The ratio of cholesterol 5,6-secosterols formed from ozone and singlet oxygen offers insight into the oxidation of cholesterol in vivo

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.

Lysine Reacts with Cholesterol Hydroperoxide to Form Secosterol Aldehydes and Lysine-Secosterol Aldehyde Adducts

Two cholesterol secosterol aldehydes, namely, 3β-hydroxy-5-oxo-5,6-secocholestan-6-al (secosterol A) and its aldolization product 3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxyaldehyde (secosterol B), are highly bioactive compounds which have been detected in human tissues and potentially contribute to the development of physiological dysfunctions such as atherosclerosis, Alzheimer’s disease, diabetes, and cancer. They were originally considered to be exclusive products of cholesterol ozonolysis and thus to be evidence for endogenous ozone formation. However, it was recently postulated that primary amines such as lysine may catalyse their formation from cholesterol-5α-hydroperoxide (Ch-5α-OOH), the main product of the oxidation of cholesterol with singlet oxygen. This involves cyclization of Ch-5α-OOH to an unstable dioxetane intermediate, which decomposes to form secosterol aldehydes with triplet carbonyl groups, whose return to the singlet state is at least partly coupled to the conversion of triplet molecular oxygen to singlet oxygen. Here, we subjected cholesterol to photosensitized oxidation, which predominantly produces Ch-5α-OOH and minor amounts of the 6α- and 6β-hydroperoxides, exposed the hydroperoxide mixture to lysine in the presence of the antioxidant 2,6-ditertiary-butyl-4-hydroxytoluene (BHT), and analysed the reaction mixture by liquid chromatography-electrospray ionization-mass spectrometry. Consistent with the postulated lysine-catalysed formation of secosterol aldehydes, we detected formation of the latter and several types of their lysine adducts, including carbinolamines, Schiff’s bases, and amide-type adducts. We propose that the amide type adducts, which are major biomarkers of lipid oxidation, are mainly formed by singlet oxygen-mediated oxidation of the carbinolamine adducts.

Cholesterol secosterol aldehyde adduction and aggregation of Cu,Zn-superoxide dismutase: Potential implications in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by degeneration of upper and lower motor neurons. While the fundamental causes of the disease are still unclear, the accumulation of Cu,Zn-superoxide dismutase (SOD1) immunoreactive aggregates is associated with familial ALS cases. Cholesterol 5,6-secosterol aldehydes (Seco A and Seco B) are reported to contribute to neurodegenerative disease pathology by inducing protein modification and aggregation. Here we have investigated the presence of secosterol aldehydes in ALS SOD1-G93A rats and their capacity to induce SOD1 aggregation. Secosterol aldehydes were analyzed in blood plasma, spinal cord and motor cortex of ALS rats at the pre-symptomatic and symptomatic stages. Seco B was significantly increased in plasma of symptomatic ALS rats compared to pre-symptomatic animals, suggesting an association with disease progression. In vitro experiments showed that both Seco A and Seco B induce the formation of high molecular weight (HMW) SOD1 aggregates with amorphous morphology. SOD1 adduction to ω-alkynyl-secosterols analyzed by click assay showed that modified proteins are only detected in the HMW region, indicating that secosterol adduction generates species highly prone to aggregate. Of note, SOD1-secosterol adducts containing up to five secosterol molecules were confirmed by MALDI-TOF analysis. Interestingly, mass spectrometry sequencing of SOD1 aggregates revealed preferential secosterol adduction to Lys residues located at the electrostatic loop (Lys 122, 128 and 136) and nearby the dimer interface (Lys 3 and 9). Altogether, our results show that secosterol aldehydes are increased in plasma of symptomatic ALS rats and represent a class of aldehydes that can potentially modify SOD1 enhancing its propensity to aggregate.

Biochemical properties of cholesterol aldehyde secosterol and its derivatives

Elevated levels of cholesterol aldehyde, 3β-hydroxy-5-oxo-5,6-secocholestan-6-al (secosterol-A, also called 5,6-secosterol), and its aldolization product (secosterol-B) have been detected in human atherosclerotic plaques and tissues samples of brains affected by neurodegeneration, such as Alzheimer’s disease and Lewy body dementia suggesting that increased formation of these compounds may be associated with inflammation-related diseases. Secosterol-A and secosterol-B, and also further oxidized products seco-A-COOH and seco-B-COOH induce several pro-inflammatory activities in vitro. Accumulating evidences demonstrate that the covalent bindings of these secosterols to target proteins seem to be critical to trigger their pro-inflammatory activities. One of the molecular mechanisms of protein adduct formations is that aldehydic function of secosterol-A and secosterol-B is reactive and form Schiff bases with ε- or N-terminal amino groups of proteins. In other cases, it is recently suggested that Michael acceptor moiety formed by the dehydration of not only secosterol-A and secosterol-B but also seco-A-COOH may react with nucleophilic site on target proteins. In this review, I summarize and provide an overview of formation mechanism of secosterols in in vitro and in vivo, patho- or physiological concentrations in biological and clinical samples, and molecular mechanisms of pro-inflammatory activities of secosterols.

The Ozonolysis of Terpenoids, a Pandora's Box of by-Products

The ozonolysis of terpenoids is described and the origins of some by-products are discussed.

The Contribution of Singlet Oxygen to Insulin Resistance

Insulin resistance contributes to the development of diabetes and cardiovascular dysfunctions. Recent studies showed that elevated singlet oxygen-mediated lipid peroxidation precedes and predicts diet-induced insulin resistance (IR), and neutrophils were suggested to be responsible for such singlet oxygen production. This review highlights literature suggesting that insulin-responsive cells such as endothelial cells, hepatocytes, adipocytes, and myocytes also produce singlet oxygen, which contributes to insulin resistance, for example, by generating bioactive aldehydes, inducing endoplasmic reticulum (ER) stress, and modifying mitochondrial DNA. In these cells, nutrient overload leads to the activation of Toll-like receptor 4 and other receptors, leading to the production of both peroxynitrite and hydrogen peroxide, which react to produce singlet oxygen. Cytochrome P450 2E1 and cytochrome c also contribute to singlet oxygen formation in the ER and mitochondria, respectively. Endothelial cell-derived singlet oxygen is suggested to mediate the formation of oxidized low-density lipoprotein which perpetuates IR, partly through neutrophil recruitment to adipose tissue. New singlet oxygen-involving pathways for the formation of IR-inducing bioactive aldehydes such as 4-hydroperoxy-(or hydroxy or oxo)-2-nonenal, malondialdehyde, and cholesterol secosterol A are proposed. Strategies against IR should target the singlet oxygen-producing pathways, singlet oxygen quenching, and singlet oxygen-induced cellular responses.

Ozone-derived Oxysterols Affect Liver X Receptor (LXR) Signaling: A POTENTIAL ROLE FOR LIPID-PROTEIN ADDUCTS

When inhaled, ozone (O₃) interacts with cholesterols of airway epithelial cell membranes or the lung-lining fluid, generating chemically reactive oxysterols. The mechanism by which O₃-derived oxysterols affect molecular function is unknown. Our data show that in vitro exposure of human bronchial epithelial cells to O₃ results in the formation of oxysterols, epoxycholesterol-α and -β and secosterol A and B (Seco A and Seco B), in cell lysates and apical washes. Similarly, bronchoalveolar lavage fluid obtained from human volunteers exposed to O₃ contained elevated levels of these oxysterol species. As expected, O₃-derived oxysterols have a pro-inflammatory effect and increase NF-κB activity. Interestingly, expression of the cholesterol efflux pump ATP-binding cassette transporter 1 (ABCA1), which is regulated by activation of the liver X receptor (LXR), was suppressed in epithelial cells exposed to O₃ Additionally, exposure of LXR knock-out mice to O₃ enhanced pro-inflammatory cytokine production in the lung, suggesting LXR inhibits O₃-induced inflammation. Using alkynyl surrogates of O₃-derived oxysterols, our data demonstrate adduction of LXR with Seco A. Similarly, supplementation of epithelial cells with alkynyl-tagged cholesterol followed by O₃ exposure causes observable lipid-LXR adduct formation. Experiments using Seco A and the LXR agonist T0901317 (T09) showed reduced expression of ABCA1 as compared with stimulation with T0901317 alone, indicating that Seco A-LXR protein adduct formation inhibits LXR activation by traditional agonists. Overall, these data demonstrate that O₃-derived oxysterols have pro-inflammatory functions and form lipid-protein adducts with LXR, thus leading to suppressed cholesterol regulatory gene expression and providing a biochemical mechanism mediating O₃-derived formation of oxidized lipids in the airways and subsequent adverse health effects.

Current Knowledge about Oxysterols: A Review

For years food consumers have been warned that a cholesterol-rich diet may result in atherosclerosis. It is also well known that consumption of large amounts of phytosterols decreases concentration of low-density lipoproteins (LDLs) in blood (LDLs are regarded a key risk factor in development of cardiovascular diseases). However, no scientific evidence has unambiguously proved any direct connection between amount of consumed cholesterol and LDL level in blood. On the other hand, concentration of cholesterol oxidation products, oxysterols, seems to be indeed relevant; for example, they significantly impact appearance of atherosclerotic lesions (plaques). Phytosterols (like sitosterol or campasterol) decrease LDL level in blood, but on the other hand products of their oxidation are toxic. Therefore, it is worth to know influence of phytosterols on living organisms, processes which lead to their formation, and their levels in popular foodstuffs. This paper is an attempt to review literature data on the above aspects, as well as on impact on living organisms of oxidation products of popular sterols.

Rearrangements of organic peroxides and related processes

This review is the first to collate and summarize main data on named and unnamed rearrangement reactions of peroxides. It should be noted, that in the chemistry of peroxides two types of processes are considered under the term rearrangements. These are conventional rearrangements occurring with the retention of the molecular weight and transformations of one of the peroxide moieties after O-O-bond cleavage. Detailed information about the Baeyer-Villiger, Criegee, Hock, Kornblum-DeLaMare, Dakin, Elbs, Schenck, Smith, Wieland, and Story reactions is given. Unnamed rearrangements of organic peroxides and related processes are also analyzed. The rearrangements and related processes of important natural and synthetic peroxides are discussed separately.

Endogenous Generation of Singlet Oxygen and Ozone in Human and Animal Tissues: Mechanisms, Biological Significance, and Influence of Dietary Components

Recent studies have shown that exposing antibodies or amino acids to singlet oxygen results in the formation of ozone (or an ozone-like oxidant) and hydrogen peroxide and that human neutrophils produce both singlet oxygen and ozone during bacterial killing. There is also mounting evidence that endogenous singlet oxygen production may be a common occurrence in cells through various mechanisms. Thus, the ozone-producing combination of singlet oxygen and amino acids might be a common cellular occurrence. This paper reviews the potential pathways of formation of singlet oxygen and ozone in vivo and also proposes some new pathways for singlet oxygen formation. Physiological consequences of the endogenous formation of these oxidants in human tissues are discussed, as well as examples of how dietary factors may promote or inhibit their generation and activity.

Chemical alterations and regulations of biomolecules in lifestyle-related diseases

We know experientially that not only nutrient factors but also non-nutritive functional food factors are playing important roles in maintenance of homeostasis, health promotion, and disease prevention. Although some of these effective behaviors are supported by accumulating scientific evidences, it is in general difficult to determine properly in human. Therefore, the discovering of novel biomarker and developments of the analytical method are one of the prudent strategies to understand disease etiology and evaluate efficacies of functional food factors via monitoring the pathophysiological alteration in live body, tissue, and cells. This review describes recent our findings on (1) formation mechanism, bioactivities, quantitative determination of cholesterol ozonolysis product, secosterol as possible biomarker for lifestyle-related disease, and (2) chemical biology approach for the investigating molecular mechanisms of most promising cancer chemopreventive food factors, isothiocyanate-inducing bioactivities.

Alternatives to the 'water oxidation pathway' of biological ozone formation

Recent studies have shown that ozone (O3) is endogenously generated in living tissues, where it makes both positive and negative physiological contributions. A pathway for the formation of both O3 and hydrogen peroxide (H2O2) was previously proposed, beginning with the antibody or amino acid-catalyzed oxidation of water by singlet oxygen ((1)O2) to form hydrogen trioxide (H2O3) as a key intermediate. A key pillar of this hypothesis is that some of the H2O2 molecules incorporate water-derived oxygen atoms. However, H2O3 decomposes extremely readily in water to form (1)O2 and water, rather than O3 and H2O2. This article highlights key literature indicating that the oxidation of organic molecules such as the amino acids methionine, tryptophan, histidine, and cysteine by (1)O2 is involved in ozone formation. Based on this, an alternative hypothesis for ozone formation is developed involving a further reaction of singlet oxygen with various oxidized organic intermediates. H2O2 having water-derived oxygen atoms is subsequently formed during ozone decomposition in water by known reactions.

Evidence for prehistoric origins of Egyptian mummification in late Neolithic burials

Traditional theories on ancient Egyptian mummification postulate that in the prehistoric period (i.e. the Neolithic and Chalcolithic periods, 5th and 4th millennia B.C.) bodies were naturally desiccated through the action of the hot, dry desert sand. Although molding of the body with resin-impregnated linen is believed to be an early Pharaonic forerunner to more complex processes, scientific evidence for the early use of resins in artificial mummification has until now been limited to isolated occurrences during the late Old Kingdom (c. 2200 B.C.), their use becoming more apparent during the Middle Kingdom (c. 2000-1600 BC). We examined linen wrappings from bodies in securely provenanced tombs (pit graves) in the earliest recorded ancient Egyptian cemeteries at Mostagedda in the Badari region (Upper Egypt). Our investigations of these prehistoric funerary wrappings using a combination of gas chromatography-mass spectrometry (GC-MS) and thermal desorption/pyrolysis (TD/Py)-GC-MS have identified a pine resin, an aromatic plant extract, a plant gum/sugar, a natural petroleum source, and a plant oil/animal fat in directly AMS-dated funerary wrappings. Predating the earliest scientific evidence by more than a millennium, these embalming agents constitute complex, processed recipes of the same natural products, in similar proportions, as those utilized at the zenith of Pharaonic mummification some 3,000 years later. The antibacterial properties of some of these ingredients and the localized soft-tissue preservation that they would have afforded lead us to conclude that these represent the very beginnings of experimentation that would evolve into the famous mummification practice of the Pharaonic period.

In vitro oxidation of LDL by ozone

Recent studies suggest that ozone is present in atherosclerotic lesions. Since these lesions are characterized by a dramatic accumulation of low-density lipoprotein (LDL), we aimed to investigate whether ozone is capable of oxidizing LDL, thereby rendering this lipoprotein atherogenic. Lipid hydroperoxide (LPO) concentrations and thiobarbituric acid reactive substances (TBARS) were measured to assess the oxidative status of the lipid part of LDL. Relative electrophoretic mobility (REM) and oxidation-specific immune epitopes were measured to assess the oxidative status of the protein part (apoB) of the LDL particle. Ozone turned out to be a potent oxidant of LDL. LPO concentrations, TBARS, REM, and oxidation-specific immune epitopes significantly increased upon ozonization. Our results suggest that ozonization of LDL may be a novel pathway which supports atherogenesis. Ozone is capable of oxidizing the lipid part of LDL, followed by immediate oxidation of the protein part of LDL, rendering the lipoprotein atherogenic.

Recent advances in cholesterol chemistry

This review article presents advances in cholesterol chemistry since 2000. Various transformations (chemical, enzymatic, electrochemical, etc.) of cholesterol are presented. A special emphasis is given to cholesterol oxidation reactions, but also substitution of the 3β-hydroxyl group, addition to the C5-C6 double bond, C-H functionalization, and C-C bond forming reactions are discussed.

Covalent binding and anchoring of cytochrome c to mitochondrial mimetic membranes promoted by cholesterol carboxyaldehyde

Mitochondrial cholesterol has been reported to be increased under specific pathological conditions associated with enhanced oxidative stress parameters. In this scenario, cholesterol oxidation would be increased, leading to the production of reactive aldehydes, including cholesterol carboxyaldehyde (ChAld). By using SDS micelles as a mitochondrial mimetic model, we have demonstrated that ChAld covalently modifies cytochrome c (cytc), a protein known to participate in electron transport and apoptosis signaling. This mimetic model induces changes in cytc structure in the same way as mitochondrial membranes do. Tryptic digestion of the cytc-ChAld adduct followed by MALDI-TOF/TOF analyses revealed that modifications occur at Lys residues (K22) localized at cytc site L, a site involved in protein-protein and protein-membrane interactions. Interestingly, ChAld ligation prevented cytc detachment from liposomes even under high ionic strength conditions. Overall, it can be concluded that ChAld ligation to Lys residues at site L creates a hydrophobic tail at cytc, which promotes cytc anchoring to the membrane. Although not investigated in detail in this study, cytc adduction to cholesterol derived aldehydes could have implications in cytc release from mitochondria under apoptotic stimuli.

A photo-triggerable drug carrier based on cleavage of PEG lipids by photosensitiser-generated reactive singlet oxygen

To circumvent the limitations of polyethylene glycol (PEG) modified carriers, a photo-triggerable liposome was prepared which was modified by cholesterol derivatives via a cleavable vinyl ether linkage so that the PEGylated coating can be efficiently removed by a photoactivated fullerene. After the photocleavage of the PEG moiety, the intracellular uptake of the photo-triggerable liposome improved.

Occurrence of cytotoxic 9-oxononanoyl secosterol aldehydes in human low-density lipoprotein

The reaction products of three major cholesteryl esters, cholesteryl palmitate (C16:0-CE), cholesteryl oleate (C18:1-CE), and cholesteryl linoleate (C18:2-CE), present in human low-density lipoprotein (LDL) treated with ozone were isolated and characterized. In vitro ozonization of C16:0-CE was found to form the palmitoyl ester of secosterol-A (3β-hydroxy-5-oxo-5,6-secocholestan-6-al) and its aldolization product secosterol-B (3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxaldehyde). On the other hand, when C18:1-CE and C18:2-CE were oxidized by ozone, the aldehyde 9-oxononanoyl cholesterol (9-ONC) was formed as a primary product, which was then further oxidized to form 9-oxononanoyl secosterol-A (9-ON-secoA) and 9-oxononanoyl secosterol-B (9-ON-secoB). The compounds 9-ON-secoA and -B, but not 9-ONC, were found to exhibit strong cytotoxicity against human leukemia HL-60 cells. An LC-ESI-MS/MS method was developed for the detection of these cholesteryl ester ozonolysis products by derivatizing them with dansyl hydrazine. Using this method, we found for the first time that low concentrations of 9-ON-secoA and -B, but not palmitoyl secosterols, were present in human LDL. These novel oxidized cholesterol esters, 9-ON-secoA and -B, probably play important roles in the pathogenesis of several inflammatory disorders such as cancer, diabetes, atherosclerosis, and neurodegenerative diseases.

Inhibition of endothelial- and neuronal-type, but not inducible-type, nitric oxide synthase by the oxidized cholesterol metabolite secosterol aldehyde: Implications for vascular and neurodegenerative diseases

The cholesterol ozonolysis products secosterol-A and its aldolization product secosterol-B were recently detected in human atherosclerotic tissues and brain specimens, and have been postulated to play pivotal roles in the pathogenesis of atherosclerosis and neurodegenerative diseases. We examined several oxidized cholesterol metabolites including secosterol-A, secosterol-B, 25-hydroxycholesterol, 5β,6β-epoxycholesterol and 7-ketocholesterol for their effects on the activities of three nitric oxide synthases. In contrast to other oxidized metabolites, secosterol-A was found to be a potent inhibitor against the neuronal- and endothelial-type, but not the inducible-type nitric oxide synthase, with IC₅₀ values of 22 ± 1 and 50 ± 5 µM, respectively. The calmodulin-binding regions of the neuronal- and endothelial-nitric oxide synthases contain lysine residues which are not present in the inducible-type nitric oxide synthase. Secosterol-A modifies proteins through the formation of a Schiff base with the lysine epsilon-amino group. It is possible that secosterol-A modifies lysine residues of constitutive nitric oxide synthases, leading to the inhibition of enzymatic activities. As nitric oxide is a critical signaling molecule in vascular function and in long-term potentiation, its reduced production through inhibition of constitutive nitric oxide synthases by secosterol-A may contribute to the development of atherosclerosis and memory impairment in particular neurodegenerative diseases.

A highly sensitive LC-ESI-MS/MS method for the quantification of cholesterol ozonolysis products secosterol-A and secosterol-B after derivatization with 2-hydrazino-1-methylpyridine

Cholesterol ozonolysis products, 3β-hydroxy-5-oxo-5,6-secocholestan-6-al (secosterol-A) and its aldolization product 3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxaldehyde (secosterol-B) have been found in atherosclerosis plaques and the brain tissues of Alzheimer's disease patients, implicating them in the pathogenesis of cardiovascular and neurodegenerative diseases. We have recently reported that when cholesterol is oxidized with an ozone-like oxidant generated by activated mouse neutrophils, secosterol-A is generated which is then converted to secosterol-B by an aldol reaction. To investigate further pathophysiological roles of secosterols, we have developed a highly sensitive method to detect secosterol-A and -B as derivatives with 2-hydrazino-1-methylpyridine (HMP) by LC-ESI-MS/MS. The limits of detection for the HMP derivatives of secosterol-A and secosterol-B were 0.05 and 0.01fmol, respectively, which were approximately 400 and 2000 times better than those for underivatized secosterol-A and -B. We also developed a highly reproducible and accurate method to extract, purify and derivatize secosterol in small volumes of biological specimens. Using this method, we determined the levels of secosterol-A and -B as 1.4 ± 0.7 and 4.3 ± 0.8 nM, respectively, in the plasma of normal C57BL/6 mice, and in the range of 10.4 ± 16.3 to 40.7 ± 20.1 pmol/g and 110.9 ± 10.6 to 161.5 ± 56.3 pmol/g, respectively, in the brain, liver and lung tissues.

Formation of cholesterol ozonolysis products in vitro and in vivo through a myeloperoxidase-dependent pathway

3β-Hydroxy-5-oxo-5,6-secocholestan-6-al (secosterol-A) and its aldolization product 3β-hydroxy-5β-hydroxy-B-norcholestane-6β-carboxaldehyde (secosterol-B) were recently detected in human atherosclerotic tissues and brain specimens, and they may play pivotal roles in the pathogenesis of atherosclerosis and neurodegenerative diseases. However, as their origin remains unidentified, we examined the formation mechanism, the stability, and the fate of secosterols in vitro and in vivo. About 40% of secosterol-A remained unchanged after 3 h incubation in the FBS-free medium, whereas 20% and 40% were converted to its aldehyde-oxidation product, 3β-hydroxy-5-oxo-secocholestan-6-oic acid, and secosterol-B, respectively. In the presence of FBS, almost all secosterol-A was converted immediately to these compounds. Secosterol-B in the medium, with and without FBS, was relatively stable, but ∼30% was converted to its aldehyde-oxidation product, 3β-hydroxy-5β-hydroxy-B-norcholestane-6-oic acid (secoB-COOH). When neutrophil-like differentiated human leukemia HL-60 (nHL-60) cells activated with PMA were cultured in the FBS-free medium containing cholesterol, significantly increased levels of secosterol-A and its aldehyde-oxidation product, but not secosterol-B, were formed. This secosterol-A formation was decreased in the culture of PMA-activated nHL-60 cells containing several reactive oxygen species (ROS) inhibitors and scavengers or in the culture of PMA-activated neutrophils isolated from myeloperoxidase (MPO)-deficient mice. Our results demonstrate that secoterol-A is formed by an ozone-like oxidant generated with PMA-activated neutrophils through the MPO-dependent mechanism.