Genistein Binding to Copper(II)-Solvent Dependence and Effects on Radical Scavenging

Pharmacological properties of extracts and prenylated isoflavonoids from the fruits of Osage orange (Maclura pomifera (Raf.) C.K.Schneid.)

Osage orange trees (Maclura pomifera (Raf.) C.K.Schneid.) are distributed worldwide, particularly in south-east states of the USA. They produce large quantities of strong yellow fruits, bigger than oranges, but these fruits are inedible, with an acid milky juice which is little consumed by birds and insects. Extracts prepared from Osage orange fruits (hedge apple) have revealed a range of pharmacological properties of interest in human and veterinary medicine. In addition, Osage orange extracts can be used in agriculture and aquaculture, and as dyeing agent for the textile industry. Extracts contain potent antioxidant compounds, notably the isoflavonoids pomiferin and auriculasin, together with other terpenoids and flavonoids. The structural characteristics and pharmacological properties of the major prenylated isoflavones isolated from M. pomifera are discussed here, with a focus on the two phenolic compounds osajin and warangalone, and the two catechol analogues pomiferin and auriculasin. The mechanisms at the origin of their potent antioxidant and anti-inflammatory effects are presented, notably inhibition of xanthine oxidase, phosphodiesterase 5A and kinases such as RKS2 and kRAS. Osajin and auriculasin display marked anticancer properties, owing to their ability to inhibit tumor cell proliferation, migration and tumor angiogenesis. Different molecular mechanisms are discussed, including osajin‑copper complexation and binding to quadruplex DNA. An overview of the mechanism of action of the prenylated isoflavones from Osage orange is presented, with the objective to promote their knowledge and to raise opportunities to better exploit the fruits of Osage orange, abundant but largely neglected at present.

Antioxidation Activity Enhancement by Intramolecular Hydrogen Bond and Non-Browning Mechanism of Active Ingredients in Rosemary: Carnosic Acid and Carnosol

Rosemary is one of the most promising, versatile, and studied natural preservatives. Carnosic acid (CA) and carnosol (CARN), as the primary active ingredients of rosemary extracts, have little difference in structure, but their antioxidant activities vary significantly, depending on the system studied. The underlying molecular mechanisms remain unclear. By means of optical spectroscopies, stopped-flow, laser photolysis, and density functional theory (DFT) calculations, we have compared CA and CARN between their reaction dynamics of radical scavenging, metal ion chelation, and oxidation inhibition in lipid emulsion and beef, as well as between their interactions with β-carotene (β-Car). For reference, 3-isopropyl catechol (IC), which is structurally similar to the active groups of CA and CARN, was studied in parallel. It is found for CA that the intramolecular hydrogen bond can boost the acidity of its phenol hydroxyl and that the synergistic effect with β-Car can substantially enhance its antioxidation activity in the model systems of lipid and meat via the CA-to-β-Car electron transfer reaction. The substitution of A and B rings on the catechol group in both CA and CARN limits browning caused by their formation of oxidative products as antioxidants.

Diosmetin derivatives as multifunctional anti-AD ligands: Design, synthesis, and biological evaluation

With the increasing aging population, rational design of drugs for Alzheimer's disease (AD) treatment has become an important research area. Based on the multifunctional design strategy, four diosmetin derivatives (1-4) were designed, synthesized, and characterized by 1H NMR, 13C NMR, and MS. Docking study was firstly applied to substantiate the design strategies and then the biological activities including cholinesterase inhibition, metal chelation, antioxidation and β-amyloid (Aβ) aggregation inhibition in vitro were evaluated. The results showed that 1-4 had good acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibition, metal chelation (selective chelation of Cu2+ ions), antioxidation, self-induced, Cu2+-induced, and AChE-induced Aβ aggregation inhibition activities, and suitable blood-brain barrier (BBB) permeability. Especially, compound 3 had the strongest inhibitory effect on AChE (10-8 M magnitude) and BuChE (10-7 M magnitude) and showed the best inhibition on AChE-induced Aβ aggregation with 66.14% inhibition ratio. Furthermore, compound 3 could also reduce intracellular reactive oxygen species (ROS) levels in Caenorhabditis elegans and had lower cytotoxicity. In summary, 3 might be considered as a potential multifunctional anti-AD ligand.

Double-Site Binding and Anti-/Pro-oxidation of Luteolin on Bovine Serum Albumin Mediated by Copper(II) Coordination

The interactions of luteolin (Lut) with bovine serum albumin (BSA) mediated by Cu(II) were investigated by spectroscopic, calorimetric, and molecular dynamic (MD) methods. Fluorescence studies showed that the binding of Lut to BSA was significantly enhanced by Cu(II) coordination with the number of binding sites and binding constant increasing from n = 1 and K _a = 3.2 × 10⁵ L·mol^-1 for Lut to n = 2 and K _a = 7.1 × 10⁵ L·mol^-1 for a 1:1 Cu(II)-luteolin complex, in agreement with the results from isothermal titration calorimetry (ITC). Site-specific experiments with warfarin and ibuprofen and MD confirmed that two binding sites of BSA were sequentially occupied by two Cu(II)-luteolin complexes. Cu(II) coordination increased the antioxidant activity of luteolin by 60% in the inhibition of carbonyl formation from the oxidation of amino groups in the side chain of BSA induced by the peroxyl radical ROO^•; however, it counteracted the antioxidant effects of luteolin and played pro-oxidative roles in BSA aggregation induced by ^•OH.

Promotion effects of flavonoids on browning induced by enzymatic oxidation of tyrosinase: structure-activity relationship

Tyrosinase, widely distributed in nature, is a copper-containing polyphenol oxidase involved in the formation of melanin. Flavonoids are most often considered as tyrosinase inhibitors but have also been confirmed to be tyrosinase substrates. Four structure-related flavonoids including flavones (apigenin and luteolin) and flavonols (kaempferol and quercetin) are found to promote not inhibit browning induced by tyrosinase catalyzed oxidation both in model systems and in mushrooms under aerobic conditions. A comparison with enzymatic oxidation and autooxidation of flavonoids alone has helped to clarify why flavonoids function as a substrate rather than an inhibitor. Flavonoids almost do not affect the kinetics of melanin formation from enzymatic oxidation of l-dopa in excess. In addition, a new brown complex formed during the reaction of flavonoid quinone and dopaquinone is suggested to enhance the browning effects by competing with isomerization and autooxidation. Structure-activity relationships of the four flavonoids in melanin formation leading to browning induced by autooxidation and enzymatic oxidation confirm the enzymatic nature of the browning.

Alkaline earth metal ion coordination increases the radical scavenging efficiency of kaempferol

Flavonoids are used as natural additives and antioxidants in foods, and after coordination to metal ions, as drug candidates, depending on the flavonoid structure. The rate of radical scavenging of the ubiquitous plant flavonoid kaempferol (3,5,7,4'-tetrahydroxyflavone, Kaem) was found to be significantly enhanced by coordination of Mg(ii), Ca(ii), Sr(ii), and Ba(ii) ions, whereas the radical scavenging rate of apigenin (5,7,4'-trihydroxyflavone, Api) was almost unaffected by alkaline earth metal (AEM) ions, as studied for short-lived β-carotene radical cations (β-Car˙⁺) formed by laser flash photolysis in chloroform/ethanol (7 : 3) and for the semi-stable 2,2-diphenyl-1-picrylhydrazyl radical, DPPH˙, in ethanol at 25 °C. A 1 : 1 Mg(ii)-Kaem complex was found to be in equilibrium with a 1 : 2 Mg(ii)-Kaem₂ complex, while for Ca(ii), Sr(ii) and Ba(ii), only 1 : 2 AEM(ii)-Kaem complexes were detected, where all complexes showed 3-hydroxyl and 4-carbonyl coordination and stability constants of higher than 10⁹ L² mol^-2. The 1 : 2 Ca(ii)-Kaem₂ complex had the highest second order rate constant for both β-Car˙⁺ (5 × 10⁸ L mol^-1 s^-1) and DPPH˙ radical (3 × 10⁵ L mol^-1 s^-1) scavenging, which can be attributed to the optimal combination of the stronger electron withdrawing capability of the (n - 1)d orbital in the heavier AEM ions and their spatially asymmetrical structures in 1 : 2 AEM-Kaem complexes with metal ion coordination of the least steric hindrance of two perpendicular flavone backbones as ligands in the Ca(ii) complex, as shown by density functional theory calculations.

Interactions of Calcium with Chlorogenic and Rosmarinic Acids: An Experimental and Theoretical Approach

Chlorogenic (CA) and rosmarinic (RA) acids are two natural bioactive hydroxycinnamic acids whose antioxidant properties can be modulated by the chelation of metal ions. In this work, the interactions of these two carboxylic phenols with calcium ions and the impact of such interactions on their antioxidant activity were investigated. UV-Vis absorbance, mass spectroscopy and 1H and 13C liquid NMR were used to identify complexes formed by CA and RA with calcium. Antioxidant activities were measured by the Bois method. Density functional theory (DFT) calculations were performed to evaluate the most stable configurations and correlated with NMR data. Taken together, these data suggest that calcium ions mainly interact with the carboxylate groups of both molecules but that this interaction modifies the reactivity of the catechol groups, especially for RA. These results highlight the complex interplay between metal chelation and antioxidant properties of natural carboxylic phenols.

Genistin: A Novel Potent Anti-Adipogenic and Anti-Lipogenic Agent

Soy isoflavones are popular ingredients with anti-adipogenic and anti-lipogenic properties. The anti-adipogenic and anti-lipogenic properties of genistein are well-known, but those of genistin and glycitein remain unknown, and those of daidzein are characterized by contrasting data. Therefore, the purpose of our study was to investigate the effects of daidzein, glycitein, genistein, and genistin on adipogenesis and lipogenesis in 3T3-L1 cells. Proliferation of 3T3-L1 preadipocytes was unaffected by genistin and glycitein, but it was affected by 50 and 100 µM genistein and 100 µM daidzein for 48 h. Among the four isoflavones, only 50 and 100 µM genistin and genistein markedly suppressed lipid accumulation during adipogenesis in 3T3-L1 cells through a similar signaling pathway in a dose-dependent manner. Genistin and genistein suppress adipocyte-specific proteins and genes, such as peroxisome proliferator-activated receptor γ (PPARγ), CCAAT-enhancer-binding protein α (C/EBPα), and adipocyte binding protein 2 (aP2)/fatty acid-binding protein 4 (FABP4), and lipogenic enzymes such as ATP citrate lyase (ACL), acetyl-CoA carboxylase 1 (ACC1), and fatty acid synthase (FAS). Both isoflavones also activate AMP-activated protein kinase α (AMPKα), an essential factor in adipocyte differentiation, and inhibited sterol regulatory element-binding transcription factor 1c (SREBP-1c). These results indicate that genistin is a potent anti-adipogenic and anti-lipogenic agent.

Kinetic Studies on Radical Scavenging Activity of Kaempferol Decreased by Sn(II) Binding

Sn(II) binds to kaempferol (HKaem, 3,4′,5,7-tetrahydroxy-2-(4-hydroxyphenyl)-4H-1-benzopyran-4-one) at the 3,4-site forming [Sn(II)(Kaem)₂] complex in ethanol. DPPH^• scavenging efficiency of HKaem is dramatically decreased by SnCl₂ coordination due to formation of acid inhibiting deprotonation of HKaem as ligands and thus reduces the radical scavenging activity of the complex via a sequential proton-loss electron transfer (SPLET) mechanism. Moderate decreases in the radical scavenging of HKaem are observed by Sn(CH₃COO)₂ coordination and by contact between Sn and HKaem, in agreement with the increase in the oxidation potential of the complex compared to HKaem, leading to a decrease in antioxidant efficiency for fruits and vegetables with Sn as package materials.

Copper(II) Coordination and Translocation in Luteolin and Effect on Radical Scavenging

Luteolin differs as a radical scavenger dramatically from apigenin in response to Cu(II) coordination despite a minor structural difference. Coordination of Cu(II) increases the radical scavenging efficiency of luteolin, especially at low pH, while decreases the efficiency of apigenin at both low and higher pH as studied by ABTS^•+ radical scavenging. Luteolin forms a 1:1 complex with Cu(II) binding to 4-carbonyl and 5-phenol for pH <6 and to 3',4'-catechol for pH >6. Apigenin forms a 1:2 complex independent of pH coordinated to 4-carbonyl and 5-hydroxylyl. Cu(II) coordinated to luteolin, as studied by pH jump stopped-flow, translocates with rate constants of 11.1 ± 0.3 s^-1 from 4,5 to 3',4' sites and 1.0 ± 0.1 s^-1 from 3',4' to 4,5 sites independent of Cu(II) concentration, pointing toward the dissociation of Cu(II) from an intermediate with two Cu(II) coordination as rate determining. 3',4'-Catechol is suggested to be a switch for Cu(II) translocation with deprotonation initiating 4,5 to 3',4' translocation and protonation initiating 3',4' to 4,5 translocation. For dicoordinated apigenin, the coordination symmetry balances an electron withdrawal effect of Cu(II) resulting in a decrease of phenol acidity and less radical scavenging efficiency compared to parent apigenin. Compared to that of parent luteolin, the radical scavenging rate of both 4,5 and 3',4' Cu(II)-coordinated luteolin is enhanced through increased phenol acidity by electron withdrawal by Cu(II), as confirmed by density functional theory (DFT) calculations. Coordination and translocation of Cu(II) accordingly increases the antioxidant activity of luteolin at pH approaching the physiological level and is discovered as a novel class of natural molecular machinery derived from plant polyphenols, which seems to be of importance for protection against oxidative stress.

Kaempferol Binding to Zinc(II), Efficient Radical Scavenging through Increased Phenol Acidity

Zinc(II) enhances radical scavenging of the flavonoid kaempferol (Kaem) most significantly for the 1:1 Zn(II)-Kaem complex in equilibrium with the 1:2 Zn(II)-Kaem complex both with high affinity at 3-hydroxyl and 4-carboxyl coordination. In methanol/chloroform (7/3, v/v), 1:1 Zn(II)-Kaem complex reduces β-carotene radical cation, β-Car^•+, with a second-order rate constant, 1.88 × 10⁸ L·mol^-1·s^-1, while both Kaem and 1:2 Zn(II)-Kaem complex are nonreactive, as determined by laser flash photolysis. In ethanol, 1:1 Zn(II)-Kaem complex reduces the 2,2-diphenyl-1-picrylhydrazyl radical, DPPH^•, with a second-order rate constant, 2.48 × 10⁴ L·mol^-1·s^-1, 16 times and 2 times as efficient as Kaem and 1:2 Zn(II)-Kaem complex, respectively, as determined by stopped-flow spectroscopy. Density functional theory calculation results indicate significantly increased acidity of Kaem as ligand in 1:1 Zn(II)-Kaem complex other than in 1:2 Zn(II)-Kaem complex. Kaem in 1:1 Zn(II)-Kaem complex loses two protons (one from 3-hydroxyl and one from phenolic hydroxyl) forming 1:1 Zn(II)-(Kaem-2H) during binding with Zn(II), while Kaem in 1:2 Zn(II)-Kaem complex loses one proton in each ligand forming Zn(II)-(Kaem-H)₂, as confirmed by UV-vis absorption spectroscopy. Zn(II)-(Kaem-2H) is a far stronger reductant than Kaem and Zn(II)-(Kaem-H)₂ as determined by cyclic voltammetry. Significant rate increases for the 1:1 complex in both β-Car^•+ scavenging by electron transfer and DPPH^• scavenging by hydrogen atom transfer were ascribed to decreases of ionization potential and of bond dissociation energy of 4'-OH for deprotonated Zn(II)-(Kaem-2H), respectively. Increased phenol acidity of plant polyphenols by 1:1 coordination with Zn(II) may explain the unique function of Zn(II) as a biological antioxidant and may help to design nontoxic metal-based drugs derived from natural bioactive molecules.