Oxidation of flavonoids by hypochlorous acid: reaction kinetics and antioxidant activity studies

Synthesis, fungal biotransformation, and evaluation of the antimicrobial potential of chalcones with a chlorine atom

Chalcones are intermediate products in the biosynthesis of flavonoids, which possess a wide range of biological properties, including antimicrobial and anticancer activities. The introduction of a chlorine atom and the glucosyl moiety into their structure may increase their bioavailability, bioactivity, and pharmacological use. The combined chemical and biotechnological methods can be applied to obtain such compounds. Therefore, 2-chloro-2'-hydroxychalcone and 3-chloro-2'-hydroxychalcone were synthesized and biotransformed in cultures of two strains of filamentous fungi, i.e. Isaria fumosorosea KCH J2 and Beauveria bassiana KCH J1.5 to obtain their novel glycosylated derivatives. Pharmacokinetics, drug-likeness, and biological activity of them were predicted using cheminformatics tools. 2-Chloro-2'-hydroxychalcone, 3-chloro-2'-hydroxychalcone, their main glycosylation products, and 2'-hydrochychalcone were screened for antimicrobial activity against several microbial strains. The growth of Escherichia coli 10,536 was completely inhibited by chalcones with a chlorine atom and 3-chlorodihydrochalcone 2'-O-β-D-(4″-O-methyl)-glucopyranoside. The strain Pseudomonas aeruginosa DSM 939 was the most resistant to the action of the tested compounds. However, chalcone aglycones and glycosides with a chlorine atom almost completely inhibited the growth of bacteria Staphylococcus aureus DSM 799 and yeast Candida albicans DSM 1386. The tested compounds had different effects on lactic acid bacteria depending on the tested species. In general, chlorinated chalcones were more effective in the inhibition of the tested microbial strains than their unchlorinated counterparts and aglycones were a little more effective than their glycosides.

Myricetin causes site-specific DNA damage via reactive oxygen species generation by redox interactions with copper ions

Myricetin (MYR), found in tea and berries, may have preventive effects on diseases, including Alzheimer's disease and cancer. However, MYR is also a mutagen, inducing DNA damage in the presence of metal ions. We have studied the molecular mechanisms of DNA damage by MYR in the presence of Cu(II) (MYR+Cu). Using 32P-5'-end-labeled DNA fragments, we analyzed site-specific DNA damage caused by MYR+Cu. MYR+Cu caused concentration-dependent DNA strand breaks and base alterations, leading to cleavage of DNA at thymine, cytosine, and guanine nucleotides. Formation of the oxidative DNA damage indicator, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG), in calf thymus DNA was increased by MYR+Cu. The production of 8-oxodG in MYR-treated HL-60 cells was significantly higher than in HP100 cells, which are more resistant to H2O2 than are HL-60 cells. Reactive oxygen species (ROS) scavengers were used to elucidate the mechanism of DNA damage. DNA damage was not inhibited by typical free hydroxyl radical (•OH) scavengers such as ethanol, mannitol, or sodium formate. However, methional, catalase, and bathocuproine inhibited DNA damage induced by MYR+Cu. These results suggest that H2O2, Cu(I), and ROS other than •OH are involved in MYR+Cu-induced DNA damage. We conclude that the Cu(I)/Cu(II) redox cycle and concomitant H2O2 production via autoxidation of MYR generate a complex of H2O2 and Cu(I), probably Cu(I)-hydroperoxide, which induces oxidative DNA damage.

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.

A low direct electrical signal attenuates oxidative stress and inflammation in septic rats

Electrical stimulation is proposed to exert an antimicrobial effect according to studies performed using bacterial and cell cultures. Therefore, we investigated the effects of electrification on inflammation in septic rats. Twenty-eight male Wistar albino rats were divided into 4 groups: healthy control (C), electrified healthy (E), sepsis (S), and electrified sepsis (SE) groups. Staphylococcus aureus (1 x 109 colonies) in 1 ml of medium was intraperitoneally injected into rats to produce a sepsis model. The rats in the E and SE groups were exposed to a low direct electrical signal (300 Hz and 2.5 volts) for 40 min and 1 and 6 h after bacterial infection. Immediately after the second electrical signal application, blood and tissue samples of the heart, lung, and liver were collected. An antibacterial effect of a low direct electrical signal was observed in the blood of rats. The effects of electrical signals on ameliorating changes in the histological structure of tissues, blood pH, gases, viscosity and cell count, activities of some important enzymes, oxidative stress parameters, inflammation and tissue apoptosis were observed in the SE group compared to the S group. Low direct electrical signal application exerts antibacterial, antioxidant, anti-inflammatory and antiapoptotic effects on septic rats due to the induction of electrolysis in body fluids without producing any tissue damage.

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.

Delocalization of the Unpaired Electron in the Quercetin Radical: Comparison of Experimental ESR Data with DFT Calculations

In the antioxidant activity of quercetin (Q), stabilization of the energy in the quercetin radical (Q•) by delocalization of the unpaired electron (UE) in Q• is pivotal. The aim of this study is to further examine the delocalization of the UE in Q•, and to elucidate the importance of the functional groups of Q for the stabilization of the UE by combining experimentally obtained spin resonance spectroscopy (ESR) measurements with theoretical density functional theory (DFT) calculations. The ESR spectrum and DFT calculation of Q• and structurally related radicals both suggest that the UE of Q• is mostly delocalized in the B ring and partly on the AC ring. The negatively charged oxygen groups in the B ring (3' and 4') of Q• have an electron-donating effect that attract and stabilize the UE in the B ring. Radicals structurally related to Q• indicate that the negatively charged oxygen at 4' has more of an effect on concentrating the UE in ring B than the negatively charged oxygen at 3'. The DFT calculation showed that an OH group at the 3-position of the AC ring is essential for concentrating the radical on the C2-C3 double bond. All these effects help to explain how the high energy of the UE is captured and a stable Q• is generated, which is pivotal in the antioxidant activity of Q.

Dietary Flavonoids Scavenge Hypochlorous Acid via Chlorination on A- and C-Rings as Primary Reaction Sites: Structure and Reactivity Relationship

Dietary flavonoids are known as scavengers of reactive oxygen species such as hypochlorous acid. In spite of the abundant scavenging capacity data reported, few reports have addressed the relationship between the scavenging capacity and structures of different flavonoids. We characterized the reaction products of five flavonoids (apigenin, quercetin, naringenin, ampelopsin, and epicatechin) with hypochlorous acid and found that primary chlorination reaction occurred on the A-ring (C6 or C8) and/or C-rings but not on B-rings. Correlation of the hypochlorous acid scavenging capacity (IC₅₀ values) and the structural features of flavonoids revealed that the hydroxyl groups in the A-ring and B-ring can enhance the scavenging capacity, whereas the C(2)C(3) double bond has a negative impact on the HClO scavenging capacity. Combining the SAR analysis and chemical study, we proposed that the reaction mechanism between flavonoids and HClO should be an electrophilic substitution reaction. Density functional theory (DFT) results are consistent with the selectivity of chlorination on the flavonoids. Our findings highlight the importance of considering specific reactive oxygen species when measuring radical scavenging capacity of dietary antioxidants.

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.