanosheets, GO quantum dots (GOQDs), as nanozymes, efficiently reduce reactive oxygen species (ROS) and H2O2 in 1-methyl-4-phenyl-pyridinium ion (MPP+)-induced PC12 cells. In addition, GOQDs exert neuroprotective effects in a neuronal cell model by decreasing apoptosis and α-synuclein. GOQDs also efficiently diminish ROS, apoptosis, and mitochondrial damage in zebrafish treated with MPP+. Furthermore, GOQDs-pretreated zebrafish shows increased locomotive activity and Nissl bodies in the brain, confirming that GOQDs ameliorate MPP+-induced neurotoxicity, in contrast to GO nanosheets. GOQDs contribute to neurotoxic amelioration by increasing amino acid metabolism, decreasing tricarboxylic acid cycle activity, and reducing steroid biosynthesis, fatty acid biosynthesis, and galactose metabolic pathway activity, which are related to antioxidation and neurotransmission. Meanwhile, H2O2 decomposition and Fenton reactions suggest the catalase-like activity of GOQDs. GOQDs can translocate into zebrafish brains and exert catalase-mimicking activity to resist oxidation in the intracellular environment. Unlike general nanomaterials, biocompatible GOQDs demonstrate their high potential for human health by reducing oxidative stress and inhibiting neurotoxicity.

Antioxidant and Cytoprotective Effects of Kukoamines A and B: Comparison and Positional Isomeric Effect

In this study, two natural phenolic polyamines, kukoamine A and B, were comparatively investigated for their antioxidant and cytoprotective effects in Fenton-damaged bone marrow-derived mesenchymal stem cells (bmMSCs). When compared with kukoamine B, kukoamine A consistently demonstrated higher IC50 values in PTIO•-scavenging (pH 7.4), Cu2+-reducing, DPPH•-scavenging, •O₂−-scavenging, and •OH-scavenging assays. However, in the PTIO•-scavenging assay, the IC50 values of each kukoamine varied with pH value. In the Fe2+-chelating assay, kukoamine B presented greater UV-Vis absorption and darker color than kukoamine A. In the HPLC⁻ESI⁻MS/MS analysis, kukoamine A with DPPH• produced radical-adduct-formation (RAF) peaks (m/z 922 and 713). The 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl (MTT) assay suggested that both kukoamines concentration-dependently increased the viabilities of Fenton-damaged bmMSCs at 56.5⁻188.4 μM. However, kukoamine A showed lower viability percentages than kukoamine B. In conclusion, the two isomers kukoamine A and B can protect bmMSCs from Fenton-induced damage, possibly through direct or indirect antioxidant pathways, including electron-transfer, proton-transfer, hydrogen atom transfer, RAF, and Fe2+-chelating. Since kukoamine B possesses higher potentials than kukoamine A in these pathways, kukoamine B is thus superior to kukoamine A in terms of cytoprotection. These differences can ultimately be attributed to positional isomeric effects.

Antifungal drug testing by combining minimal inhibitory concentration testing with target identification by gas chromatography-mass spectrometry

Fungal infections and their increasing resistance to antibiotics are an emerging threat to public health. Novel antifungal drugs, as well technologies that can help us bolster the antimicrobial pipeline and understand resistance mechanisms, are needed. The ergosterol biosynthetic pathway is one potential target for antifungal drugs. Here we describe how antifungal susceptibility testing can be combined with target identification in distal ergosterol biosynthesis by means of gas chromatography-mass spectrometry. The fungi are treated with sublethal doses of active components that block ergosterol biosynthesis, and the ergosterol biosynthesis intermediates are analyzed in a targeted metabolomics manner after derivatization (trimethylsilylation). Drug treatment results in distinct sterol patterns that are characteristic of the affected enzyme. Sterol identification based on relative retention times and electron ionization (EI) mass spectra, as well as semiquantitative assessment of ergosterol intermediates, is described. The protocol is applicable to yeasts and molds. The overall analysis time from incubation to test result is not more than 3 d. The assay can be used to determine whether an antifungal compound of interest targets sterol biosynthesis, and, if so, to determine which enzyme in the pathway it targets.

Lipid Peroxidation: Kinetics, Mechanisms, and Products

Recent advances in our understanding of lipid peroxidation, a degenerative process that is believed to play a key role in the pathogenesis of many diseases, are highlighted. In particular, the factors that control the kinetics and regio-/stereochemical outcomes of the autoxidation of both polyunsaturated fatty acids and sterols and the subsequent decomposition of the hydroperoxide products to cytotoxic derivatives are discussed. These advances promise to help clarify the role of lipid peroxidation in cell death and human disease.

Probes for protein adduction in cholesterol biosynthesis disorders: Alkynyl lanosterol as a viable sterol precursor

The formation of lipid electrophile-protein adducts is associated with many disorders that involve perturbations of cellular redox status. The identities of adducted proteins and the effects of adduction on protein function are mostly unknown and an increased understanding of these factors may help to define the pathogenesis of various human disorders involving oxidative stress. 7-Dehydrocholesterol (7-DHC), the immediate biosynthetic precursor to cholesterol, is highly oxidizable and gives electrophilic oxysterols that adduct proteins readily, a sequence of events proposed to occur in Smith-Lemli-Opitz syndrome (SLOS), a human disorder resulting from an error in cholesterol biosynthesis. Alkynyl lanosterol (a-Lan) was synthesized and studied in Neuro2a cells, Dhcr7-deficient Neuro2a cells and human fibroblasts. When incubated in control Neuro2a cells and control human fibroblasts, a-Lan completed the sequence of steps involved in cholesterol biosynthesis and alkynyl-cholesterol (a-Chol) was the major product formed. In Dhcr7-deficient Neuro2a cells or fibroblasts from SLOS patients, the biosynthetic transformation was interrupted at the penultimate step and alkynyl-7-DHC (a-7-DHC) was the major product formed. When a-Lan was incubated in Dhcr7-deficient Neuro2a cells and the alkynyl tag was used to ligate a biotin group to alkyne-containing products, protein-sterol adducts were isolated and identified. In parallel experiments with a-Lan and a-7-DHC in Dhcr7-deficient Neuro2a cells, a-7-DHC was found to adduct to a larger set of proteins (799) than a-Lan (457) with most of the a-Lan protein adducts (423) being common to the larger a-7-DHC set. Of the 423 proteins found common to both experiments, those formed from a-7-DHC were more highly enriched compared to a DMSO control than were those derived from a-Lan. The 423 common proteins were ranked according to the enrichment determined for each protein in the a-Lan and a-7-DHC experiments and there was a very strong correlation of protein ranks for the adducts formed in the parallel experiments.

Propagation rate constants for the peroxidation of sterols on the biosynthetic pathway to cholesterol

The free radical chain autoxidation of cholesterol and the oxidation products formed, i.e. oxysterols, have been the focus of intensive study for decades. The peroxidation of sterol precursors to cholesterol such as 7-dehydrocholesterol (7-DHC) and desmosterol as well as their oxysterols has received less attention. The peroxidation of these sterol precursors can become important under circumstances in which genetic conditions or exposures to small molecules leads to an increase of these biosynthetic intermediates in tissues and fluids. 7-DHC, for example, has a propagation rate constant for peroxidation some 200 times that of cholesterol and this sterol is found at elevated levels in a devastating human genetic condition, Smith-Lemli-Opitz syndrome (SLOS). The propagation rate constants for peroxidation of sterol intermediates on the biosynthetic pathway to cholesterol were determined by a competition kinetic method, i.e. a peroxyl radical clock. In this work, propagation rate constants for lathosterol, zymostenol, desmosterol, 7-dehydrodesmosterol and other sterols in the Bloch and Kandutsch-Russell pathways are assigned and these rate constants are related to sterol structural features. Furthermore, potential oxysterols products are proposed for sterols whose oxysterol products have not been determined.

Free radical oxidation of cholesterol and its precursors: Implications in cholesterol biosynthesis disorders

Free radical oxidation of cholesterol and its precursors contribute significantly to the pathophysiology of a number of human diseases. This review intends to summarize recent developments and provide a perspective on the reactivities of sterols toward free radical oxidation, the free radical reaction mechanism, and the biological consequences of oxysterols derived from the highly oxidizable cholesterol precursor, 7-dehydrocholesterol. We propose that the rigid structures, additional substituents on the double bonds, and the well-aligned reactive C-H bonds in sterols make them more prone to free radical oxidation than their acyclic analogs found in unsaturated fatty acids. The mechanism of sterol peroxidation follows some well-established reaction pathways found in the free radical peroxidation of polyunsaturated fatty acids, but sterols also undergo some reactions that are unique to these compounds. Peroxidation of 7-dehydrocholesterol gives arguably the most diverse set of oxysterol products that have been observed to date. The metabolism of these oxysterols in cells and the biological consequences of their formation will be discussed in the context of the pathophysiology of the human disease Smith-Lemli-Opitz syndrome. Considering the high reactivity of sterols, we propose that a number of other cholesterol biosynthesis disorders may be associated with oxidative stress.

Tunneling in tocopherol-mediated peroxidation of 7-dehydrocholesterol

The peroxidation of 7-dehydrocholesterol (7-DHC), a biosynthetic precursor to vitamin D3 and cholesterol, has been linked to the pathophysiology of Smith-Lemli-Optiz syndrome (SLOS), a devastating human disorder. In SLOS, 7-DHC plasma and tissue levels are elevated because of defects in the enzyme that convert it to cholesterol. α-Tocopherol can mediate the peroxidation of 7-DHC under certain circumstances and this prompted us to investigate the kinetic isotope effect (KIE) during this process. Thus, 9,14-d2-7-DHC was synthesized using a photochemical cyclization of deuterium-reinforced previtamin D3 (retro to its biosynthesis). Subsequently, we carried out co-oxidation of 9,14-h2-25,26,26,26,27,27,27-d7- and 9,14-d2-7-DHC in the presence of α-tocopherol under conditions that favor TMP. By monitoring the products formed from each precursor using mass spectrometry, the KIE for the hydrogen (deuterium) atom removal at C9 was found to be 21 ± 1. This large KIE value indicates that tunneling plays a role in the hydrogen atom transfer step in the tocopherol-mediated peroxidation of 7-DHC.

Oxidation of 7-dehydrocholesterol and desmosterol by human cytochrome P450 46A1

Cytochrome P450 (P450 or CYP) 46A1 is expressed in brain and has been characterized by its ability to oxidize cholesterol to 24S-hydroxycholesterol. In addition, the same enzyme is known to further oxidize 24S-hydroxycholesterol to the 24,25- and 24,27-dihydroxy products, as well as to catalyze side-chain oxidations of 7α-hydroxycholesterol and cholestanol. As precursors in the biosynthesis of cholesterol, 7-dehydrocholesterol has not been found to be a substrate of P450 46A1 and desmosterol has not been previously tested. However, 24-hydroxy-7-dehydrocholesterol was recently identified in brain tissues, which prompted us to reexamine this enzyme and its potential substrates. Here we report that P450 46A1 oxidizes 7-dehydrocholesterol to 24-hydroxy-7-dehydrocholesterol and 25-hydroxy-7-dehydrocholesterol, as confirmed by LC-MS and GC-MS. Overall, the catalytic rates of formation increased in the order of 24-hydroxy-7-dehydrocholesterol < 24-hydroxycholesterol < 25-hydroxy-7-dehydrocholesterol from their respective precursors, with a ratio of 1:2.5:5. In the case of desmosterol, epoxidation to 24S,25-epoxycholesterol and 27-hydroxylation was observed, at roughly equal rates. The formation of these oxysterols in the brain may be of relevance in Smith-Lemli-Opitz syndrome, desmosterolosis, and other relevant diseases, as well as in signal transduction by lipids.