Hypochlorous acid-induced heme damage of hemoglobin and its inhibition by flavonoids

Rational Design of Activatable Lanthanide NIR-IIb Emissive Nanoprobe for In Situ Specific Imaging of HOCl In Vivo

Hypochlorous acid (HOCl), as an indispensable signaling molecule in organisms, is one of the key members of reactive oxygen species (ROS). However, in vivo, real-time dynamic near-infrared fluorescence imaging of HOCl levels in the 1400-1700 nm sub-window (NIR-IIb) remains a major challenge due to the lack of suitable detection methods. Herein, a general design of HOCl-responsive NIR-IIb fluorescence nanoprobe is proposed by integrating NaLuF4Yb/Er@NaLuF4 downshift nanoparticles (DSNPs) and HOCl recognition/NIR-IIb emissive modulation unit of M2-xS (M = Cu, Co, Pb) nanodots for real-time monitoring of HOCl levels. The fluorescence modulation unit of M2-xS nanodots presents remarkably enhanced absorption than Yb sensitizer at 980 nm and greatly inhibits the NIR-IIb fluorescence emission via competitive absorption mechanism. While, the M2-xS nanodots are easily degraded after triggering by HOCl, resulting in HOCl responsive turn-on (≈ten folds) NIR-IIb emission at 1532 nm. More importantly, in vivo highly precise and specific monitoring of inflammatory with abnormal HOCl expression is successfully achieved. Thus, the explored competitive absorption mediated quenching-activation mechanism provides a new general strategy of designing HOCl-responsive NIR-IIb fluorescence nanoprobe for highly specific and sensitive HOCl detection.

The adducts of cyano- and aquacobalamin with hypochlorite

Hypochlorite is known to oxidatively degrade the corrin ring of cobalamin. Here, transient reaction intermediates are described in the reaction of aqua as well as of cyano-cobalamin with hypochlorite, using stopped-flow UV-vis kinetics. For aqua-cobalamin, the intermediate is assigned as arising from substitution of the aqua ligand with hypochlorite. For cyano-cobalamin, the intermediate is proposed to arise from substitution of the benzimidazole ligand trans to the cyanide. In both cases, the intermediates would feature a new Co(III)-OCl-bond-which is also supported by density functional theory (DFT) calculations.

Quercetin Attenuates Vascular Endothelial Dysfunction in Atherosclerotic Mice by Inhibiting Myeloperoxidase and NADPH Oxidase Function

Myeloperoxidase (MPO) exhibits a unique property to use H₂O₂ to oxidize chloride and lead to the generation of a strong oxidant, hypochlorous acid (HOCl), which plays important roles in atherosclerosis. A lot of evidence indicates that quercetin, a natural polyphenol derived from human diet, effectively contributes to cardiovascular health. Herein, we found that dietary quercetin significantly inhibited vascular endothelial dysfunction and atherosclerosis in apolipoprotein E-deficient (ApoE^-/-) mice. Mechanistic studies revealed that dietary quercetin effectively suppressed the MPO level and activity in the vessels of ApoE^-/- animals, and p47phox expression and NADPH oxidase activity were simultaneously attenuated after quercetin treatment. In vascular endothelial cells, NADPH oxidase was demonstrated to be the major source of H₂O₂ formation. Moreover, quercetin effectively attenuated MPO/H₂O₂-mediated HOCl production and toxicity to human vascular endothelial cells, and this compound was not toxic. The inhibitory effect on MPO activity was likely attributed to that quercetin significantly inhibited NADPH oxidase-derived H₂O₂ formation in human endothelial cells and could act as an effective mediator for MPO intermediates, subsequently preventing HOCl production by the MPO/H₂O₂ system. Collectively, it was suggested that quercetin effectively suppressed endothelial dysfunction in atherosclerotic vasculature through the reduction of MPO/NADPH oxidase-mediated HOCl production.

The design of fluorescein-ferrocene derivatives as HOCl-triggered turn-on fluorescent probes and anticancer prodrugs

Overexpressed HOCl in tumors can behave as an activator for imaging-guided precision therapy. Herein, a new kind of HOCl-activated molecular platform has been developed aiming at the integration of detection, imaging, and anticancer functions. The design strategy uses a five-membered heterocyclic ring to bridge the fluorescent fluorescein part (FL) and the anticancer ferrocene part (Fc). Three derivatives, namely FL-Fc, FL-NP-Fc and FL-TEG-Fc, were designed with different grafted chains on the fluorescein mother to modulate the hydrophilic and biocompatible capacity. In these molecular platforms, the ferrocene unit serves as the fluorescence emission quencher and masked prodrug. These three could respond to HOCl with good selectivity and sensitivity, showing a turn-on fluorescence signal and anticancer efficacy. FL-TEG-Fc with the highest sensitivity (6.5 × 10^-6 M) was successfully used for imaging endogenous HOCl in AGS cells, in which it presented strong toxicity IC₅₀ = 9.5 ± 0.3 μM. The mechanistic study revealed that the five-membered heterocyclic ring of FL-TEG-Fc was broken specifically and effectively by HOCl to release strongly fluorescent fluorescein and a bioactive ferrocene derivative; the obtained ferrocene derivative further generated cytotoxic ˙OH through a Fenton-like reaction. This study provides a potential theranostic strategy against HOCl-overexpressing cancers.

Ratiometric Detection of Hypochlorous Acid in Brain Tissues of Neuroinflammation and Maternal Immune Activation Models with a Deep-Red/Near-Infrared Emitting Probe

Reactive oxygen species (ROS) produced by an inflammatory response in the brain are associated with various neurological disorders. To investigate ROS-associated neuroinflammatory diseases, fluorescent probes with practicality are in demand. We have investigated hypochlorous acid, an important ROS, in the brain tissues of neuroinflammation and maternal immune activation (MIA) model mice, using a new fluorescent probe. The probe has outstanding features over many known probes, such as providing two bright ratio signals in cells and tissues in deep-red/near-infrared wavelength regions with a large spectral separation, in addition to being strongly fluorescent, photo- and chemo-stable, highly selective and sensitive, fast responding, and biocompatible. We have found that the level of hypochlorous acid in the brain tissue of a neuroinflammatory mouse model was higher (2.7-4.0-fold) compared with that in normal brain tissue. Furthermore, the level of hypochlorous acid in the brain tissue of a MIA mouse model was higher (1.2-1.3-fold) compared with that in the normal brain tissue. The "robust" probe provides a practical tool for studying ROS-associated neurological disorders.

Melatonin interferes with COVID-19 at several distinct ROS-related steps

Recent studies have shown a correlation between COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, and the distinct, exaggerated immune response titled "cytokine storm". This immune response leads to excessive production and accumulation of reactive oxygen species (ROS) that cause clinical signs characteristic of COVID-19 such as decreased oxygen saturation, alteration of hemoglobin properties, decreased nitric oxide (NO) bioavailability, vasoconstriction, elevated cytokines, cardiac and/or renal injury, enhanced D-dimer, leukocytosis, and an increased neutrophil to lymphocyte ratio. Particularly, neutrophil myeloperoxidase (MPO) is thought to be especially abundant and, as a result, contributes substantially to oxidative stress and the pathophysiology of COVID-19. Conversely, melatonin, a potent MPO inhibitor, has been noted for its anti-inflammatory, anti-oxidative, anti-apoptotic, and neuroprotective actions. Melatonin has been proposed as a safe therapeutic agent for COVID-19 recently, having been given with a US Food and Drug Administration emergency authorized cocktail, REGEN-COV2, for management of COVID-19 progression. This review distinctly highlights both how the destructive interactions of HOCl with tetrapyrrole rings may contribute to oxygen deficiency and hypoxia, vitamin B12 deficiency, NO deficiency, increased oxidative stress, and sleep disturbance, as well as how melatonin acts to prevent these events, thereby improving COVID-19 prognosis.

Quercetin Attenuated Myeloperoxidase-Dependent HOCl Generation and Endothelial Dysfunction in Diabetic Vasculature

Myeloperoxidase (MPO)-dependent hypochlorous acid (HOCl) generation plays crucial roles in diabetic vascular complications. As a natural polyphenol, quercetin has antioxidant properties in various diabetic models. Herein, we investigated the therapeutic mechanism for quercetin on MPO-mediated HOCl generation and endothelial dysfunction in diabetic vasculature. In vitro, the presence of MPO could amplify high glucose-induced endothelial dysfunction which was significantly inhibited by the NADPH oxidase inhibitor, HOCl or H₂O₂ scavengers, revealing the contribution of MPO/H₂O₂/HOCl to vascular endothelial injury. Furthermore, quercetin effectively inhibited MPO/high glucose-mediated HOCl generation and cytotoxicity to vascular endothelial cells. The inhibitive effect on MPO activity was related to the fact that quercetin reduced high glucose-induced H₂O₂ generation in endothelial cells and directly acted as a competitive substrate for MPO, thus limiting MPO/H₂O₂-dependent HOCl production. Moreover, quercetin could attenuate HOCl-caused endothelial dysfunction in endothelial cells and isolated aortas. In vivo, dietary quercetin significantly inhibited aortic endothelial dysfunction in diabetic mice, while this compound simultaneously suppressed vascular MPO expression and activity. Therefore, it was demonstrated herein that quercetin inhibited endothelial injury in diabetic vasculature via suppression of MPO/high glucose-dependent HOCl formation.

Luminescent probes for hypochlorous acid in vitro and in vivo

HClO/ClO^- is the most effective antibacterial active oxygen in neutrophils. However, its excessive existence often leads to the destruction of human physiological mechanisms. In recent years, the developed luminescent probes for the detection of HClO/ClO^- are not only conducive to improve the sensitivity and selectivity of HClO/ClO^- detection, but also play a crucial role in understanding the biological functions of HClO/ClO^-. In addition, luminescent probe-based biological imaging for HClO/ClO^- at sub-cellular resolution has become a powerful tool for biopathology and medical diagnostic research. This article reviews a variety of luminescent probes for the detection of HClO/ClO^-in vitro and in vivo with different design principles and mechanisms, including fluorescence, phosphorescence, and chemiluminescence. The photophysical/chemical properties and biological applications of these luminescent probes were outlined. Finally, we summarized the merits and demerits of the developed luminescent probes and discussed their challenges and future development trends. It is hoped that this review can provide some inspiration for the development of luminescent probe-based strategies and to promote the further research of biomedical luminescent probes for HClO/ClO^-.

Inhibitive Effects of Quercetin on Myeloperoxidase-Dependent Hypochlorous Acid Formation and Vascular Endothelial Injury

Myeloperoxidase (MPO) from activated neutrophils plays important roles in multiple human inflammatory diseases by catalyzing the formation of powerful oxidant hypochlorous acid (HOCl). As a major flavonoid in the human diet, quercetin has been suggested to act as antioxidant and anti-inflammatory agent in vitro and in vivo. In this study, we showed that quercetin inhibited MPO-mediated HOCl formation (75.0 ± 6.2% for 10 μM quercetin versus 100 ± 5.2% for control group, P < 0.01) and cytotoxicity to endothelial cells in vitro, while this flavonoid was nontoxic to endothelial cell cultures ( P > 0.05, all cases). Moreover, quercetin inhibited HOCl generation by stimulated neutrophils (a rich source of MPO) and protected endothelial cells from neutrophils-induced injury. Furthermore, quercetin could inhibit HOCl-induced endothelial dysfunction such as loss of cell viability, and decrease of nitric oxide formation in endothelial cells ( P < 0.05, all cases). Consistent with these in vitro data, quercetin attenuated lipopolysaccharide-induced endothelial dysfunction and increase of MPO activity in mouse aortas, while this flavonoid could protect against HOCl-mediated endothelial dysfunction in isolated aortas ( P < 0.05). Therefore, it was proposed that quercetin attenuated endothelial injury in inflammatory vasculature via inhibition of vascular-bound MPO-mediated HOCl formation or scavenging of HOCl. These data indicate that quercetin is a nontoxic inhibitor of MPO activity and MPO/neutrophils-induced cytotoxicity in endothelial cells and may be useful for targeting MPO-dependent vascular disease and inflammation.

Melatonin prevents hypochlorous acid-mediated cyanocobalamin destruction and cyanogen chloride generation

Hypochlorous acid (HOCl) is a potent cytotoxic oxidant generated by the enzyme myeloperoxidase (MPO) in the presence of hydrogen peroxide (H₂ O₂ ) and chloride (Cl^- ). Elevated levels of HOCl play an important role in various pathological conditions through oxidative modification of several biomolecules. Recently, we have highlighted the ability of HOCl to mediate the destruction of the metal-ion derivatives of tetrapyrrole macrocyclic rings such as hemoproteins and vitamin B₁₂ (VB₁₂ ) derivatives. Destruction of cyanocobalamin, a common pharmacological form of VB₁₂ mediated by HOCl, results in the generation of toxic molecular products such as chlorinated derivatives, corrin ring cleavage products, the toxic blood agents cyanide (CN^- ) and cyanogen chloride (CNCl), and redox-active free cobalt. Here, we show that melatonin prevents HOCl-mediated cyanocobalamin destruction, using a combination of UV-Vis spectrophotometry, high-performance liquid chromatography analysis, and colorimetric CNCl assay. Identification of several melatonin oxidation products suggests that the protective role of melatonin against HOCl-mediated cyanocobalamin destruction and subsequent CNCl generation is at the expense of melatonin oxidation. Collectively, this work highlights that, in addition to acting as an antioxidant and as a MPO inhibitor, melatonin can also prevent VB₁₂ deficiency in inflammatory conditions such as cardiovascular and neurodegenerative diseases, among many others.

A fast response fluorescence probe specific for hypochlorous acid detection and its applications in bioimaging

The features ofDNPH-NA, including its high sensitivity, selectivity, and reliability at physiological pH, together with a rapid response, enable its successful application in the detection of endogenous HOClin vitroandin vivo.

Effect of pomegranate juice consumption on biochemical parameters and complete blood count

Pomegranate has been used therapeutically for centuries. The aim of the present study was to examine the effects of pomegranate juice (PJ) supplementation on complete blood count (CBC), glucose, blood lipids and C-reactive protein (CRP) in healthy subjects. A total of 5 males and 5 females (aged 31.8±6.6 years, weighing 66.2±12.9 kg) were randomly assigned into one of two groups and either consumed 500 ml PJ/day or no PJ for 14 days. Blood samples were obtained from participants prior to and following the experimental period. PJ consumption resulted in a significant increase in red blood cell count (P<0.05), hemoglobin levels (P<0.001) and hematocrit levels (P<0.05). Other CBC parameters, glucose, cholesterol, triglycerides, high-density lipoprotein, low-density lipoprotein and CRP levels did not significantly change following PJ consumption. These results indicate that PJ intake for a short period of time may result in increased erythropoiesis or decreased degradation without any significant alterations in factors associated with metabolic health and inflammation in healthy individuals.

Inhibition of Myeloperoxidase- and Neutrophil-Mediated Hypochlorous Acid Formation in Vitro and Endothelial Cell Injury by (-)-Epigallocatechin Gallate

Myeloperoxidase (MPO) plays important roles in various diseases through its unique chlorinating activity to catalyze excess hypochlorous acid (HOCl) formation. Epidemiological studies indicate an inverse correlation between plant polyphenol consumption and the incidence of cardiovascular diseases. Here we showed that (-)-epigallocatechin gallate (EGCG), the main flavonoid present in green tea, dose-dependently inhibited MPO-mediated HOCl formation in vitro (chlorinating activities of MPO: 50.2 ± 5.7% for 20 μM EGCG versus 100 ± 5.6% for control, P < 0.01). UV-vis spectral and docking studies indicated that EGCG bound to the active site (heme) of MPO and resulted in the accumulation of compound II, which was unable to produce HOCl. This flavonoid also effectively inhibited HOCl generation in activated neutrophils (HOCl formation: 65.0 ± 5.6% for 20 μM EGCG versus 100 ± 6.2% for control, P < 0.01) without influencing MPO and Nox2 release and superoxide formation, suggesting that EGCG specifically inhibited MPO but not NADPH oxidase activity in activated neutrophils. Moreover, EGCG inhibited MPO (or neutrophil)-mediated HOCl formation in human umbilical vein endothelial cells (HUVEC) culture and accordingly protected HUVEC from MPO (or neutrophil)-induced injury (P < 0.05, all cases), although it did not induce cytotoxicity to HUVEC (P > 0.05, all cases). Our results indicate that dietary EGCG is an effective and specific inhibitor of MPO activity and may participate in the regulation of immune responses at inflammatory sites.

A two-photon fluorescent probe for ratiometric imaging of endogenous hypochlorous acid in live cells and tissues

A fluorescent probe enables two-photon ratiometric imaging of endogenous hypochlorous acid, a reactive oxygen species, in cells and tissues.

Analysis of Chlorination, Nitration, and Nitrosylation of Tyrosine and Oxidation of Methionine and Cysteine in Hemoglobin from Type 2 Diabetes Mellitus Patients by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

The post-translational modification (PTM) of proteins by endogenous reactive chlorine, nitrogen, and oxygen species is implicated in certain pathological conditions, including diabetes mellitus. Evidence showed that the extents of modifications on a number of proteins are elevated in diabetic patients. Measuring modification on hemoglobin has been used to monitor the extent of exposure. This study develops an assay for simultaneous quantification of the extent of chlorination, nitration, and oxidation in human hemoglobin and to examine whether the level of any of these modifications is higher in poorly controlled type 2 diabetic mellitus patients. This mass spectrometry-based assay used the bottom-up proteomic strategy. Due to the low amount of endogenous modification, we first characterized the sites of chlorination at tyrosine in hypochlorous acid-treated hemoglobin by an accurate mass spectrometer. The extents of chlorination, nitration, and oxidation of a total of 12 sites and types of modifications in hemoglobin were measured by nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry under the selected reaction monitoring mode. Relative quantification of these PTMs in hemoglobin extracted from blood samples shows that the extents of chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in diabetic patients (n = 19) than in nondiabetic individuals (n = 18). After excluding the factor of smoking, chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in the nonsmoking diabetic patients (n = 12) than in normal nonsmoking subjects (n = 11). Multiple regression analysis performed on the combined effect of age, body-mass index (BMI), and HbA1c showed that the diabetes factor HbA1c contributes significantly to the extent of chlorination at α-Tyr-24 in nonsmokers. In addition, age contributes to oxidation at α-Met-32 significantly in all subjects and in nonsmokers. These results suggest the potential of using chlorination at α-Tyr-24-containing peptide to evaluate protein damage in nonsmoking type 2 diabetes mellitus.

Kinetic studies on the reaction of cob(II)alamin with hypochlorous acid: Evidence for one electron oxidation of the metal center and corrin ring destruction

Kinetic and mechanistic studies on the reaction of a major intracellular vitamin B₁₂ form, cob(II)alamin (Cbl(II)), with hypochlorous acid/hypochlorite (HOCl/OCl^-) have been carried out. Cbl(II) (Co(II)) is rapidly oxidized by HOCl to predominately aquacobalamin/hydroxycobalamin (Cbl(III), Co(III)) with a second-order rate constant of 2.4×10⁷M^-1s^-1 (25.0°C). The stoichiometry of the reaction is 1:1. UHPLC/HRMS analysis of the product mixture of the reaction of Cbl(II) with 0.9mol equiv. HOCl provides support for HOCl being initially reduced to Cl and subsequent H atom abstraction from the corrin macrocycle occurring, resulting in small amounts of corrinoid species with two or four H atoms fewer than the parent cobalamin. Upon the addition of excess (H)OCl further slower reactions are observed. Finally, SDS-PAGE experiments show that HOCl-induced damage to bovine serum albumin does not occur in the presence of Cbl(II), providing support for Cbl(II) being an efficient HOCl trapping agent.

Recent progress in chromogenic and fluorogenic chemosensors for hypochlorous acid

Due to the biological and industrial importance of hypochlorous acid, the development of optical probes for HOCl has been an active research area. Hypochlorous acid and hypochlorite can oxidize electron-rich analytes with accompanying changes in molecular sensor spectroscopic profiles. Probes for such processes may monitor HOCl levels in the environment or in an organism and via bio-labeling or bioimaging techniques. This review summarizes recent developments in the area of chromogenic and fluorogenic chemosensors for HOCl.

An ICT based ultraselective and sensitive fluorescent probe for detection of HClO in living cells

An ICT based ultraselective and sensitive probe for colorimetric and fluorescent detection of HClOviaoxidative cleavage of an alkene linker to epoxide and then to aldehydes was developed through the conjugation of pyridinium with vanilline.

Antioxidant effects of quercetin and naringenin are associated with impaired neutrophil microbicidal activity

Naringenin and quercetin are considered antioxidant compounds with promising activity against oxidative damage in human cells. However, no reports have described their effects on reactive oxygen species (ROS) production by phagocytes during microbicidal activity. Thus, the present study evaluated the effects of naringenin and quercetin on ROS production, specifically hypochlorous acid (HOCl), and their involvement in the microbicidal activity of neutrophils. Naringenin and quercetin inhibited HOCl production through different systems, but this inhibition was more pronounced for quercetin, even in the cell-free systems. With regard to the microbicidal activity of neutrophils, both naringenin and quercetin completely inhibited the killing of Staphylococcus aureus. Altogether, these data indicate that the decrease in the oxidant activity of neutrophils induced by these compounds directly impaired the microbicidal activity of neutrophils. Naringenin and quercetin exerted their effects by controlling the effector mechanisms of ROS production, with both positive and negative effects of these antioxidant agents in oxidative stress conditions and on ROS in the microbicidal activity of phagocytes. The present results challenge the traditional view of antioxidants as improvers of pathological conditions.

Catalase is inhibited by flavonoids

Catalases, heme enzymes, which catalyze decomposition of hydrogen peroxide to water and molecular oxygen, belong to the antioxidant defense system of the cell. In this work we have shown that catalase from bovine liver is inhibited by flavonoids. The inhibition is, at least partially, due to the formation of hydrogen bonds between catalase and flavonoids. In the presence of some flavonoids the formation of unreactive catalase compound II has been detected. The most potent catalase inhibitors among the tested flavonoids have appeared myricetin, epicatechin gallate and epigallocatechin gallate. The relationship between the degree of enzyme inhibition and molecular structure of flavonoids has been analyzed.

Mechanisms of neuroprotection by hemopexin: modeling the control of heme and iron homeostasis in brain neurons in inflammatory states

Hemopexin provides neuroprotection in mouse models of stroke and intracerebral hemorrhage and protects neurons in vitro against heme or reactive oxygen species (ROS) toxicity via heme oxygenase-1 (HO1) activity. To model human brain neurons experiencing hemorrhages and inflammation, we used human neuroblastoma cells, heme-hemopexin complexes, and physiologically relevant ROS, for example, H(2)O(2) and HOCl, to provide novel insights into the underlying mechanism whereby hemopexin safely maintains heme and iron homeostasis. Human amyloid precursor protein (hAPP), needed for iron export from neurons, is induced ~twofold after heme-hemopexin endocytosis by iron from heme catabolism via the iron-regulatory element of hAPP mRNA. Heme-hemopexin is relatively resistant to damage by ROS and retains its ability to induce the cytoprotective HO1 after exposure to tert-butylhydroperoxide, although induction is impaired, but not eliminated, by exposure to high concentrations of H(2)O(2) in vitro. Apo-hemopexin, which predominates in non-hemolytic states, resists damage by H(2)O(2) and HOCl, except for the highest concentrations likely in vivo. Heme-albumin and albumin are preferential targets for ROS; thus, albumin protects hemopexin in biological fluids like CSF and plasma where it is abundant. These observations provide strong evidence that hemopexin will be neuroprotective after traumatic brain injury, with heme release in the CNS, and during the ensuing inflammation. Hemopexin sequesters heme, thus preventing unregulated heme uptake that leads to toxicity; it safely delivers heme to neuronal cells; and it activates the induction of proteins including HO1 and hAPP that keep heme and iron at safe levels in neurons.