Mutagenicity of natural naphthoquinones and benzoquinones in the Salmonella/microsome test

The mutagenicities of naturally occurring naphthoquinones and benzoquinones were tested by the pre-incubation method with Salmonella typhimurium strains TA98, TA100 and TA2637, which all contain plasmid pKM101. 6 of the 16 naphthoquinones tested, i.e., plumbagin, naphthazarin, 2-hydroxy-naphthoquinone, vitamin K3 (menadione), juglone and 7-methyljuglone, were mutagenic to strain TA2637 with metabolic activation. Except for juglone and 7-methyl-juglone, these compounds also had slight mutagenic effects on strain TA98 with S9 mix. All the mutagenic naphthoquinones contain one or two hydroxyl and/or methyl substituents. The naphthoquinone mompain, which has four hydroxyl groups, was not mutagenic. Unsubstituted beta-naphthoquinone, naphthoquinones with a prenyl side chain and all bi-naphthoquinone derivatives tested were non-mutagenic. None of the 13 benzoquinones examined was mutagenic to any of the strains used with or without metabolic activation. These results show that natural naphthoquinones are mutagenic when they have only one or two hydroxyl and/or methyl substituents.     

Herbicidal 4-hydroxyphenylpyruvate dioxygenase inhibitors—A review of the triketone chemistry story from a Syngenta perspective

A review, outlining the origins and subsequent development of the triketone class of herbicidal 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors.     

Occurrence, biosynthesis and function of isoprenoid quinones

Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K₁, a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.     

Interaction of (4-hydroxyphenyl)pyruvate dioxygenase with the specific inhibitor 2-[2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione

(4-Hydroxyphenyl)pyruvate dioxygenase (HPPD) is a non-heme Fe(II) enzyme that catalyzes the conversion of (4-hydroxyphenyl)pyruvate (HPP) to homogentisate as part of the tyrosine catabolism pathway. Inhibition of HPPD by the triketone 2-[2-nitro-4-(trifluoromethyl)benzoyl]-1,3-cyclohexanedione (NTBC) is used to treat type I tyrosinemia, a rare but fatal defect in tyrosine catabolism. Although triketones have been used for many years as HPPD inhibitors for both medical and herbicidal purposes, the mechanism of inhibition is not well understood. The following work provides mechanistic insight into NTBC binding. The tautomeric population of NTBC in aqueous solution is dominated by a single enol as determined by NMR spectroscopy. NTBC preferentially binds to the complex of HPPD and FeII [HPPD.Fe(II)] as evidenced by a visible absorbance feature centered at 450 nm. The binding of NTBC to HPPD.Fe(II) was observed using a rapid mixing method and was shown to occur in two phases and comprise three steps. A hyperbolic dependence of the first observable process with NTBC concentration indicates a pre-equilibrium binding step followed by a limiting rate (K(1) = 1.25 +/- 0.08 mM, k(2) = 8.2 +/- 0.2 s(-1)), while the second phase (k(3) = 0.76 +/- 0.02 s(-1)) had no dependence on NTBC concentration. Neither K(1),k(2), nor k(3) was influenced by pH in the range of 6.0-8.0. Isotope effects on both k(2) and k(3) were observed when D(2)O is used as the solvent (for k(2), k(h)/k(d) = 1.3; for k(3), k(h)/k(d) = 3.2). It is therefore proposed that the bidentate association of NTBC with the active site metal ion (k(2)) precedes the Lewis acid-assisted conversion of the bound enol to the enolate (k(3)). Although the native enzyme without substrate reacts with molecular oxygen to form the oxidized holoenzyme, the HPPD.Fe(II).NTBC complex does not. When the complex is exposed to atmospheric oxygen, the absorbance feature associated with NTBC binding does not diminish over the course of 2 days. This means not only that the HPPD.Fe(II).NTBC complex does not oxidize but also that the dissociation rate constant for NTBC is essentially zero because any HPPD.Fe(II) that formed would readily oxidize in the presence of dioxygen. Consistent with this observation, EPR spectroscopy has shown that only 2% of the HPPD.Fe(II).NTBC complex forms an NO complex as compared to the holoenzyme.     

p-Hydroxyphenylpyruvate dioxygenase is a herbicidal target site for beta-triketones from Leptospermum scoparium

p-Hydroxyphenylpyruvate dioxygenase (HPPD) is a key enzyme in tyrosine catabolism and is the molecular target site of β-triketone pharmacophores used to treat hypertyrosinemia in humans. In plants, HPPD is involved in the biosynthesis of prenyl quinones and tocopherols, and is the target site of β-triketone herbicides. The β-triketone-rich essential oil of manuka (Leptospermum scoparium), and its components leptospermone, grandiflorone and flavesone were tested for their activity in whole-plant bioassays and for their potency against HPPD. The achlorophyllous phenotype of developing plants exposed to manuka oil or its purified β-triketone components was similar to that of plants exposed to the synthetic HPPD inhibitor sulcotrione. The triketone-rich fraction and leptospermone were approximatively 10 times more active than that of the crude manuka oil, with I₅₀ values of 1.45, 0.96 and 11.5 μg mL⁻¹, respectively. The effect of these samples on carotenoid levels was similar. Unlike their synthetic counterpart, steady-state O₂ consumption experiments revealed that the natural triketones were competitive reversible inhibitors of HPPD. Dose-response curves against the enzyme activity of HPPD provided apparent I₅₀ values 15.0, 4.02, 3.14, 0.22 μg mL⁻¹ for manuka oil, triketone-rich fraction, leptospermone and grandiflorone, respectively. Flavesone was not active. Structure-activity relationships indicate that the size and lipophilicity of the side-chain affected the potency of the compounds. Computational analysis of the catalytic domain of HPPD indicates that a lipophilic domain proximate from the Fe²⁺ favors the binding of ligands with lipophilic moieties.     

Insect growth regulatory activity of naturally occurring quinones and their derivatives in Dysdercus koenigii Fabr. (Hem., Pyrrhocoridae)

Plumbagin, a naphthoquinone occurring in the plants belonging to Plumbago sp. is known to have insect growth disrupting activities. In the present study, a comparative evaluation of bioactivities of related naphthoquinones, juglone and menadione and benzoquinones, 2,6-dimethylbenzoquinone and 2,3,6-trimethylbenzoquinone along with 2,6-dimethylhydroquinone have been undertaken using Dysdercus koenigii . The LD₅₀ values of the compounds showed wide variation. Although all the test compounds disrupted the normal growth at sublethal doses, 2,6-dimethylbenzoquinone and menadione showed activity at low doses.     

Structure of the ferrous form of (4-hydroxyphenyl)pyruvate dioxygenase from Streptomyces avermitilis in complex with the therapeutic herbicide, NTBC

Di- and triketone inhibitors of (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) are both effective herbicides and therapeutics. The inhibitory activity is used to halt the production of lipophilic redox cofactors in plants and also in humans to prevent accumulation of toxic metabolic byproducts that arise from specific inborn defects of tyrosine catabolism. The three-dimensional structure of the Fe(II) form of HPPD from Streptomyces avermitilis in complex with the inhibitor 2-[2-nitro-4-(triflouromethyl)benzoyl]-1,3-cyclohexanedione (NTBC) has been determined at a resolution of 2.5 A. NTBC coordinates to the active site metal ion, located at the bottom of a wide solvent-accessible cavity in the C-terminal domain of the protein. The iron is liganded in a predominantly five-coordinate, distorted square-pyramidal arrangement in which Glu349, His187, and His270 are protein-derived ligands and two other ligands are from the 5' and 7' oxygens of NTBC. There is a low-occupancy water molecule in the sixth coordination site in one of the protomers. The distance to His270 is unusually long at 2.5 A, and its orientation is somewhat distorted from ideal ligand geometry to within 2.8 A of the inhibitor nitro group. In contrast to the tetrameric quartenary structure observed for HPPD from other bacterial sources, the asymmetric unit is composed of two weakly associated protomers with a buried surface area of 1266 A(2) and a total of 12 hydrogen-bonding and no electrostatic interactions. The overall tertiary structure is similar to that of HPPD from Pseudomonas fluorescens (Serre et al., (1999) Structure 7, 977-988), although the position of the C-terminal alpha-helix is dramatically shifted. This C-terminal alpha-helix provides Phe364, which in combination with Phe336 sandwiches the phenyl ring of the bound NTBC; no other significant hydrogen-bonding or charge-pairing interactions are observed. Moreover, the structure reveals that, with the exception of Val189, NTBC makes contacts to only fully conserved amino acids. The combination of bidentate metal-ion coordination and pi-stacked aromatic rings is suggestive of a binding mode for the substrate and/or a transition state, which may be the origin of the exceedingly high affinity these inhibitors have for HPPD.     

Characterization of the quinone reductase activity of the ferric reductase B protein from Paracoccus denitrificans

The ferric reductase B (FerB) protein of Paracoccus denitrificans exhibits activity of an NAD(P)H: Fe(III) chelate, chromate and quinone oxidoreductase. Sequence analysis places FerB in a family of soluble flavin-containing quinone reductases. The enzyme reduces a range of quinone substrates, including derivatives of 1,4-benzoquinone and 1,2- and 1,4-naphthoquinone, via a ping-pong kinetic mechanism. Dicoumarol and Cibacron Blue 3GA are competitive inhibitors of NADH oxidation. In the case of benzoquinones, FerB apparently acts through a two-electron transfer process, whereas in the case of naphthoquinones, one-electron reduction takes place resulting in the formation of semiquinone radicals. A ferB mutant strain exhibited an increased resistance to 1,4-naphthoquinone, attributable to the absence of the FerB-mediated redox cycling. The ferB promoter displayed a high basal activity throughout the growth of P. denitrificans, which could not be further enhanced by addition of different types of naphthoquinones. This indicates that the ferB gene is expressed constitutively.     

Regulation of 1-dehydrogenation and 20 beta-reduction of ketosteroids by microorganisms]

Regulation of two microbiological processes--1-dehydrogenation and 20 beta-reduction of ketosteroids--was studied using several exogeneous quinones which (a) stimulated 1-dehydrogenation and (b) inhibited the accompanying reaction, the reduction of 20-ketogroup of the steroid molecule. The least active compounds were benzoquinones. The best regulators are 1,4-naphthoquinone, 1,2-naphthoquinone, and 7-methoxy-1,2-naphthoquinone. Possible mechanism of the action of naphthoquinones on Mycobacterium globiforme is discussed.     

Heterogenized polymetallic catalysts Part II. Oxidation of 3,5-di-t-butylphenol by Cu(II), Fe(III) and Pd(II) complexed to a polyphenylene polymer containing ß-di- and triketone surface ligands; an improved catalyst system

Polyphenylene polymer preparation involves the cyclic trimerization polymerization of acetylated methyl benzoate with diacetyl benzene. Since the methyl benzoate groups do not take part in the polymerization they are present in high concentration. The ß-diketone ligands were placed on the surface by reaction of the methylbenzoate group with base and a methyl ketone and the triketone by reaction with base to give the ß-triketone. The ß-triketones can bind two metal ions in a known geometry that is suitable for bimetallic catalysis of the rapid polyelectron oxidation of catechols. The final catalytic surfaces were prepared by treating the chemically modified polymer with copper(II), iron(II) and palladium(II) acetonitrile complexes with non-coordinating BF₄⁻ as the anion. Since the metal ions contain no strongly coordinating ligand, they are very reactive species. These surfaces catalyzed the rapid air oxidation of 3,5-di-tert-butylcatechol (DTBC). The diketone surfaces gave only 3,5-di-tert-butyl-o-quinone (DTBQ) while the triketone surfaces gave ring-cleaved products, confirming the special catalytic effect of the triketone surface. Also, only the triketone catalysts showed any activity for ring cleavage oxidation of DTBQ. These catalysts were much more reactive than previous ones using the same polyphenylene polymer but without the methyl benzoate groups. With these polymers the di- and triketone groups were placed on the surface by chemical modification of the unpolymerized acetyl groups.     

Inhibitory effects of flavonoids on rabbit heart carbonyl reductase

The inhibitory effects of flavonoids (galangin, kaempferol, quercetin, myricetin, morin, and taxifolin) on rabbit heart carbonyl reductase (RHCR) were investigated using 4-benzoylpyridine (4BP) as the substrate. The stereochemical characteristics of the flavonoids were found to be a factor determining their inhibitory potencies toward RHCR. Furthermore, the lipophilicity, and the scavenging or antioxidative effects of the flavonoids were likely to complicate the structure-activity relationship of their inhibitory effects on RHCR. Quercetin inhibited RHCR uncompetitively with respect to NADPH and competitively with respect to 4BP, suggesting that it competes with 4BP at the substrate-binding site of RHCR. RHCR efficiently reduced benzoquinones (1,4-benzoquinone and 2-methyl-1, 4-benzoquinone) and naphthoquinones (1,4-naphthoquinone and menadione). Galangin was a potent inhibitor of RHCR when menadione was used as the substrate, and prevented the formation of the superoxide anion radical in the presence of RHCR, NADPH, and menadione. Flavonoids may be useful compounds for suppressing the cardiotoxicity of quinones by inhibiting RHCR.     

The dependence on quinone specificity of terminal electron transport of bacteria

Bacterial quinones were extracted with pentane, and homologues or other quinones were reincorporated. In spite of the redox potential difference of 110 mV, menaquinone and demethylmenaquinone could replace each other in aerobic electron transport and fumarate respiration ofHaemophilus influenzae RAMC 18 Bensted andProteus mirabilis Harding & Nicholson. The enzymes involved may recognize the naphthoquinone structure and are not specific for menaquinone or demethylmenaquinone. Ubiquinone was not replaced in aerobic electron transport by naphthoquinones withPseudomonas fluorescens 28/5 Rhodes orAcinetobacter sp. 661/60 Mannheim, probably owing to the specificity for benzoquinones of the enzymes involved, since the redox potential difference between demethylmenaquinone and ubiquinone is only 76 mV.Haemophilus parainfluenzae 429 Pittman, which resembles aerobic bacteria with respect to the terminal electron transport system, could incorporate demethylmenaquinone or menaquinone. This organism seems to be defective in the synthesis of naphthoquinones but possesses the enzyme system for fumarate respiration.Haemophilus influenzae RAMC 18 Bensted, which produces only demethylmenaquinone, seems to be defective in synthesizing ubiquinone, but it also possesses the enzymes for a ubiquinonemediated aerobic respiration.     

Natural products inspired synthesis of neuroprotective agents against H2O2-induced cell death

Stroke is a debilitating disease and the third leading cause of death in the USA, where over 2000 new stroke cases are diagnosed every day. Treatment options for stroke-related brain damage are very limited and there is an urgent need for effective neuroprotective agents to treat these conditions. Comparison of the structures of several classes of neuroprotective natural products such as limonoids and cardiac glycosides revealed the presence of a common structural motif which may account for their observed neuroprotective activity. Several natural product mimics that incorporate this shared structural motif were synthesized and were found to possess significant neuroprotective activity. These compounds enhanced cell viability against H₂O₂ induced oxidative stress or cell death in PC12 neuronal cells. The compounds were also found to enhance and modulate Na⁺/K⁺-ATPase activity of PC12 cells, which may suggest that the observed neuroprotective activity is mediated, at least partly, through interaction with Na⁺/K⁺-ATPase.     

Novel biologically active natural and unnatural products

Natural products have been used as medicinal agents for many years. In addition, these compounds have served as the starting points for semisynthetic analogs with improved properties. This review highlights work on several classes of natural products and their derivatives, including both well established and emerging structural classes that are in, or nearing, clinical use for a variety of important indications.     

Indole alkaloid marine natural products: an established source of cancer drug leads with considerable promise for the control of parasitic, neurological and other diseases

The marine environment produces natural products from a variety of structural classes exhibiting activity against numerous disease targets. Historically marine natural products have largely been explored as anticancer agents. The indole alkaloids are a class of marine natural products that show unique promise in the development of new drug leads. This report reviews the literature on indole alkaloids of marine origin and also highlights our own research. Specific biological activities of indole alkaloids presented here include: cytotoxicity, antiviral, antiparasitic, anti-inflammatory, serotonin antagonism, Ca-releasing, calmodulin antagonism, and other pharmacological activities.     

Chemosterilant activity of naturally occurring quinones and their analogues in the red cotton bug, Dysdercus koenigii (Het., Pyrrhocoridae)

The chemosterilant activity of two naturally occurring napthaquinones, plumbagin from Plumbago sp. and juglone from Juglans regia has been evaluated using the red cotton bug Dysdercus koenigii . Their activity were compared with synthetic napthaquinones, menadione, two benzoquinones, 2,6-dimethyl and 2,3,6-trimethylbenzoquinone and a hydroquinone 2,6-dimethylhydroquinone. As far as the authors are aware, the present investigation is the first systematic attempt to investigate the effects of quinones on different aspects of reproduction, namely mating behaviour, fecundity and fertility. All the above types of quinones have revealed adverse effects on the reproduction of D. koenigii , which were topically treated as 1-day-old adults with different doses of the compounds depending on their 50% lethality (LD₅₀) values. Analysis of the data revealed a highly statistically significant reduction in the number of eggs, their hatching and further development up to the final instar stage. The sterility index increased with the increase in the dose of any of the above compounds and was highest when both sexes were treated. Among the natural products, juglone induced more sterilizing effects than plumbagin at 5–10  μ g/insect. Menadione caused sterility at a very low dose such as 0.5  μ g. Of the two benzoquinones, the dimethylbenzoquinone acted at a much lower dose.     

Terpenoids from Marine Organisms: Unique Structures and their Pharmacological Potential

Marine organisms produce a wide array of fascinating terpenoid structures distinguished by characteristic structural features. Certain structural classes, e.g. cembrane, chamigrene, amphilectane skeletons, and unusual functional groups such as isonitrile, isothiocyanate, isocyanate, dichloroimine and halogenated functionalities occur predominantly in marine metabolites. Especially striking is the frequent occurrence of sesterterpenes in marine organisms, and sponges must be considered as one of the prime sources of these C₂₅ terpenoid compounds. In most cases however, these structural features are not strictly unique for marine natural products. The prominent biological activity of marine terpenes is evident in their ecological role in the marine environment, and makes them interesting as potential drugs. Several terpenoid compounds, e.g. eleutherobin, sarcodictyin, contignasterol derivatives, are in preclinical or clinical development. Despite the many structures known and their ecological and pharmacological importance, only a few biosynthetic studies on marine terpenoid compounds have been performed.     

Microbial production of small medicinal molecules and biologics: From nature to synthetic pathways

Natural products are promising chemicals due to their structural diversity and bioactivities. Over the decades, a vast variety of gene clusters encoding natural products have been identified and overexpressed in microbes. Recently, the development of metabolic engineering, synthetic biology and bioinformatics strategies have facilitated target discovery and design. Microbial cells have been therefore constantly engineered for product accumulation. This review summarizes approaches of domesticating microbial hosts in producing major classes of natural products, with an emphasis on recent advances.     

Synthesis of 3-[(N-carboalkoxy)ethylamino]-indazole-dione derivatives and their biological activities on human liver carbonyl reductase

A series of indazole-dione derivatives were synthesized by the 1,3-dipolar cycloaddition reaction of appropriate substituted benzoquinones or naphthoquinones and N-carboalkoxyamino diazopropane derivatives. These compounds were evaluated for their effects on human carbonyl reductase. Several of the analogs were found to serve as substrates for carbonyl reductase with a wide range of catalytic efficiencies, while four analogs display inhibitory activities with IC₅₀ values ranging from 3–5 μM. Two of the inhibitors were studied in greater detail and were found to be noncompetitive inhibitors against both NADPH and menadione with K_I values ranging between 2 and 11 μM. Computational studies suggest that conformation of the compounds may determine whether the indazole-diones bind productively to yield product or nonproductively to inhibit the enzyme.