Novel lipoxygenase inhibitors, tetrapetalone B, C, and D from Streptomyces sp

Discovery and Heterologous Production of Tetrapetalones Provide Insights into the Formation of the Tetracyclic System

Tetrapetalones make up a unique class of pentaketide ansamycins that feature a tetracyclic skeleton and exhibit potent inhibitory activities against soybean lipoxygenase. However, a detailed biosynthetic route to tetrapetalones has not been published. Herein we report the activation of the tetrapetalones' biosynthetic gene cluster (tpt) in Streptomyces sp. S10 by promoter engineering along with constitutive expression of pathway-specific regulator genes, leading to the discovery of seven new derivatives, tetrapetalones E-K (2-8), and the known tetrapetalone A (1). In vivo gene deletion experiments and heterologous expression of the minimized tpt cluster in Streptomyces albus J1074 suggest that the tetracyclic system of tetrapetalones is probably formed spontaneously, and the regioselective glycosylation of tetrapetalones at the C-9 hydroxy group with d-rhamnose or d-rhodinose was catalyzed by the glycosyltransferase Tpt14.

Iron salt-promoted oxidation of steroidal phenols by m-chloroperbenzoic acid: a route to possible antitumor agents

A new oxidant, containing m-chloroperoxybenzoic acid (MCPBA) and an iron salt, was developed and used for oxidation of steroidal phenols to a quinol/epoxyquinol mixture. Reaction was optimized for estrone, by varying initiators (Fe-salts), reaction temperature, time and mode of MCPBA application. A series of five more substrates (17β-estradiol and its hydrophobized derivatives) was subjected to the optimized oxidation, providing corresponding p-quinols and 4β,5β-epoxyquinols in good to moderate yields. The obtained epoxyquinols were additionally transformed by oxidation, as well as the acid-catalyzed oxirane opening. In a preliminary study of the antiproliferative activity against human cancer cell lines, all newly synthesized compounds expressed moderate to high activity.

Iron salt-promoted reaction of estrone and its derivatives with meta-chloroperoxybenzoic acid was developed and epoxyquinols were further transformed. Most compounds showed in vitro antiproliferative activity.

Synthetic Studies toward the Tetrapetalones: Diastereoselective Construction of a Putative Intermediate

A strategy toward tetrapetalones was explored including a site-selective ethylenation of the silyl enol ether A to afford a quaternary stereocenter that serves in a stereogenic capacity. Regio- and diastereoselective reactions were observed in conjunction with the oxidative formation of cation B, which included subsequent selective formation of either carbon-oxygen or carbon-carbon bonds at the δ or ζ position on the seven-membered ring. The fourth ring was formed using a Stetter reaction.

Recent Advances in the Total Synthesis of Tetramic Acid-Containing Natural Products

With incredible bioactivities and fascinating structural complexities, tetramic acid- (TA-) containing natural products have attracted favorable attention among the organic chemistry community. Although the construction of the TA core is usually straightforward, the intricate C3-side chain sometimes asks for some deliberative strategy so as to fulfill an elegant total synthesis. This review mainly covers some exceptional synthetic examples for each type of natural product in recent years, showcasing the great achievements as well as unsettled obstacles in this area, in the hope of accelerating the synthetic and biological investigations for this unique type of natural product.

A comprehensive review of glycosylated bacterial natural products

A systematic analysis of all naturally-occurring glycosylated bacterial secondary metabolites reported in the scientific literature up through early 2013 is presented. This comprehensive analysis of 15 940 bacterial natural products revealed 3426 glycosides containing 344 distinct appended carbohydrates and highlights a range of unique opportunities for future biosynthetic study and glycodiversification efforts.

Studies toward the AB ring system of the tetrapetalone natural products

Synthetic efforts toward the rapid assembly of the AB ring system of the tetrapetalones is described. Key to this work was the use of [3+2] cycloaddition/oxidative extrusion methodology to furnish functionalized aryl enones. The Nazarov cyclization of these substrates was examined, and optimized to generate the AB ring carbon skeleton. Then, Pd-catalyzed cross-coupling were conducted, and conditions were identified that enabled installation of the requisite C14-N bond.

Bioassay-guided isolation, identification and molecular ligand-target insight of lipoxygenase inhibitors from leaves of Anisomeles malabarica R.Br

Background:

Anisomeles malabarica R. Br. (Lamiaceae) is extensively used in traditional medicine in major parts of India for several medicinal purposes, including their use in rheumatism.

Materials and Methods:

The air-dried leaves of A. malabarica were extracted with ethanol, defatted with n-hexane and then successively partitioned into chloroform and n-butanol fractions. Bioassay-guided fractionation and purification of chloroform fraction from A. malabarica lead to the isolation of lipoxygenase (LOX) inhibitors. The structures of isolated compounds were elucidated by ultraviolet, infrared, ¹H nuclear magnetic resonance (NMR), ¹³C NMR and mass spectrometry spectroscopic techniques and assessed further by in vitro soybean lipoxygenase (sLOX) assay. In addition, the enzyme type inhibition was evaluated through molecular docking technique as a part of computational study.

Results:

The bioactive compounds 3, 4 dihydroxy benzoic acid (1) and 4’, 5, 7-trihydroxyflavone (2) were isolated from chloroform fraction of A. malabarica, whose bioactivity was observed to be dose-dependent compared to n-butanol fraction. Among the compounds, 3, 4 dihydroxy benzoic acid showed significant sLOX inhibitory activity with 74.04% ±2.6% followed by 4’, 5, 7-trihydroxyflavone (34.68% ±1.9%). The computational analysis of compounds showed their molecular interaction with important amino acid residues and nonheme iron atom in the catalytic site of LOX by enlightening their potential binding mode at molecular level.

Conclusions:

The LOX inhibitory constituents were identified from A. malabarica by means of bioassay-guided fractionation process. The results derived from in vitro and computational experiments confirm the potential of the isolated compounds and provide additional evidence for its traditional use in inflammatory disorders.

Inhibitory effects of the ethyl acetate extract from bulbs of Scilla scilloides on lipoxygenase and hyaluronidase activities

CONTEXT

Scilla scilloides Druce (Liliaceae) is a folk medicine to treat dermal inflammation; however, the medicinal properties of this plant have not been completely established.

OBJECTIVE

The current study investigates the potent anti-inflammatory effects of S. scilloides bulbs for its traditional usage using lipoxygenase and hyaluronidase as the inflammation model. To gain insight into the active constituents, nine homoisoflavones (1-9) were subsequently tested.

MATERIALS AND METHODS

Lipoxygenase and hyaluronidase inhibition of ethyl acetate extract from the bulbs of this plant within 2000 µg/mL or homoisoflavones within 1000 µM were determined by colorimetric methods. RAW264.7 cells were incubated with 10 or 50 µM homoisoflavones plus lipopolysaccharide (LPS) for 24 h. The culture media were collected and analyzed for determination of the nitric oxide (NO) level by the colorimetric Griess method to measure the extent of inflammation.

RESULTS

The extract exhibited inhibitory effects on lipoxygenase and hyaluronidase activities with IC50 values 31.5 and 169 µg/mL, respectively. Among the nine homoisoflavones tested, four (1 and 3-5) resulted in 79.3-97.9% higher lipoxygenase inhibition than 6.7-32.7% of the others at 500 µM. Calculated IC50 values indicated 5 as the compound responsible for strong lipoxygenase inhibition with 15.8 µM as the IC50 value. In the hyaluronidase assay, all homoisoflavones tested at 1000 µM demonstrated 16.2-58.0% inhibition. Incubating the cells in the presence of all nine homoisoflavones tested at 50 µM significantly suppressed the NO production, downward to 1.5-66.0%, in the LPS-activated macrophage cells as a model.

DISCUSSION AND CONCLUSION

These results may indicate a potential role of S. scilloides for anti-inflammatory purposes.

A general diastereoselective catalytic vinylogous aldol reaction among tetramic acid-derived pyrroles

A catalytic diastereoselective aldol reaction has been developed for N1-arylated/C2-O-silylated/C3-methylated and brominated/C4-O-methylated pyrroles in its reactions with various aldehydes. Syn adducts emerge with regard to the vicinal nitrogen and oxygen heteroatom substituents. The N1-aryl residue undergoes oxidative cleavage, and the C3-bromine atom undergoes palladium-mediated coupling reactions, both without disturbing the newly created stereocenters.

Naturally occurring tetramic acid products: isolation, structure elucidation and biological activity

Natural products containing the tetramic acid core scaffold have been isolated from an assortment of terrestrial and marine species and often display wide ranging and potent biological activities including antibacterial, antiviral and antitumoral activities.

6'-Hydroxyoxosorbicillinol, a new lipoxygenase inhibitor and PGD2/LTB4 release suppressor from Penicillium sp

A new lipoxygenase inhibitor, 6'-hydroxyoxosorbicillinol (1, C(14) H(16)O(6)), was identified from a culture of Penicillium sp. A known compound, oxosorbicillinol (2, C(14)H(16)O(5)), was also isolated. Compound 1 showed an approximately 10 times greater inhibitory effect on soybean lipoxygenase (IC(50), 16 µM) than 2 (IC(50), 150 µM), and also showed prostaglandin D(2) (PGD(2)) and leucotriene B(4) (LTB(4)) release suppression activity (IC(50), 10 µM for PGD(2) and 100 µM for LTB(4)).

Lipoxygenase inhibitory activity of crude bark extracts and isolated compounds from Commiphora berryi

ETHNOPHARMACOLOGICAL RELEVANCE

Commiphora berryi is traditionally used for the treatment of cold and fever as well as for wound healing in the southern parts of India.

AIM OF STUDY

The present study was designed to investigate in vitro soybean lipoxygenase inhibitory activity of crude extracts and compounds isolated from Commiphora berryi.

MATERIALS AND METHODS

The bark of Commiphora berryi was extracted with different organic solvents and subjected to chromatographic separation for isolation of bioactive compounds. Structures of isolated compounds were elucidated by spectroscopic methods. The anti-inflammatory activity of bark extracts and bioactive compounds were assessed by in vitro soybean lipoxygenase (SBL) assay.

RESULTS

3β-Hydroxyglutin-5-ene (1), friedelin (2), cycloeucaneol (3) nimbiol (4), sugiol (5), surianol (6), daucosterol (7) and ursolic acid (8) were isolated from crude bark extracts of the Commiphora berryi. The structure of nimbiol (4) was also confirmed by single crystal X-ray analysis. The petroleum ether, methanol, chloroform and ethyl acetate extracts of bark of Commiphora berryi showed SBL inhibitory activity with the IC(50) values of 15.3, 54.2, 71.5 and 87.8 μg/ml respectively. Among all the isolates, friedelin (2) showed significant SBL inhibitory activity with IC(50) 35.8 μM.

CONCLUSION

The overall results provide evidence that the studied plant might be a potential source of anti-inflammatory agents.

A novel role for iron in modulating the activity and membrane-binding ability of a trimmed soybean lipoxygenase-1

Lipoxygenases (LOXs) are iron-containing enzymes that play critical roles in plants and animals. As yet, metal atom extraction, reconstitution, and substitution have not been successfully applied to soybean LOX-1 [Glycine max (L.) Merrill], a prototype member of the LOX family that is widely used in structural and kinetic studies. Here, tryptic digestion of native LOX-1, used as a control, allowed us to isolate the 60-kDa C-terminal region (termed miniLOX), that retains the catalytically active iron in a more accessible position. Then, iron was removed to obtain an unprecedented apo-miniLOX, which was reconstituted and substituted with different metal ions. These forms of miniLOX were characterized vs. native LOX-1 by kinetic analysis, near UV circular dichroism, steady-state fluorescence, and fluorescence resonance energy transfer. MiniLOX showed a 2-fold increase in the membrane-binding affinity compared with native LOX-1 and a remarkable 4-fold increase compared with apo-miniLOX (K(d)=9.2+/-1.0, 17.9+/-2.0, and 45.4+/-4.3 microM, respectively). Furthermore, miniLOX reconstituted with Fe(II) or Fe(III) partially recovered its membrane-binding ability (K(d)=21.4+/-2.4 and 18.9+/-5.5 microM, respectively), overall supporting a novel noncatalytic role for iron in the LOX family.

A highly reactive dicationic iridium(III) catalyst for the polarized Nazarov cyclization reaction

Pushing the Nazarov Envelope

A new electrophilic complex [IrBr(CO)(diethylisopropylidene malonate)((R )-(+)-BINAP)](SbF6)2 (2 ) exhibits unusual activity in the catalysis of polarized Nazarov cyclization. Aryl vinyl ketones that show poor reactivity with well-known catalysts such as [Ir(CH3)(CO)(1,2-diiodobenzene)(dppe)](B(Arf)4−)2 (1 ), Sc(OTf)3 + LiClO4 and Cu(ClO4)2, can be cyclized with 2 + AgSbF6 (1:1) under mild conditions with concurrent AgBr precipitation.

Pyrrolidinones from the ascomycete fungus Albonectria rigidiuscula

The new pyrrolidinones, rigidiusculamides A-D (1-4), have been isolated from the crude extract of the ascomycete fungus Albonectria rigidiuscula. The structures of these compounds were elucidated primarily by NMR experiments. The absolute configuration of the 3,4-diol moieties in 1 and 4 was assigned using Snatzke's method. Compounds 1 and 2 showed modest cytotoxicity against the human tumor cell lines HeLa and MCF-7.

Biosynthesis of tetrapetalones

The biosynthesis of tetrapetalones (tetrapetalones A, B, C, and D) in Streptomyces sp. USF-4727 was studied by feeding experiments with [1-13C] sodium propanoate, [1-13C] sodium butanoate, [carbonyl-13C] 3-amino-5-hydroxybenzoic acid (AHBA) hydrochloride, and [1-13C] glucose, followed by analysis of the 13C-NMR spectra. These feeding experiments revealed that the four tetrapetalones were polyketide compounds constructed from propanoate, butanoate, AHBA, and glucose. The tetrapetalone biosynthetic pathway was also suggested in this study. In this pathway, tetrapetalone A (1) is synthesized by polyketide synthase (PKS) using AHBA as a starter unit, then the side chain of 1 is subjected to acetoxylation to produce tetrapetalone B (2). Additionally, 1 is oxidized and transformed into tetrapetalone C (3). In a similar way, 2 is converted to tetrapetalone D (4). Therefore, the biosynthetic relationship of the four tetrapetalones was indicated.