Two Prenylated Chalcones, 4-Hydroxyderricin, and Xanthoangelol Prevent Postprandial Hyperglycemia by Promoting GLUT4 Translocation via the LKB1/AMPK Signaling Pathway in Skeletal Muscle Cells

SCOPE

Stimulation of glucose uptake in the skeletal muscle is crucial for the prevention of postprandial hyperglycemia. Insulin and certain polyphenols enhance glucose uptake through the translocation of glucose transporter 4 (GLUT4) in the skeletal muscle. The previous study reports that prenylated chalcones, 4-hydroxyderricin (4-HD), and xanthoangelol (XAG) promote glucose uptake and GLUT4 translocation in L6 myotubes, but their underlying molecular mechanism remains unclear. This study investigates the mechanism in L6 myotubes and confirms antihyperglycemia by 4-HD and XAG.

METHODS AND RESULTS

In L6 myotubes, 4-HD and XAG promote glucose uptake and GLUT4 translocation through the activation of adenosine monophosphate-activated protein kinase (AMPK) and liver kinase B1 (LKB1) signaling pathway without activating phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) and Janus kinases (JAKs)/signal transducers and activators of transcriptions (STATs) pathways. Moreover, Compound C, an AMPK-specific inhibitor, as well as siRNA targeting AMPK and LKB1 completely canceled 4-HD and XAG-increased glucose uptake. Consistently, oral administration of 4-HD and XAG to male ICR mice suppresses acute hyperglycemia in an oral glucose tolerance test.

CONCLUSION

In conclusion, LKB1/AMPK pathway and subsequent GLUT4 translocation in skeletal muscle cells are involved in Ashitaba chalcone-suppressed acute hyperglycemia.

Synthesis, spectroscopic characterization, and antibacterial activity of chalcone (2E)-1-(3'-aminophenyl)-3-(4-dimethylaminophenyl)-prop-2-en-1-one against multiresistant Staphylococcus aureus carrier of efflux pump mechanisms and β-lactamase

BACKGROUND

The bacterium Staphylococcus aureus has stood out for presenting a high adaptability, acquiring resistance to multiple drugs. The search for natural or synthetic compounds with antibacterial properties capable of reversing the resistance of S. aureus is the main challenge to be overcome today. Natural products such as chalcones are substances present in the secondary metabolism of plants, presenting important biological activities such as antitumor, antidiabetic, and antimicrobial activity.

OBJECTIVES

In this context, the aim of this work was to synthesize the chalcone (2E)-1-(3'-aminophenyl)-3-(4-dimethylaminophenyl)-prop-2-en-1-one with nomenclature CMADMA, confirm its structure by nuclear magnetic resonance (NMR), and evaluate its antibacterial properties.

METHODS

The synthesis methodology used was that of Claisen-Schmidt, and spectroscopic characterization was performed by NMR. For microbiological assays, the broth microdilution methodology was adopted in order to analyze the antibacterial potential of chalcones and to analyze their ability to act as a possible inhibitor of β-lactamase and efflux pump resistance mechanisms, present in S. aureus strain K4100.

RESULTS

The results obtained show that CMADMA does not show direct antibacterial activity, expressing a MIC of ≥1024 μg/mL, or on the enzymatic mechanism of β-lactamase; however, when associated with ethidium bromide in efflux pump inhibition assays, CMADMA showed promising activity by reducing the MIC of the bromide from 64 to 32 μg/mL.

CONCLUSION

We conclude that the chalcone synthesized in this study is a promising substance to combat bacterial resistance, possibly acting in the inhibition of the QacC efflux pump present in S. aureus strain K4100, as evidenced by the reduction in the MIC of ethidium bromide.

Furan-based Chalcone Annihilates the Multi-Drug-Resistant Pseudomonas aeruginosa and Protects Zebra Fish Against its Infection

The emergence of carbapenem-resistant Pseudomonas aeruginosa, a multi-drug-resistant bacteria, is becoming a serious public health concern. This bacterium infects immunocompromised patients and has a high fatality rate. Both naturally and synthetically produced chalcones are known to have a wide array of biological activities. The antibacterial properties of synthetically produced chalcone were studied against P. aeruginosa. In vitro, study of the compound (chalcone derivative named DKO1), also known as (2E)-1-(5-methylfuran-2-yl)-3-(4-nitrophenyl) prop-2-en-1-one, had substantial antibacterial and biofilm disruptive action. DKO1 effectively shielded against P. aeruginosa-induced inflammation, oxidative stress, lipid peroxidation, and apoptosis in zebrafish larvae. In adult zebrafish, the treatment enhanced the chances of survivability and reduced the sickness-like behaviors. Gene expression, biochemical analysis, and histopathology studies found that proinflammatory cytokines (TNF-α, IL-1β, IL-6, iNOS) were down regulated; antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT) levels increased, and histoarchitecture was restored in zebrafish. The data indicate that DKO1 is an effective antibacterial agent against P. aeruginosa demonstrated both in vitro and in vivo.

Low temperature inhibits anthocyanin accumulation in strawberry fruit by activating FvMAPK3-induced phosphorylation of FvMYB10 and degradation of Chalcone Synthase 1

Low temperature causes poor coloration of strawberry (Fragaria sp.) fruits, thus greatly reducing their commercial value. Strawberry fruits accumulate anthocyanins during ripening, but how low temperature modulates anthocyanin accumulation in plants remains largely unknown. We identified MITOGEN-ACTIVATED PROTEIN KINASE3 (FvMAPK3) as an important negative regulator of anthocyanin accumulation that mediates the poor coloration of strawberry fruits in response to low temperature. FvMAPK3 activity was itself induced by low temperature, leading to the repression of anthocyanin accumulation via two mechanisms. Activated FvMAPK3 acted as the downstream target of MAPK KINASE4 (FvMKK4) and SUCROSE NONFERMENTING1-RELATED KINASE2.6 (FvSnRK2.6) to phosphorylate the transcription factor FvMYB10 and reduce its transcriptional activity. In parallel, FvMAPK3 phosphorylated CHALCONE SYNTHASE1 (FvCHS1) to enhance its proteasome-mediated degradation. These results not only provide an important reference to elucidate the molecular mechanisms underlying low-temperature-mediated repression of anthocyanin accumulation in plants, but also offer valuable candidate genes for generating strawberry varieties with high tolerance to low temperature and good fruit quality.

Chalcone Scaffolds, Bioprecursors of Flavonoids: Chemistry, Bioactivities, and Pharmacokinetics

Chalcones are secondary metabolites belonging to the flavonoid (C6-C3-C6 system) family that are ubiquitous in edible and medicinal plants, and they are bioprecursors of plant flavonoids. Chalcones and their natural derivatives are important intermediates of the flavonoid biosynthetic pathway. Plants containing chalcones have been used in traditional medicines since antiquity. Chalcones are basically α,β-unsaturated ketones that exert great diversity in pharmacological activities such as antioxidant, anticancer, antimicrobial, antiviral, antitubercular, antiplasmodial, antileishmanial, immunosuppressive, anti-inflammatory, and so on. This review provides an insight into the chemistry, biosynthesis, and occurrence of chalcones from natural sources, particularly dietary and medicinal plants. Furthermore, the pharmacological, pharmacokinetics, and toxicological aspects of naturally occurring chalcone derivatives are also discussed herein. In view of having tremendous pharmacological potential, chalcone scaffolds/chalcone derivatives and bioflavonoids after subtle chemical modification could serve as a reliable platform for natural products-based drug discovery toward promising drug lead molecules/drug candidates.

Chalcones identify cTXNPx as a potential antileishmanial drug target

With current drug treatments failing due to toxicity, low efficacy and resistance; leishmaniasis is a major global health challenge that desperately needs new validated drug targets. Inspired by activity of the natural chalcone 2’,6’-dihydroxy-4’-methoxychalcone (DMC), the nitro-analogue, 3-nitro-2’,4’,6’- trimethoxychalcone (NAT22, 1c) was identified as potent broad spectrum antileishmanial drug lead. Structural modification provided an alkyne containing chemical probe that labelled a protein within the parasite that was confirmed as cytosolic tryparedoxin peroxidase (cTXNPx). Crucially, labelling is observed in both promastigote and intramacrophage amastigote life forms, with no evidence of host macrophage toxicity. Incubation of the chalcone in the parasite leads to ROS accumulation and parasite death. Deletion of cTXNPx, by CRISPR-Cas9, dramatically impacts upon the parasite phenotype and reduces the antileishmanial activity of the chalcone analogue. Molecular docking studies with a homology model of in-silico cTXNPx suggest that the chalcone is able to bind in the putative active site hindering access to the crucial cysteine residue. Collectively, this work identifies cTXNPx as an important target for antileishmanial chalcones.

Leishmaniasis is an insect vector-borne parasitic disease. With >350 million people world wide considered at risk, 12 million people currently infected and an economic cost that can be estimated in terms of >3.3 million working life years lost, leishmaniasis is a major global health challenge. The disease is of particular importance in Brazil. Current treatment of leishmaniasis is difficult requiring a long, costly course of drug treatment using old drugs with poor safety indications requiring close medical supervision. Moreover, resistance to current antileishmanials is growing, emphasising a major need for new drug targets. In earlier work we had identified a naturally inspired chalcone which had promising antileishmanial activity but with no known mode of action. In this work we use an analogue of this molecule as an activity based probe to identify a protein target of the chalcone. This protein, cTXNPx, has a major role in protecting the parasite against attack by reactive oxygen species in the host cell. By inhibiting this protein the parasite can no longer survive in the host. Collectively this work validates cTXNPx as a drug target with the chalcone as a lead structure for future drug discovery programmes.

Adenosine Receptor Ligands: Coumarin-Chalcone Hybrids as Modulating Agents on the Activity of hARs

Adenosine receptors (ARs) play an important role in neurological and psychiatric disorders such as Alzheimer’s disease, Parkinson’s disease, epilepsy and schizophrenia. The different subtypes of ARs and the knowledge on their densities and status are important for understanding the mechanisms underlying the pathogenesis of diseases and for developing new therapeutics. Looking for new scaffolds for selective AR ligands, coumarin–chalcone hybrids were synthesized (compounds 1–8) and screened in radioligand binding (hA₁, hA_2A and hA₃) and adenylyl cyclase (hA_2B) assays in order to evaluate their affinity for the four human AR subtypes (hARs). Coumarin–chalcone hybrid has been established as a new scaffold suitable for the development of potent and selective ligands for hA₁ or hA₃ subtypes. In general, hydroxy-substituted hybrids showed some affinity for the hA₁, while the methoxy counterparts were selective for the hA₃. The most potent hA₁ ligand was compound 7 (K_i = 17.7 µM), whereas compound 4 was the most potent ligand for hA₃ (K_i = 2.49 µM). In addition, docking studies with hA₁ and hA₃ homology models were established to analyze the structure–function relationships. Results showed that the different residues located on the protein binding pocket could play an important role in ligand selectivity.

Antiproliferative Effect of Acridine Chalcone Is Mediated by Induction of Oxidative Stress

Chalcones are naturally occurring phytochemicals with diverse biological activities including antioxidant, antiproliferative, and anticancer effects. Some studies indicate that the antiproliferative effect of chalcones may be associated with their pro-oxidant effect. In the present study, we evaluated contribution of oxidative stress in the antiproliferative effect of acridine chalcone 1C ((2 E)-3-(acridin-9-yl)-1-(2,6-dimethoxyphenyl)prop-2-en-1-one) in human colorectal HCT116 cells. We demonstrated that chalcone 1C induced oxidative stress via increased reactive oxygen/nitrogen species (ROS/RNS) and superoxide production with a simultaneous weak adaptive activation of the cellular antioxidant defence mechanism. Furthermore, we also showed chalcone-induced mitochondrial dysfunction, DNA damage, and apoptosis induction. Moreover, activation of mitogen activated phosphokinase (MAPK) signalling pathway in 1C-treated cancer cells was also observed. On the other hand, co-treatment of cells with strong antioxidant, N-acetyl cysteine (NAC), significantly attenuated all of the above-mentioned effects of chalcone 1C, that is, decreased oxidant production, prevent mitochondrial dysfunction, DNA damage, and induction of apoptosis, as well as partially preventing the activation of MAPK signalling. Taken together, we documented the role of ROS in the antiproliferative/pro-apoptotic effects of acridine chalcone 1C. Moreover, these data suggest that this chalcone may be useful as a promising anti-cancer agent for treating colon cancer.

Hydroxysafflor yellow A actives BKCa channels and inhibits L-type Ca channels to induce vascular relaxation

Hydroxy-safflor yellow A (HSYA) can exert a variety of effects upon the vascular system. However, the underlying mechanisms are not clear. The present study is to investigate its vasodilating effect and the mechanisms. Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) were enrolled for studying effects of HSYA on blood pressure, vasodilation, intracellular Ca²⁺ transient and membrane ion channels. Vasodilation and intracellular Ca²⁺ transient were measured by using vasomotor assay and fluorescence imaging system, respectively. The effect of HSYA on the large conductance Ca²⁺ activated and voltage-gated potassium channel (BK_Ca channel) currents in rat mesentery artery and on L-type calcium channel (Ca-L) currents in HEK293cells expressed with Ca-L were investigated using patch clamp techniques. Blood pressure of SHR and WKY rats were concentration dependently reduced by HSYA with a larger effect of HSYA in SHR than that in WKY rats. The tension of mesenteric arteries induced by 3 μM phenylephrine was attenuated by HSYA (IC₅₀ = 90.8 μΜ). Patch clamp study showed that HSYA could activate BK_Ca channels and suppress Ca-L channels in a concentration dependent manner. The results of calcium signaling assays indicated that HSYA could reduce the intracellular free Ca²⁺ level. These findings demonstrate that HSYA could activate BK_Ca channels and inhibit Ca-L channels and reduce intracellular free Ca²⁺ level, which are probably important for its vasodilatory effect.

A comprehensive in vitro and in vivo metabolism study of hydroxysafflor yellow A

As the most important marker component in Carthamus tinctorius L., hydroxysafflor yellow A (HSYA) was widely used in the prevention and treatment of cardiovascular diseases, due to its effect of improving blood supply, suppressing oxidative stress, and protecting against ischemia/reperfusion. In this paper, both an in vitro microsomal incubation and an in vivo animal experiment were conducted, along with an LC-Q-TOF/MS instrument and a 3-step protocol, to further explore the metabolism of HSYA. As a result, a total of 10 metabolites were searched and tentatively identified in plasma, urine, and feces after intravenous administration of HSYA to male rats, although no obvious biotransformation was found in the simulated rat liver microsomal system. The metabolites detected involving both phase I and phase II metabolism including dehydration, deglycosylation, methylation, and glucuronic acid conjugation. A few of the metabolites underwent more than one-step metabolic reactions, and some have not been reported before. The study would contribute to a further understanding of the metabolism of HSYA and provide scientific evidence for its pharmacodynamic mechanism research and clinical use.

Plasma membrane depolarization precedes photosynthesis damage and long-term leaf bleaching in (E)-chalcone-treated Arabidopsis shoots

The plant phenolic compound (E)-chalcone has been previously found to induce noticeable seedling size reduction and progressive de-greening (bleaching) in shoots of Arabidopsis thaliana seedlings. In this work, we demonstrate that this progressive de-greening occurring on Arabidopsis shoots after (E)-chalcone treatment, is directly linked to early plasma membrane depolarization and dramatic effects on chloroplasts structure and function. Later effects in chalcone-treated seedlings included ROS accumulation, pigment degradation, reduced photosynthetic activity, bleaching, and eventually cell death. De-greening and pigment degradation induced by (E)-chalcone were partially reversed when NaCl was added together with chalcone, which could be related to restoration of altered pH gradients. All these results suggest that rapid alteration of plasma membrane potential after chalcone treatment is a major component of the mode of action of (E)-chalcone on Arabidopsis metabolism.

Flavokawain A inhibits Cytochrome P450 in in vitro metabolic and inhibitory investigations

ETHNOPHARMACOLOGICAL RELEVANCE

Flavokawain A, the major chalcone in kava extracts, was served as beverages for informal social occasions and traditional ceremonials in most South Pacific islands. It exhibited strong antiproliferative and apoptotic effects against human prostate and urinary bladder cancer cells.

AIM OF THE STUDY

The current study was purposed to investigate the interaction between Flavokawain A and Cytochrome P450, including the inhibitory effects of Flavokawain A on predominant CYP450 isotypes and further clarified the inhibitory mechanism of FKA on CYP450 enzymes. Besides, study about identifying the key CYP450 isotypes responsible for the metabolism of FKA was also performed.

MATERIALS AND METHODS

In this study, probe-based assays with rat liver microsome system were used to characterize the inhibitory effects of FKA. Molecular docking study was performed to further explore the binding site of FKA on CYP450 isoforms. In addition, chemical inhibition experiments using specific inhibitors (a-naphthoflavone, quinidine, sulfamethoxazde, ketoconazole, omeprazole) were performed to clarify the individual CYP450 isoform that are responsible for the metabolism of FKA.

RESULTS

FKA showed significant inhibition on CYP1A2, CYP2D1, CYP2C6 and CYP3A2 activities with IC50 values of 102.23, 20.39, 69.95, 60.22μmol/L, respectively. The inhibition model was competitive, mixed-inhibition, uncompetitive, and noncompetitive for CYP1A2, CYP2D1, CYP2C6 and CYP3A2 enzymes. Molecular docking study indicated the ligand-binding conformation of FKA in the active site of CYP450 isoforms. The chemical inhibition experiments showed that the metabolic clearance rate of Flavokawain A decreased to 19.84%, 50.38%, and 67.02% of the control in the presence of ketoconazole, sulfamethoxazde and a-naphthoflavone.

CONCLUSION

The study showed that Flavokawain A has varying inhibitory effect on CYP450 enzymes and CYP3A2 was the principal CYP isoform contributing to the metabolism of Flavokawain A. Besides, CYP2C6 and CYP1A2 isoforms also play important roles in the metabolism of FKA. Our results provided a basis for better understanding the biotransformation of FKA and prediction of drug-drug interaction of FKA.

[Separation and evaluation of antioxidant constituents from Carthamus tinctorius]

Bio-active components from Carthamus tinctorius were separated on the basis of antioxidant capacities in vitro. The antioxidant capacity was investigated on the basis of the ability to scavenge DPPH radical, ABTS radical and reduce Fe3+ of different polar fractions. Furthermore, the chemical compounds were isolated from bio-active fraction, and were evaluated for the antioxidative effects. Five major components were isolated and identified from water extract as 6-hydroxykaempferol 3,6,7-tri-O-β-D-glucoside(1), 6-hydroxykaempferol 3-O-β-rutinoside-6-O-β-D-glucoside (2), 6-hydroxykaempferol 3-O-β-D-glucoside (3), hydroxysafflor yellow A (4) and anhydrosafflor yellow B (5). By evaluating and comparing the antioxidative effects of different fractions and obtained compounds, the results showed that water extract displayed significantly high antioxidative activities and 6-hydroxykaempferol glycosides and quinochalcone C-glycosides were found as main contribution for antioxidant property.

[C-ring cleavage of liquiritigenin extracted from licorice roots by an oxygen-tolerant bovine rumen bacterium strain Aeroto-Niu-O16]

Aeroto-Niu-O16, an oxygen-tolerant bovine rumen bacterium, is capable of aerobically reducing isoflavones daidzein and genistein to dihydrodaidzein and dihydrogenistein through catalytic hydrogenation. In this study, it was found that bacterium strain Aeroto-Niu-O16 was able to cleavage the C-ring of liquiritigenin (LG), which is one of the main biologically active components of licorice roots, in the presence of atmospheric oxygen. LG was prepared by acid hydrolysis of the crude extract of licorice roots. The metabolite of LG obtained in strain Aeroto-Niu-O16 was identified as davidigenin (DG) based on the data of UV, MS, 1H and 13C NMR. The maximal concentration of LG that the strain Aeroto-Niu-O16 was able to transform effectively was 0.8 mmol x L(-1) and the average productivity of the metabolite DG was 71.7%. Furthermore, when 0.1% (m/v) of L-cysteine or sodium thiosulfate was added in the cultural medium, the average bioconversion rate of LG was increased from 71.7% to 78.3% and 77.2%, respectively. The in vitro antioxidant investigation showed that 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity of DG was significantly or extremely significantly higher than that of LG at the concentrations from 0.2 mmol x L(-1) to 1.6 mmol x L(-1). We discoverd for the first time that LG can be converted to DG, which has stronger and wider biological activities, through microbial biotransformation method.

Relationship between the composition of flavonoids and flower colors variation in tropical water lily (Nymphaea) cultivars

Water lily, the member of the Nymphaeaceae family, is the symbol of Buddhism and Brahmanism in India. Despite its limited researches on flower color variations and formation mechanism, water lily has background of blue flowers and displays an exceptionally wide diversity of flower colors from purple, red, blue to yellow, in nature. In this study, 34 flavonoids were identified among 35 tropical cultivars by high-performance liquid chromatography (HPLC) with photodiode array detection (DAD) and electrospray ionization mass spectrometry (ESI-MS). Among them, four anthocyanins: delphinidin 3-O-rhamnosyl-5-O-galactoside (Dp3Rh5Ga), delphinidin 3-O-(2"-O-galloyl-6"-O-oxalyl-rhamnoside) (Dp3galloyl-oxalylRh), delphinidin 3-O-(6"-O-acetyl-β-glucopyranoside) (Dp3acetylG) and cyanidin 3- O-(2"-O-galloyl-galactopyranoside)-5-O-rhamnoside (Cy3galloylGa5Rh), one chalcone: chalcononaringenin 2'-O-galactoside (Chal2'Ga) and twelve flavonols: myricetin 7-O-rhamnosyl-(1 → 2)-rhamnoside (My7RhRh), quercetin 7-O-galactosyl-(1 → 2)-rhamnoside (Qu7GaRh), quercetin 7-O-galactoside (Qu7Ga), kaempferol 7-O-galactosyl-(1 → 2)-rhamnoside (Km7GaRh), myricetin 3-O-galactoside (My3Ga), kaempferol 7-O-galloylgalactosyl-(1 → 2)-rhamnoside (Km7galloylGaRh), myricetin 3-O-galloylrhamnoside (My3galloylRh), kaempferol 3-O-galactoside (Km3Ga), isorhamnetin 7-O-galactoside (Is7Ga), isorhamnetin 7-O-xyloside (Is7Xy), kaempferol 3-O-(3"-acetylrhamnoside) (Km3-3"acetylRh) and quercetin 3-O-acetylgalactoside (Qu3acetylGa) were identified in the petals of tropic water lily for the first time. Meanwhile a multivariate analysis was used to explore the relationship between pigments and flower color. By comparing, the cultivars which were detected delphinidin 3-galactoside (Dp3Ga) presented amaranth, and detected delphinidin 3'-galactoside (Dp3'Ga) presented blue. However, the derivatives of delphinidin and cyanidin were more complicated in red group. No anthocyanins were detected within white and yellow group. At the same time a possible flavonoid biosynthesis pathway of tropical water lily was presumed putatively. These studies will help to elucidate the evolution mechanism on the formation of flower colors and provide theoretical basis for outcross breeding and developing health care products from this plant.

Simultaneous post-transcriptional gene silencing of two different chalcone synthase genes resulting in pure white flowers in the octoploid dahlia

Garden dahlias (Dahlia variabilis) are autoallooctoploids with redundant genes producing wide color variations in flowers. There are no pure white dahlia cultivars, despite its long breeding history. However, the white areas of bicolor flower petals appear to be pure white. The objective of this experiment was to elucidate the mechanism by which the pure white color is expressed in the petals of some bicolor cultivars. A pigment analysis showed that no flavonoid derivatives were detected in the white areas of petals in a star-type cultivar 'Yuino' and the two seedling cultivars 'OriW1' and 'OriW2' borne from a red-white bicolor cultivar, 'Orihime', indicating that their white areas are pure white. Semi-quantitative RT-PCR showed that in the pure white areas, transcripts of two chalcone synthases (CHS), DvCHS1 and DvCHS2 which share 69% nucleotide similarity with each other, were barely detected. Premature mRNA of DvCHS1 and DvCHS2 were detected, indicating that these two CHS genes are silenced post-transcriptionally. RNA gel blot analysis revealed that small interfering RNAs (siRNAs) derived from CHSs were produced in these pure white areas. By high-throughput sequence analysis of small RNAs in the pure white areas with no mismatch acceptance, small RNAs were mapped to two alleles of DvCHS1 and two alleles of DvCHS2 expressed in 'Yuino' petals. Therefore, we concluded that simultaneous siRNA-mediated post-transcriptional gene silencing of redundant CHS genes results in the appearance of pure white color in dahlias.

Simultaneous post-transcriptional gene silencing of two different chalcone synthase genes resulting in pure white flowers in the octoploid dahlia

Garden dahlias (Dahlia variabilis) are autoallooctoploids with redundant genes producing wide color variations in flowers. There are no pure white dahlia cultivars, despite its long breeding history. However, the white areas of bicolor flower petals appear to be pure white. The objective of this experiment was to elucidate the mechanism by which the pure white color is expressed in the petals of some bicolor cultivars. A pigment analysis showed that no flavonoid derivatives were detected in the white areas of petals in a star-type cultivar 'Yuino' and the two seedling cultivars 'OriW1' and 'OriW2' borne from a red-white bicolor cultivar, 'Orihime', indicating that their white areas are pure white. Semi-quantitative RT-PCR showed that in the pure white areas, transcripts of two chalcone synthases (CHS), DvCHS1 and DvCHS2 which share 69% nucleotide similarity with each other, were barely detected. Premature mRNA of DvCHS1 and DvCHS2 were detected, indicating that these two CHS genes are silenced post-transcriptionally. RNA gel blot analysis revealed that small interfering RNAs (siRNAs) derived from CHSs were produced in these pure white areas. By high-throughput sequence analysis of small RNAs in the pure white areas with no mismatch acceptance, small RNAs were mapped to two alleles of DvCHS1 and two alleles of DvCHS2 expressed in 'Yuino' petals. Therefore, we concluded that simultaneous siRNA-mediated post-transcriptional gene silencing of redundant CHS genes results in the appearance of pure white color in dahlias.

Study of DNA interactions with cyclic chalcone derivatives by spectroscopic techniques

A series of chalcone derivatives (1-4) were studied. The interaction between these ligands and calf thymus DNA was studied with UV-vis spectrophotometry, fluorescence and circular dichroism spectroscopy. The binding constants K were estimated at 0.5-4.6×10(5) M(-1). All these measurements indicated that the compounds behave as effective DNA-intercalating agents. Electrophoretic separation proved that ligands inhibited topoisomerase I at a concentration of 60 μM.

Cell line studies of glioblastoma and malignant glioma using a newly discovered prenylated chalcone

CONTEXT

A newly discovered geranyl prenylated chalcone, semisynthesized from naturally occurring nymphaeol C, has the ability to inhibit the growth of CNS1 (glioblastoma) and 13-06 (malignant glioma) cells. A second-order regression model was established to predict the normalized cell viability of CNS1 and 13-06 cells.

OBJECTIVE

The goal of this study is to evaluate the influence of prenylated chalcone on the glioblastoma and malignant glioma cell lines. For the first time, response surface methodology (RSM) has been introduced to perform a cell line study.

MATERIALS AND METHODS

A newly discovered prenylated chalcone was used. This compound is a member of the flavonoid family and possesses a common phenylbenzopyrone structure. Two independent factors, including prenylated chalcone concentration and uptake time, were carefully evaluated by a 2² factorial design. RSM was introduced as a new method for CNS1 and 13-06 cell line studies.

RESULTS

For CNS1 cells, the least inhibition uptake time was 20.7 h, and the least inhibition dose was 12.4 μg/ml. For 13-06 cells, the best inhibition uptake time was 26.2 h, and the least inhibition dose was 12.0 μg/ml.

DISCUSSION AND CONCLUSION

The RSM model successfully predicted the normalized cell viability of CNS1 and 13-06 cells through the use of prenylated chalcone. The results obtained in this study will be useful for further studies on the use of prenylated chalcone.

A continuous spectrophotometric assay for determination of the aureusidin synthase activity of tyrosinase

INTRODUCTION

Aurones (aureusidin glycosides) are plant flavonoids that provide yellow colour to the flowers of some ornamental plants. In this study we analyse the capacity of tyrosinase to catalyse the synthesis of aureusidin by tyrosinase from the chalcone THC (2',4',6',4-tetrahydroxychalcone).

OBJECTIVE

To develop a simple continuous spectrophotometric assay for the analysis of the spectrophotometric and kinetic characteristics of THC oxidation by tyrosinase.

METHODOLOGY

THC oxidation was routinely assayed by measuring the increase in absorbance at 415 nm vs. reaction time.

RESULTS

According to the mechanism proposed for tyrosinase, the enzymatic reaction involves the o-hydroxylation of the monophenol THC to the o-diphenol (PHC, 2',4',6',3,4 - pentahydroxychalcone), which is then oxidised to the corresponding o-quinone in a second enzymatic step. This product is highly unstable and thus undergoes a series of fast chemical reactions to produce aureusidin. In these experimental conditions, the optimum pH for THC oxidation is 4.5. The progress curves obtained for THC oxidation showed the appearance of a lag period. The following kinetic parameters were also determined: K(m )= 0.12 mM, V(m )= 13 microM/min, V(m)/K(m )= 0.11/min.

CONCLUSION

This method has made it possible to analyse the spectrophotometric and kinetic characteristics of THC by tyrosinase. This procedure has the advantages of a short analysis time, straightforward measurement techniques and reproducibility. In addition, it also allows the study of tyrosinase inhibitors, such as tropolone.

Hypoxia-inducible factor-1 inhibitory benzofurans and chalcone-derived diels-alder adducts from Morus species

Hypoxia-inducible factor-1 (HIF-1) is the central mediator of cellular responses to low oxygen concentrations and vital to many aspects of cancer biology. Bioassay-guided fractionation of the chloroform-soluble extracts of Morus species using a hypoxia response element (HRE)-dependent reporter assay led to identification of six benzofurans (1-6) and two chalcone-derived Diels-Alder adducts (7, 8) from Mori Cortex Radicis and three prenylated benzofurans (9-11) and four chalcone-derived Diels-Alder adducts (12-15) from Morus bombycis. The structure of the new 2-arylbenzofuran-type compound, moracin Q (3), was elucidated by spectroscopic methods, and the absolute configuration of 2 was determined for the first time. The selected compounds (1-3, 5, 7, 9, 10, and 12) from the cell-based reporter assay were found to inhibit hypoxia-induced HIF-1alpha accumulation in a dose-dependent manner in human hepatocelluar carcinoma cell-line Hep3B cells. Furthermore, these compounds were also active against hypoxia-induced vascular endothelial growth factor (VEGF) secretion in Hep3B cells.

Microbial metabolism of the prenylated chalcone xanthohumol

Microbial metabolism of xanthohumol (1), a prenylated chalcone isolated from hops, gave three novel glucosylated derivatives (2-4) and a known compound, isoxanthohumol (5). The structures of the new compounds were identified as xanthohumol 4'-O-beta-glucopyranoside (2), xanthohumol 4,4'-O-beta-diglucopyranoside (3), and 5-methoxy-8-prenylnaringenin 7-O-beta-glucopyranoside (4) on the basis of spectroscopic methods.

Yellow flowers generated by expression of the aurone biosynthetic pathway

Flower color is most often conferred by colored flavonoid pigments. Aurone flavonoids confer a bright yellow color on flowers such as snapdragon (Antirrhinum majus) and dahlia (Dahlia variabilis). A. majus aureusidin synthase (AmAS1) was identified as the key enzyme that catalyzes aurone biosynthesis from chalcones, but transgenic flowers overexpressing AmAS1 gene failed to produce aurones. Here, we report that chalcone 4'-O-glucosyltransferase (4'CGT) is essential for aurone biosynthesis and yellow coloration in vivo. Coexpression of the Am4'CGT and AmAS1 genes was sufficient for the accumulation of aureusidin 6-O-glucoside in transgenic flowers (Torenia hybrida). Furthermore, their coexpression combined with down-regulation of anthocyanin biosynthesis by RNA interference (RNAi) resulted in yellow flowers. An Am4'CGT-GFP chimeric protein localized in the cytoplasm, whereas the AmAS1(N1-60)-RFP chimeric protein was localized to the vacuole. We therefore conclude that chalcones are 4'-O-glucosylated in the cytoplasm, their 4'-O-glucosides transported to the vacuole, and therein enzymatically converted to aurone 6-O-glucosides. This metabolic pathway is unique among the known examples of flavonoid, including anthocyanin biosynthesis because, for all other compounds, the carbon backbone is completed before transport to the vacuole. Our findings herein not only demonstrate the biochemical basis of aurone biosynthesis but also open the way to engineering yellow flowers for major ornamental species lacking this color variant.

Biocatalytic synthesis of butein and sulfuretin by Aspergillus alliaceus

Aspergillus alliaceus UI315 was examined for its potential to catalyze biotransformation reactions of chalcones that mimic plant biosynthetic processes. 3-(4' '-Hydroxyphenyl)-1-(2',4'-dihydroxyphenyl)propenone (4,2',4'-trihydroxychalcone, isoliquiritigein) (1) was efficiently transformed to two major metabolites that were isolated chromatographically and identified by spectroscopic methods as 3-(3' ',4' '-dihydroxyphenyl)-1-(2',4'-dihydroxyphenyl)propenone (butein) (7) and 2-[(3,4-dihydroxyphenyl)methylene]-6-hydroxy-3(2H)benzofuranone (7,3',4'-trihydroxyaurone, sulfuretin) (10). Inhibition experiments suggested that initial C-3 hydroxylation of 1 to 7 was catalyzed by a cytochrome P450 enzyme system. A second A. alliaceus enzyme, partially purified and identified as a catechol oxidase, catalyzed the oxidation of the catechol butein (7) likely through an ortho-quinone (8) that cyclized to the aurone product 10. This work showed that A. alliaceus UI315 contains oxidative enzyme systems capable of converting phenolic chalcones such as 1 into aurones such as 10 in a process that mimics plant biosynthetic pathways.

Synthesis and PPAR-gamma ligand-binding activity of the new series of 2'-hydroxychalcone and thiazolidinedione derivatives

Fifteen chalcones and three thiazolidinedione (TZD) chalcones were prepared to evaluate their peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligand-binding activities. Among the three TZDs, one compound possessed PPAR-gamma transactivation potential, while the others showed antagonistic activity against PPAR-gamma transactivation. Among the chalcones, compound 5 was the most potent, and structure-activity relationship studies indicated that a methoxyl group in position C-4 and hydroxyl group in position C-4' or 5' in chalcone plays a key role in determining the potency of PPAR-gamma activation.

Genista tinctoria hairy root cultures for selective production of isoliquiritigenin

Hairy root cultures were established after inoculation of Genista tinctoria in vitro shoots with Agrobacterium rhizogenes, strain ATCC 15834. In transformed roots of G. tinctoria grown in Schenk-Hildebrandt medium without growth regulators the biosynthesis of isoflavones, derivatives of genistein and daidzein, and flavones, derivatives of luteolin and apigenin, characteristic for the intact plant, was completely inhibited. The only compound synthesized in G. tinctoria hairy roots was isoliquiritigenin (2.3 g/100 g DW), a daidzein precursor absent in the intact plant. This compound was stored entirely within cells and it was not until abscisic acid was added (37.8 microM supplement on day 42) that approx. 80% of it was released into the experimental medium. The paper discusses the effect of abscisic acid on the growth of G. tinctoria hairy root cultures, the biosynthesis of isoliquiritigenin and the way it is stored. A prototype basket-bubble bioreactor was designed and built to upgrade the scale of the G. tinctoria hairy root cultures. With immobilized roots and a new aeration system, large amounts of biomass were obtained (FWmax 914.5 g l(-1)) which produced high contents of isoliquiritigenin (2.9 g/100 g DW). The abscisic acid-induced release of the metabolite from the tissue into the growth medium greatly facilitated subsequent extraction and purification of isoliquiritigenin.

Asymmetric enone epoxidation by short solid-phase bound peptides: Further evidence for catalyst helicity and catalytic activity of individual peptide strands

In the presence of short solid-phase bound peptide catalysts, the Juliá-Colonna epoxidation of enones (such as chalcone) with hydrogen peroxide can be performed with high enantiomeric excess (> or = 95% ee). It was proposed earlier (A. Berkessel, N. Gasch, K. Glaubitz, C. Koch, Organic Letters, 2001, Vol. 3, pp. 3839-3842) that this remarkable catalysis is governed by the N-terminus of individual and helical peptide strands. This mechanistic proposal was scrutinized further. (i) Nonaggregation of the peptide catalysts: five solid-phase bound statistic mixtures (0/100; 30/70; 50/50; 70/30; 100/0) of D-Leu and L-Leu heptamers were generated and assayed as catalysts. A linear dependence of the epoxide ee on the enantiomeric composition of the catalysts resulted. (ii) Catalyst helicity [introduction of the helix-stabilizing C(alpha)-methyl-L-leucine, L-(alphaMe)Leu]: solid-phase bound Leu/(alphaMe)Leu-pentamers of composition TentaGel-NH-[(alphaMe)-L-Leu]n-(L-Leu)m-H (n = 0-4; m = 5-n) were prepared and assayed as catalysts. The introduction of up to two (alphaMe)-L-Leu residues (n = 1, 2) significantly enhanced the catalyst activity relative to the L-Leu homopentamer (n = 0). Higher (alphaMe)-L-Leu contents (n = 3, 4) led to a decrease in both catalyst activity and enantiopurity of the product epoxide. In summary, both the individual catalytic action of the peptide strands and the helical conformation as the catalytically competent state of the peptide catalysts were further supported.

Flavonoid components and flower color change in transgenic tobacco plants by suppression of chalcone isomerase gene

A cDNA encoding chalcone isomerase (CHI) was isolated from the petals of Nicotiana tabacum and the effect of its suppression on flavonoid biosynthesis was analyzed in transgenic tobacco plants. CHI-suppression by RNA interference (RNAi) showed reduced pigmentation and change of flavonoid components in flower petals. The plants also accumulated high levels of chalcone in pollen, showing a yellow coloration. Our results first demonstrated that suppression of CHI by genetic transformation is possible in higher plants. This suggests that CHI plays a major part in the cyclization reaction from chalcone to flavanone, and that spontaneous reactions are few, if any, in tobacco plants.

Modifying specific cysteines of the electrophile-sensing human Keap1 protein is insufficient to disrupt binding to the Nrf2 domain Neh2

The risks of cancer and other degenerative diseases caused by reactive oxygen species and electrophiles can be reduced by the up-regulation of detoxifying enzymes. A major mechanism whereby these protective enzymes are induced occurs through activation of the antioxidant response element (ARE) by the oxidative-stress sensor protein Kelch-like ECH-associated protein 1 (Keap1) and the transcription factor NF-E2-related factor 2 (Nrf2). Under basal conditions, Keap1 sequesters Nrf2 in the cytoplasm by binding to its Neh2 domain. Chemical inducers such as sulforaphane are known to react with Keap1 cysteine residues, thereby promoting Nrf2 nuclear accumulation and hence ARE activation. A widely accepted model for Nrf2 nuclear accumulation is that modification of Keap1 cysteines leads directly to dissociation of the Keap1-Nrf2 complex. This model is based on studies with mouse proteins and has served as the experimental basis and hypothesis for numerous investigations. Through a combination of chemical, mass spectrometry, and isothermal titration calorimetry methods, we have tested the direct-dissociation model using a series of ARE inducers: sulforaphane, isoliquiritigenin, 15-deoxy-Delta12,14-prostaglandin-J2, menadione, 1-Cl-2,4-dinitrobenzene, and biotinylated iodoacetamide. Surprisingly, these data suggest that the direct disruption model for Keap1-Nrf2 is incorrect. The relative reactivity of human Keap1 cysteines was determined. In addition to the same five cysteines identified for mouse Keap1, two highly reactive and previously unobserved cysteines were identified. Based on these results, a model is proposed that should aid in the understanding of Keap1-Nrf2 signaling mechanisms.

Structural elucidation of chalcone reductase and implications for deoxychalcone biosynthesis

4,2',4',6'-Tetrahydroxychalcone (chalcone) and 4,2',4'-trihydroxychalcone (deoxychalcone) serve as precursors of ecologically important flavonoids and isoflavonoids. Deoxychalcone formation depends on chalcone synthase and chalcone reductase; however, the identity of the chalcone reductase substrate out of the possible substrates formed during the multistep reaction catalyzed by chalcone synthase remains experimentally elusive. We report here the three-dimensional structure of alfalfa chalcone reductase bound to the NADP+ cofactor and propose the identity and binding mode of its substrate, namely the non-aromatized coumaryl-trione intermediate of the chalcone synthase-catalyzed cyclization of the fully extended coumaryl-tetraketide thioester intermediate. In the absence of a ternary complex, the quality of the refined NADP+-bound chalcone reductase structure serves as a template for computer-assisted docking to evaluate the likelihood of possible substrates. Interestingly, chalcone reductase adopts the three-dimensional structure of the aldo/keto reductase superfamily. The aldo/keto reductase fold is structurally distinct from all known ketoreductases of fatty acid biosynthesis, which instead belong to the short-chain dehydrogenase/reductase superfamily. The results presented here provide structural support for convergent functional evolution of these two ketoreductases that share similar roles in the biosynthesis of fatty acids/polyketides. In addition, the chalcone reductase structure represents the first protein structure of a member of the aldo/ketoreductase 4 family. Therefore, the chalcone reductase structure serves as a template for the homology modeling of other aldo/keto-reductase 4 family members, including the reductase involved in morphine biosynthesis, namely codeinone reductase.

METABOLISM OF THE α,β-UNSATURATED KETONES, CHALCONE AND TRANS-4-PHENYL-3-BUTEN-2-ONE, BY RAT LIVER MICROSOMES AND ESTROGENIC ACTIVITY OF THE METABOLITES

When chalcone and trans-4-phenyl-3-buten-2-one (PBO) were incubated with liver microsomes of untreated rats in the presence of NADPH, 4-hydroxychalcone and trans-4-(4-hydroxyphenyl)-3-buten-2-one (4-OH-PBO), respectively, were formed as major metabolites. Two minor metabolites of chalcone, 4'-hydroxychalcone and 2-hydroxychalcone, were also observed. The oxidase activity affording 4-hydroxychalcone was inhibited by SKF 525-A, disulfiram, ketoconazole, and alpha-naphthoflavone. The oxidase activities leading to 4-hydroxychalcone and 4'-hydroxychalcone were enhanced in liver microsomes of 3-methylcholanthrene- and phenobarbital-treated rats, respectively. The activity generating 2-hydroxychalcone was enhanced in liver microsomes of 3-methylcholanthrene- and dexamethasone-treated rats. The oxidation of PBO to 4-OH-PBO was inhibited by SKF 525-A, ketoconazole, disulfiram, and sulfaphenazole. This activity was enhanced in liver microsomes of 3-methylcholanthrene-, acetone- and phenobarbital-treated rats. 4-Hydroxylation, 4'-hydroxylation, and 2-hydroxylation of chalcone were catalyzed by rat recombinant cytochrome P450 1A1, 1A2, and 2C6; by 1A1 and 2C6; and by 1A1 and 3A1, respectively. PBO was oxidized by cytochrome P450 1A1, 1A2, 2C6, and 2E1. Chalcone and PBO were negative in an estrogen reporter assay using estrogen-responsive human breast cancer cell line MCF-7. However, 4-hydroxychalcone, 2-hydroxychalcone, 4'-hydroxychalcone, and 4-OH-PBO exhibited estrogenic activity.

Degradation of neohesperidin dihydrochalcone by human intestinal bacteria

The degradation of neohesperidin dihydrochalcone by human intestinal microbiota was studied in vitro. Human fecal slurries converted neohesperidin dihydrochalcone anoxically to 3-(3-hydroxy-4-methoxyphenyl)propionic acid or 3-(3,4-dihydroxyphenyl)propionic acid. Two transient intermediates were identified as hesperetin dihydrochalcone 4'-beta-d-glucoside and hesperetin dihydrochalcone. These metabolites suggest that neohesperidin dihydrochalcone is first deglycosylated to hesperetin dihydrochalcone 4'-beta-d-glucoside and subsequently to the aglycon hesperetin dihydrochalcone. The latter is hydrolyzed to the corresponding 3-(3-hydroxy-4-methoxyphenyl)propionic acid and probably phloroglucinol. Eubacterium ramulus and Clostridium orbiscindens were not capable of converting neohesperidin dihydrochalcone. However, hesperetin dihydrochalcone 4'-beta-d-glucoside was converted by E. ramulus to hesperetin dihydrochalcone and further to 3-(3-hydroxy-4-methoxyphenyl)propionic acid, but not by C. orbiscindens. In contrast, hesperetin dihydrochalcone was cleaved to 3-(3-hydroxy-4-methoxyphenyl)propionic acid by both species. The latter reaction was shown to be catalyzed by the phloretin hydrolase from E. ramulus.

Probing biosynthesis of plant polyketides with synthetic N-acetylcysteamine thioesters

Recombinant chalcone synthase (CHS) from Scutellaria baicalensis accepted cinnamoyl diketide-NAC and cinnamoyl-NAC as a substrate, and carried out sequential condensations with malonyl-CoA to produce 2',4',6'-trihydroxychalcone. Steady-state kinetic analysis revealed that the CHS accepted the diketide-NAC with less efficiency, while cinnamoyl-NAC primed the enzyme reaction almost as efficiently as cinnamoyl-CoA. On the other hand, it was for the first time demonstrated that the diketide-NAC was also a substrate for recombinant polyketide reductase (PKR) from Glycyrrhiza echinata, and converted to the corresponding beta-ketohemithioester. Furthermore, by co-action of the CHS and the PKR, the NAC-thioesters were converted to 6'-deoxychalcone in the presence of NADPH and malonyl-CoA.

Isoliquiritigenin induces apoptosis and cell cycle arrest through p53-dependent pathway in Hep G2 cells

Isoliquiritigenin (ISL) is a natural pigment with the simple chalcone structure 4,2',4'-trihydroxychalcone. In this study, we report ISL induced inhibition in the proliferation of human hepatoma cells (Hep G2) for the first time. The cell proliferation inhibition achieved by ISL treatment resulted in a G2/M-phase arrest and programmed cell death. ISL treatment was found to result in the upregulation of p53, p21/WAF1, Fas/APO-1 receptor, Fas ligand, Bax and NOXA, but not in Bad. To elevate the role of p53 in these functions, we generated Hep G2 cells that express the dominant negative p53, which blocks the transcriptional activity of p53. The enhancement of p21/WAF1, Fas/APO-1, Bax and NOXA were decreased in Hep G2 cells that lack functional p53. Furthermore, Hep G2 cells were significantly more resistant to ISL when the activity of p53 was blocked. These results demonstrated that ISL-inducible p53 plays a key apoptotic role, and may do so by regulating the expression of specific target molecules that promotes efficient apoptotic cell death following G2/M-cell cycle arrest.

First bacterial chalcone isomerase isolated from Eubacterium ramulus

The human fecal anaerobe Eubacterium ramulus is capable of degrading various flavonoids, including the flavone naringenin. The first step in the proposed degradation pathway is the isomerization of naringenin to the corresponding chalcone. Cell-free extracts of E. ramulus displayed chalcone isomerase activity. The enzyme from E. ramulus was purified to homogeneity. Its apparent molecular mass was estimated to be 136 and 129 kDa according to gel filtration and native polyacrylamide gel electrophoresis, respectively. Chalcone isomerase is composed of one type of subunit of 30 kDa. The purified enzyme catalyzed the isomerization of naringenin chalcone, isoliquiritigenin, and butein, three chalcones that differ in their hydroxylation pattern. N-bromosuccinimide, but also naringenin and phloretin, inhibited the purified enzyme considerably. This is the first report on a bacterial chalcone isomerase. The physiological function of the purified enzyme is unclear, but an involvement in the conversion of the flavanone naringenin to the chalcone is proposed.

Microbial transformations of chalcones: hydroxylation, O-demethylation, and cyclization to flavanones

Microorganisms were examined for their potential to catalyze biotransformation reactions that mimic plant biosynthetic processes. Specifically, microorganisms were screened for their abilities to transform selected chalcones to flavonoid and other products. Aspergillus alliaceus UI 315 efficiently transformed 3-(2' ',3' '-dimethoxyphenyl)-1-(2'-hydroxyphenyl)propenone (2'-hydroxy-2,3-dimethoxychalcone) (1) to several products, all of which were characterized by UV, NMR, and mass spectral analyses. A. alliaceus cyclized 1 to three flavanones and to O-demethylated and hydroxylated chalcones, some of which functioned as intermediates in the cyclization process. Inhibition studies using SKF525A, metyrapone, and phenylthiocarbamide with whole cell reactions showed that as many as three cytochrome P450 enzymes may be involved in these reactions. One enzyme catalyzed chalcone cyclization; another, O-demethylation; and a third, hydroxylation of chalcones. Flavonoid products are racemic, unlike the same products that are stereoselectively cyclized in plants. This work shows that microorganisms are capable of cyclizing chalcones to form flavonoid products, thus affording a mimic of plant biosynthetic processes.

Natural antimycobacterial metabolites: current status

Over the years the introduction of very effective drugs has revolutionized the treatment of tuberculosis. In recent years, however, emerging multiple drug resistance has become a major threat and thus calls for an urgent search for new and effective treatments for this deadly disease. This review is complementary to earlier reviews and covers more recent reports of naturally occurring compounds, and in some cases synthetic analogs, largely from plants, fungi and marine organisms that demonstrate significant activity in the in vitro bioassays against Mycobacterium tuberculosis, and other mycobacterial species. Included also are traditional medicinal uses of specific plants when utilized to treat tuberculosis and other pulmonary diseases.

Chalcone dimethylallyltransferase fromMorus nigracell cultures. Substrate specificity studies

A new prenyltransferase (PT) enzyme derived from the microsomal fractions of cell cultures of Morus nigra was shown to be able to prenylate exclusively chalcones with a 2',4'-dihydroxy substitution and the isoflavone genistein. Computational studies were performed to shed some light on the relationship between the structure of the substrate and the enzymatic activity. PT requires divalent cations, particularly Mg(2+), to be effective. The apparent K(m) values for gamma,gamma-dimethylallyldiphosphate and 2',4'-dihydroxychalcone were 63 and 142 microM, respectively. The maximum activity of the enzyme was expressed during the first 10 days of cell growth.

Influence of industrial processing on orange juice flavanone solubility and transformation to chalcones under gastrointestinal conditions

Orange juice manufactured at industrial scale was subjected to digestion under in vitro gastrointestinal conditions (pH, temperature, and enzyme and chemical conditions) to evaluate the influence of individual industrial processing treatments on flavanone solubility, stability, and ability to permeate through a membrane under simulated physiological conditions. Four industrial processes including squeezing, standard pasteurization, concentration, and freezing were evaluated. Hand squeezing was compared with industrial squeezing. After in vitro gastrointestinal digestion of the orange juices, the flavanones able to permeate through a dialysis membrane, and those remaining in the retentate were evaluated by HPLC as were those present in the insoluble fraction. In all of the assayed orange juices, a high content of precipitated chalcones ( approximately 70% of the total flavanones) was formed under the physiological conditions of the gastrointestinal tract. Hand squeezing provided a higher concentration of flavanones in the permeated fraction and lower transformation to chalcones than industrial squeezing. Standard pasteurization did not influence the solubility and permeability of the orange juice flavanones and chalcones. Industrial concentration did not affect the amount of flavanones able to permeate but decreased the chalcones produced. Juices produced from frozen orange juice contained considerably smaller amounts of both soluble flavanones and insoluble chalcones.

Enzymes do what is expected (chalcone isomerase versus chorismate mutase)

Madicago sativa chalcone isomerase (CI) catalyzes the isomerization of chalcone to flavanone, whereas E. coli chorismate mutase (CM) catalyzes the pericyclic rearrangement of chorismate to prephenate. Covalent intermediates are not formed in either of the enzyme-catalyzed reactions, K(M) and k(cat) are virtually the same for both enzymes, and the rate constants (k(o)) for the noncatalyzed reactions in water are also the same. This kinetic identity of both the enzymatic and the nonenzymatic reactions is not shared by a similarity in driving forces. The efficiency (DeltaG(o)() - DeltaG(cat)()) for the CI mechanism involves transition-state stabilization through general-acid catalysis and freeing of three water molecules trapped in the E.S species. The contribution to lowering DeltaG(cat)() by an increase in near attack conformer (NAC) formation in E.S as compared to S in water is not so important. In the CM reaction, the standard free energy for NAC formation in water is 8.4 kcal/mol as compared to 0.6 kcal/mol in E.S. Because the value of (DeltaG(o)() - DeltaG(cat)()) is 9 kcal/mol, the greater percentage of NACs accounts for approximately 90% of the kinetic advantage of the CM reaction. There is no discernible transition-state stabilization in the CM reaction. These results are discussed. In anthropomorphic terms, each enzyme has had to do what it must to have a biologically relevant rate of reaction.

Stilbenecarboxylate biosynthesis: a new function in the family of chalcone synthase-related proteins

Chalcone (CHS), stilbene (STS) synthases, and related proteins are key enzymes in the biosynthesis of many secondary plant products. Precursor feeding studies and mechanistic rationalization suggest that stilbenecarboxylates might also be synthesized by plant type III polyketide synthases; however, the enzyme activity leading to retention of the carboxyl moiety in a stilbene backbone has not yet been demonstrated. Hydrangea macrophylla L. (Garden Hortensia) contains stilbenecarboxylates (hydrangeic acid and lunularic acid) that are derived from 4-coumaroyl and dihydro-4-coumaroyl starter residues, respectively. We used homology-based techniques to clone CHS-related sequences, and the enzyme functions were investigated with recombinant proteins. Sequences for two proteins were obtained. One was identified as CHS. The other shared 65-70% identity with CHSs and other family members. The purified recombinant protein had stilbenecarboxylate synthase (STCS) activity with dihydro-4-coumaroyl-CoA, but not with 4-coumaroyl-CoA or other substrates. We propose that the enzyme is involved in the biosynthesis of lunularic acid. It is the first example of a STS-type reaction that does not lose the terminal carboxyl group during the ring folding to the end product. Comparisons with CHS, STS, and a pyrone synthase showed that it is the only enzyme exerting a tight control over decarboxylation reactions. The protein contains unusual residues in positions highly conserved in other CHS-related proteins, and mutagenesis studies suggest that they are important for the structure or/and the catalytic activity. The formation of the natural products in vivo requires a reducing step, and we discuss the possibility that the absence of a reductase in the in vitro reactions may be responsible for the failure to obtain stilbenecarboxylates from substrates like 4-coumaroyl-CoA.

Electrochemical biosensor for the interaction of DNA with the alkylating agent 4,4'-dihydroxy chalcone based on guanine and adenine signals

The interaction of an alkylating agent, 4,4'-dihydroxy chalcone (DHC) with calf thymus double stranded DNA (dsDNA) and calf thymus single stranded DNA (ssDNA) was studied electrochemically based on the oxidation signals of guanine and adenine by using differential pulse voltammetry (DPV) at carbon paste electrode (CPE). As a result of the alkylation of DHC between the base pairs in dsDNA, the voltammetric signal of guanine and adenine greatly decreased. After the interaction of DHC with ssDNA, a higher decrease in the oxidation signals of guanine and adenine was observed under the same conditions. The partition coefficients of DHC at dsDNA and ssDNA modified CPEs were calculated. The interactions of DHC with synthetic polynucleotides, such as polyguanylic acid and polyadenylic acid were also observed. In addition, the detection limit and the reproducibility were determined by using DPV. The interaction of DHC with dsDNA in solution-phase was also investigated and the results were compared with the ones obtained by surface immobilized dsDNA. The application of electrochemical DNA biosensor for monitoring the DNA-alkylating agent interactions was explored.

Enzymatic formation of an unnatural C(6)-C(5) aromatic polyketide by plant type III polyketide synthases

[reaction: see text] Substrate specificities of plant polyketide synthases (PKSs) were investigated using analogues of malonyl-CoA, the extension unit of the polyketide chain elongation reactions. When incubated with methylmalonyl-CoA and 4-coumaroyl-CoA, plant PKSs (chalcone synthase from Scutellaria baicalensis, stilbene synthase from Arachis hypogaea, and benzalacetone synthase from Rheum palmatum) afforded an unnatural C(6)-C(5) aromatic polyketide, 1-(4-hydroxyphenyl)pent-1-en-3-one, formed by one-step decarboxylative condensation of the two substrates. In contrast, succinyl-CoA was not accepted as a substrate by the enzymes.

Increasing antioxidant levels in tomatoes through modification of the flavonoid biosynthetic pathway

Flavonoids are a diverse group of phenolic secondary metabolites that occur naturally in plants and therefore form an integral component of the human diet. Many of the compounds belonging to this group are potent antioxidants in vitro and epidemiological studies suggest a direct correlation between high flavonoid intake and decreased risk of cardiovascular disease, cancer and other age-related diseases. Enhancing flavonoid biosynthesis in chosen crops may provide new raw materials that have the potential to be used in foods designed for specific benefits to human health. Using genetic modification, it was possible to generate several tomato lines with significantly altered flavonoid content and to probe the role and importance of several key enzymatic steps in the tomato flavonoid biosynthetic pathway. Most notably an up to 78-fold increase in total fruit flavonols was achieved through ectopic expression of a single biosynthetic enzyme, chalcone isomerase. In addition, chalcone synthase and flavonol synthase transgenes were found to act synergistically to up-regulate flavonol biosynthesis significantly in tomato flesh tissues.

Excision of transposable elements from the chalcone isomerase and dihydroflavonol 4-reductase genes may contribute to the variegation of the yellow-flowered carnation (Dianthus caryophyllus)

In the "Rhapsody" cultivar of the carnation, which bears white flowers variegated with red flecks and sectors, a transposable element, dTdic1, belonging to the Ac/Ds superfamily, was found within the dihydroflavonol 4-reductase (DFR) gene. The red flecks and sectors of "Rhapsody" may be attributable to a reversion to DFR activity after the excision of dTdic1. The yellow color of the carnation petals is attributed to the synthesis and accumulation of chalcone 2'-glucoside. In several of the carnation cultivars that bear yellow flowers variegated with white flecks and sectors, both the chalcone isomerase (CHI) and DFR genes are disrupted by dTdic1.

Role of hydrogen bonds in the reaction mechanism of chalcone isomerase

In flavonoid, isoflavonoid, and anthocyanin biosynthesis, chalcone isomerase (CHI) catalyzes the intramolecular cyclization of chalcones into (S)-flavanones with a second-order rate constant that approaches the diffusion-controlled limit. The three-dimensional structures of alfalfa CHI complexed with different flavanones indicate that two sets of hydrogen bonds may possess critical roles in catalysis. The first set of interactions includes two conserved amino acids (Thr48 and Tyr106) that mediate a hydrogen bond network with two active site water molecules. The second set of hydrogen bonds occurs between the flavanone 7-hydroxyl group and two active site residues (Asn113 and Thr190). Comparison of the steady-state kinetic parameters of wild-type and mutant CHIs demonstrates that efficient cyclization of various chalcones into their respective flavanones requires both sets of contacts. For example, the T48A, T48S, Y106F, N113A, and T190A mutants exhibit 1550-, 3-, 30-, 7-, and 6-fold reductions in k(cat) and 2-3-fold changes in K(m) with 4,2',4'-trihydroxychalcone as a substrate. Kinetic comparisons of the pH-dependence of the reactions catalyzed by wild-type and mutant enzymes indicate that the active site hydrogen bonds contributed by these four residues do not significantly alter the pK(a) of the intramolecular cyclization reaction. Determinations of solvent kinetic isotope and solvent viscosity effects for wild-type and mutant enzymes reveal a change from a diffusion-controlled reaction to one limited by chemistry in the T48A and Y106F mutants. The X-ray crystal structures of the T48A and Y106F mutants support the assertion that the observed kinetic effects result from the loss of key hydrogen bonds at the CHI active site. Our results are consistent with a reaction mechanism for CHI in which Thr48 polarizes the ketone of the substrate and Tyr106 stabilizes a key catalytic water molecule. Hydrogen bonds contributed by Asn113 and Thr190 provide additional stabilization in the transition state. Conservation of these residues in CHIs from other plant species implies a common reaction mechanism for enzyme-catalyzed flavanone formation in all plants.

Nitric oxide-scavenging properties of some chalcone derivatives

The implication of NO in many inflammatory diseases has been well documented. We have previously reported that some chalcone derivatives can control the iNOS pathway in inflammatory processes. In the present study, we have assessed the NO-scavenging capacity of three chalcone derivatives (CH8, CH11, and CH12) in a competitive assay with HbO(2), a well-known physiologically relevant NO scavenger. Our data identify these chalcones as new NO scavengers. The estimated second-order rate constants (k(s)) for the reaction of the three derivatives with NO is in the same range as the value obtained for HbO(2), with CH11 exerting the greatest effect. These results suggest an additional action of these compounds on NO regulation.

Reaction mechanism of chalcone isomerase. pH dependence, diffusion control, and product binding differences

Chalcone isomerase (CHI) catalyzes the intramolecular cyclization of bicyclic chalcones into tricyclic (S)-flavanones. The activity of CHI is essential for the biosynthesis of flavanone precursors of floral pigments and phenylpropanoid plant defense compounds. We have examined the spontaneous and CHI-catalyzed cyclization reactions of 4,2',4',6'-tetrahydroxychalcone, 4,2',4'-trihydroxychalcone, 2',4'-dihydroxychalcone, and 4,2'-dihydroxychalcone into the corresponding flavanones. The pH dependence of flavanone formation indicates that both the non-enzymatic and enzymatic reactions first require the bulk phase ionization of the substrate 2'-hydroxyl group and subsequently on the reactivity of the newly formed 2'-oxyanion during C-ring formation. Solvent viscosity experiments demonstrate that at pH 7.5 the CHI-catalyzed cyclization reactions of 4,2',4',6'-tetrahydroxychalcone, 4,2',4'-trihydroxychalcone, and 2',4'-dihydroxychalcone are approximately 90% diffusion-controlled, whereas cyclization of 4,2'-dihydroxychalcone is limited by a chemical step that likely reflects the higher pK(a) of the 2'-hydroxyl group. At pH 6.0, the reactions with 4,2',4',6'-tetrahydroxychalcone and 4,2',4'-trihydroxychalcone are approximately 50% diffusion-limited, whereas the reactions of both dihydroxychalcones are limited by chemical steps. Comparisons of the 2.1-2.3 A resolution crystal structures of CHI complexed with the products 7,4'-dihydroxyflavanone, 7-hydroxyflavanone, and 4'-hydroxyflavanone show that the 7-hydroxyflavanones all share a common binding mode, whereas 4'-hydroxyflavanone binds in an altered orientation at the active site. Our functional and structural studies support the proposal that CHI accelerates the stereochemically defined intramolecular cyclization of chalcones into biologically active (2S)-flavanones by selectively binding an ionized chalcone in a conformation conducive to ring closure in a diffusion-controlled reaction.

Structure and mechanism of chalcone synthase-like polyketide synthases

Polyketide synthases (PKS) produce an array of natural products with different biological activities and pharmacological properties by varying the starter and extender molecules that form the final polyketide. Recent studies of the simplest PKS, the chalcone synthase (CHS)-like enzymes involved in the biosynthesis of flavonoids, anthocyanin pigments, and antimicrobial phytoalexins, have yielded insight on the molecular basis of this biosynthetic versatility. Understanding the structure-function relationship in these PKS provides a foundation for manipulating polyketide formation and suggests strategies for further increasing the scope of polyketide biosynthetic diversity.