A soluble, magnesium-independent prenyltransferase catalyzes reverse and regular C-prenylations and O-prenylations of aromatic substrates

Fnq26 from Streptomyces cinnamonensis DSM 1042 is a new member of the recently identified CloQ/Orf2 class of prenyltransferases. The enzyme was overexpressed in E. coli and purified to apparent homogeneity, resulting in a soluble, monomeric protein of 33.2 kDa. The catalytic activity of Fnq26 is independent of the presence of Mg(2+) or other divalent metal ions. With flaviolin (2,5,7-trihydroxy-1,4-naphthoquinone) as substrate, Fnq26 catalyzes the formation of a carbon-carbon-bond between C-3 (rather than C-1) of geranyl diphosphate and C-3 of flaviolin, i.e. an unusual "reverse" prenylation. With 1,3-dihydroxynaphthalene and 4-hydroxybenzoate as substrates Fnq26 catalyzes O-prenylations.     

Identification of a cryptic type III polyketide synthase (1,3,6,8-tetrahydroxynaphthalene synthase) from Streptomyces peucetius ATCC 27952

We identified a 1,134-bp putative type III polyketide synthase from the sequence analysis of Streptomyces peucetius ATCC 27952, named Sp-RppA, which is characterized as 1,3,6,8-tetrahydroxynaphthalene synthase and shares 33% identity with SCO1206 from S. coelicolor A3(2) and 32% identity with RppA from S. griseus. The 1,3,6,8-tetrahydroxynaphthalene synthase is known to catalyze the sequential decarboxylative condensation, intramolecular cyclization, and aromatization of an oligoketide derived from five units of malonyl-CoA to give 1,3,6,8-tetrahydroxynaphthalene, which spontaneously oxidizes to form 2,5,7-trihydroxy-1,4-naphthoquinone (flaviolin). In this study, we report the in vivo expression and in vitro synthesis of flaviolin from purified gene product (Sp-RppA).     

Degradation of 1,4-naphthoquinones by Pseudomonas putida

Pseudomonas putida J1 and J2, enriched from soil with juglone, are capable of a total degradation of 1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, and 2-chloro-1,4-naphthoquinone. Naphthazerin and plumbagin are only converted into the hydroxyderivatives 2-hydroxynaphthazerin and 3-hydroxyplumbagin, respectively, whereas 2-amino-1,4-naphthoquinone is not attacked at all. The degradation of 1,4-naphthoquinone begins with a hydroxylation of the quinoid ring, yielding 2-hydroxy-1,4-naphthoquinone (lawsone). Lawsone is reduced to 1,2,4-trihydroxynaphthalene with consumption of NADH. The fission product of the quinol could not be detected by direct means because of its instability. However, the presence of 2-chromonecarboxylic acid, a secondary product of lawsone degradation, leads to the conclusion, that the cleavage of the quinol takes place in the meta-position. The resulting ring fission product is converted into salicylic acid by removal of the side chain, presumably as pyruvate. Further degradation of salicyclic acid leads to the formation of catechol, which is then cleaved in the ortho-position and then metabolized via the 3-oxoadipate pathway. The initial steps in the degradation of 2-chloro-1,4-naphthoquinone, namely, the hydroxylation of the quinone to 2-chloro-3-hydroxy-1,4-naphthoquinone, followed by the elimination of the chlorine substituent lead to lawsone, which is further degraded through the pathway described. The degradation steps could be verified by the accumulation products of mutant strains blocked in different steps of lawsone metabolism. Generation of mutants was carried out by chemical and by transposon mutagenesis. The regulation of the first steps of the pathway catalysed by juglone hydroxylase and lawsone reductase, was investigated by induction experiments.     

Role of active site water molecules and substrate hydroxyl groups in oxygen activation by cytochrome P450 158A2: a new mechanism of proton transfer

From the x-ray crystal structure of CYP158A2 (Zhao, B., Guengerich, F. P., Bellamine, A., Lamb, D. C., Izumikawa, M., Lei, L., Podust, L. M., Sundaramoorthy, M., Reddy, L. M., Kelly, S. L., Kalaitzis, J. A., Stec, D., Voehler, M., Falck, J. R., Moore, B. S., Shimada, T., and Waterman, M. R. (2005) J. Biol. Chem. 280, 11599-11607), one of 18 cytochrome P450 (CYP) genes in the actinomycete Streptomyces coelicolor, ordered active site water molecules (WAT505, WAT600, and WAT640), and hydroxyl groups of the substrate flaviolin were proposed to participate in proton transfer and oxygen cleavage in this monooxygenase. To probe their roles in catalysis, we have studied the crystal structures of a substrate analogue (2-hydroxy-1,4-naphthoquinone) complex with ferric CYP158A2 (2.15 A) and the flaviolin ferrous dioxygen-bound CYP158A2 complex (1.8 A). Catalytic activity toward 2-hydroxy-1,4-naphthoquinone was approximately 70-fold less than with flaviolin. In the ferrous dioxygen-bound flaviolin complex, the three water molecules in the ferric flaviolin complex still occupy the same positions and form hydrogen bonds to the distal dioxygen atom. These findings suggest that CYP158A2 utilizes substrate hydroxyl groups to stabilize active site water and further assist in the iron-linked dioxygen activation. A continuous hydrogen-bonded water network connecting the active site to the protein surface (bulk solvent) not present in the other two ferrous dioxygen-bound P450 structures (CYP101A1/P450cam and CYP107A1/P450eryF) is proposed to participate in the proton-delivery cascade, leading to dioxygen bond scission. This ferrous-dioxygen structure suggests two classes of P450s based on the pathway of proton transfer, one using the highly conserved threonine in the I-helix (CYP101A1) and the other requiring hydroxyl groups of the substrate molecules either directly transferring protons (CYP107A1) or stabilizing a water pathway for proton transfer (CYP158A2).     

鹊肾树树皮的化学成分和抗炎作用研究

通过药理实验确定了鹊肾树树皮的乙酸乙酯萃取物为抗炎活性部位,并利用色谱技术从该部位中分离得到了7个化合物,通过UV、IR、NMR、MS等现代谱学方法鉴定化合物的结构分别为:丁二酸(1)、6-乙基-5-羟基-2,7-二甲氧基-1,4-萘醌(2)、3β,20-二羟基-5β-孕甾烷(3)、5-羟基麦芽酚(4)、双[5-甲酰基...     

Isolation and characterization of a novel vitamin-K from Eubacterium lentum

A novel fat-soluble vitamin K like molecule was isolated from the prokaryote, Eubacterium lentum, and its structure investigated by mass spectrometry and proton nuclear magnetic resonance spectrometry. On the basis of these studies the novel quinone is shown to be 2,5 and 6- or 2,7 and 8-trimethyl-3-farnesylfarnesyl-1,4-naphthoquinone.     

Effect of hydroxy substituent position on 1,4-naphthoquinone toxicity to rat hepatocytes.

The effect of hydroxy substitution on 1,4-naphthoquinone toxicity to cultured rat hepatocytes was studied. Toxicity of the quinones decreased in the series 5,8-dihydroxy-1,4-naphthoquinone greater than 5-hydroxy-1,4-naphthoquinone greater than 1,4-naphthoquinone greater than 2-hydroxy-1,4-naphthoquinone, and intracellular GSSG formation decreased in the order 5,8-dihydroxy-1,4-naphthoquinone greater than 5-hydroxy-1,4-naphthoquinone much greater than 1,4-naphthoquinone much greater than 2-hydroxy-1,4-naphthoquinone. The electrophilicity of the quinones decreased in the order 1,4-naphthoquinone much greater than 5-hydroxy-1,4-naphthoquinone greater than 5,8-dihydroxy-1,4-naphthoquinone much greater than 2-hydroxy-1,4-naphthoquinone. Treatment of the hepatocytes with BSO (buthionine sulfoximine) or BCNU (1,3-bis-2-chloroethyl-1-nitrosourea) increased 5-hydroxy-1, 4-naphthoquinone and 5,8-dihydroxy-1,4-naphthoquinone toxicity, whereas neither BSO nor BCNU largely affected 1,4-naphthoquinone and 2-hydroxy-1, 4-naphthoquinone toxicity. Dicumarol increased the toxicity of 1,4-naphthoquinone dramatically and somewhat the toxicity of 2-hydroxy-1,4- naphthoquinone, whereas 5-hydroxy-1,4-naphthoquinone and 5,8-dihydroxy-1,4-naphthoquinone toxicity increased only slightly. The toxicity of 5,8-dihydroxy-1,4-naphthoquinone decreased dramatically in reduced O2 concentration, whereas 1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone, and 2-hydroxy-1,4-naphthoquinone toxicity was not largely affected. It was concluded that 5,8-dihydroxy-1,4-naphthoquinone toxicity is due to free radical formation, whereas the toxicity of 1,4-naphthoquinone and of 5-hydroxy-1,4-naphthoquinone also has an electrophilic addition component. The toxicity of 2-hydroxy-1,4-naphthoquinone could not be fully explained by either of these phenomena.     

Cytotoxicity of new polyfluorinated 1,4-naphtoquinones with diverse substituents in the quinone moiety

Fluorinated derivatives of 1,4-naphthoquinones are highly potent inhibitors of Cdc25A and Cdc25B phosphatases and growth of tumor cells. Eight new derivatives of polyfluoro-1,4-naphthoquinone were synthesized and their cytotoxicity in human myeloma, human mammary adenocarcinoma, mouse fibroblasts and primary mouse fibroblast cells as well as their mutagenic and antioxidant properties in a Salmonella tester strain were studied. The efficiency of suppressing the growth of two lines of tumor cells decreased in the order: 2-(2-hydroxy-ethylamino)-3,5,6,7,8-pentafluoro-1,4-naphthoquinone (1), 2,3-dimethoxy-5,6,7,8-tetrafluoro-1,4-naphthoquinone (2), 2-[2-hydroxyethyl(methyl)amino]-3,5,6,7,8-pentafluoro-1,4-naphthoquinone (3), 2-morpholino-3,5,6,7,8-pentafluoro-1,4-naphthoquinone (4), 2-[bis-(2-hydroxyethyl)amino]-3,5,6,7,8-pentafluoro-1,4-naphthoquinone (5), 2-[(2-hydroxy)ethylsulfanyl)]-5,6,7,8-tetrafluoro-1,4-naphthoquinone (6), 2-methoxy-3,5,6,7,8-pentafluoro-1,4-naphthoquinone (7), and 1,4-dioxo-3-(1-pyridinio)-1,4-dihydro-5,6,7,8-tetrafluoronaphthalene-2-olate (8). Taking into account these data together with the better cytotoxic effect against cancer cells as compared with normal mammalian cells, protecting of bacterial cells from spontaneous and H₂O₂-dependent mutagenesis, and lower general toxicity of the compounds towards different cells, one can propose that compounds 3-5 may be considered as useful potential inhibitors of growth of tumor cells.     

Prenylation and methylation reactions in phylloquinone (vitamin K1) synthesis in Capsicum annuum plastids

The biosynthesis of phylloquinone (vitamin K₁) was examined using Capsicum fruit chloroplasts and chromoplasts (apparently phylloquinone free). In both cases, the synthesis of phylloquinone from -naphthoquinone, dihydro--naphthoquinone, 1,4-dihydroxy-2-naphthoic acid (as precursors of the ring moiety) and (S)-adenosyl-L-methionine was achieved. In the presence of phytylpyrophosphate, the biosynthesis of phylloquinone in both organelles is particularly enhanced when 1,4-dihydroxy-2-naphthoic acid is used.     

DT-diaphorase-catalysed reduction of 1,4-naphthoquinone derivatives and glutathionyl-quinone conjugates. Effect of substituents on autoxidation rates.

DT-diaphorase catalysed the reduction of 1,4-naphthoquinones with hydroxy, methyl, methoxy and glutathionyl substituents at the expense of reducing equivalents from NADPH. The initial rates of quinone reduction did not correlate with either the half-wave reduction potential (E1/2) value (determined by h.p.l.c. with electrochemical detection against an Ag/AgCl reference electrode) or the partition coefficient of the quinones. After their reduction by DT-diaphorase the 1,4-naphthoquinone derivatives autoxidized at distinct rates, the extent of which was influenced by the nature of the substituents. Thus for the 1,4-naphthoquinone series the following order of rate of autoxidation was found: 5-hydroxy-1,4-naphthoquinone greater than 3-glutathionyl-1,4-naphthoquinone greater than 5-hydroxy-3-glutathionyl-1,4-naphthoquinone greater than 1,4-naphthoquinone greater than 2-hydroxy-1,4-naphthoquinone. For the 2-methyl-1,4-naphthoquinone (menadione) series the following order was observed: 5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-5-hydroxy-2-methyl-1,4-naphthoquinone greater than 3-glutathionyl-2-methyl-1,4-naphthoquinone greater than 2-methyl-1,4-naphthoquinone greater than 3-hydroxy-2-methyl-1,4-naphthoquinone. The autoxidized naphthohydroquinone derivatives were re-reduced by DT-diaphorase, thus closing a cycle of enzymic reduction in equilibrium autoxidation. This was expressed as an excess of NADPH oxidized over the initial concentration of quinone present as well as H2O2 formation. These findings demonstrate that glutathionyl conjugates of 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone and those of their respective 5-hydroxy derivatives are able to act as substrates for DT-diaphorase and that they also autoxidize at rates higher than those for the unsubstituted parent compounds. These results are discussed in terms of the cellular role of DT-diaphorase in the reduction of hydroxy- or glutathionyl-substituted naphthoquinones as well as the further conjugation of these hydroquinones with glucuronide or sulphate within the cellular milieu, thereby facilitating their disposal from the cells.     

Synthesis and MEK1 inhibitory activities of imido-substituted 2-chloro-1,4-naphthoquinones

Mitogen activated protein kinases are of interest as research tools and as therapeutic target for certain physiological disorders. In this study, we found 2-chloro-3-(N-succinimidyl)-1,4-naphthoquinone 6 to be a selective inhibitor of MEK1 with an IC(50) of 0.38 microM. An open-chain homologue, 10, showed selective cytotoxicity against renal cancer in the NCI in vitro tumor screening. Structure-activity relationship study of eight compounds showed the cyclic imido-substituted chloro-1,4-naphthoquinone as more potent and selective MEK1 inhibitors than the open chain homologues. The imido-substituted chloro-1,4-naphthoquinones were synthesized in a straightforward fashion by refluxing 2-amino-3-chloro-1,4-naphthoquinone with the appropriate acid chloride or diacyl dichloride.     

Identification and cloning of a type III polyketide synthase required for diffusible pigment biosynthesis in Saccharopolyspora erythraea

The soluble, diffusible red-brown pigment produced by a Saccharopolyspora erythraea "red variant" has been shown to contain glycosylated and polymerized derivatives of 2,5,7-trihydroxy-1,4-naphthoquinone (flaviolin). Flaviolin is a spontaneous oxidation product of 1,3,6,8-tetrahydroxynaphthalene (THN), which is biosynthesized in bacteria by a chalcone synthase-like (CS-like) type III polyketide synthase (PKS). A fragment of the gene responsible for THN biosynthesis in S. erythraea E_8-7 was amplified by polymerase chain reaction (PCR) using degenerate primers based on conserved regions of known plant CS and bacterial CS-like genes. From the isolated fragment, a suicide vector was prepared, which was subsequently used to disrupt the red-brown pigment-producing (rpp) locus in S. erythraea, generating a mutant that displayed an albino phenotype. Chromosomal DNA from the albino mutant was subsequently used in a vector-recapture protocol to isolate a plasmid that contained an insert spanning the entire rpp locus. Sequencing of the insert revealed that the disrupted open reading frame (ORF) encodes a CS-like protein displaying 69% sequence identity to the rppA gene of Streptomyces griseus. The S. griseus rppA gene encodes RppA, the first characterized bacterial CS-like protein, which is sufficient in vitro for the synthesis of THN from malonyl-CoA. The rppA disruption mutant and rppA sequence provided a means by which to address the mechanism of diffusible pigment biosynthesis, as well as to investigate any link between this and the modulation of erythromycin A titre, which has been observed for S. erythraea variants.     

青龙衣中细胞毒活性成分的研究

利用各种化学及色谱技术从青龙衣中分离得到11个化合物,通过理化性质和波谱学手段分别鉴定为2-羟基-1,4-萘醌(2-hydroxy-1,4-naphthoquinone,1)、5-羟基-1,4-萘醌(5-hydroxy-1,4-naphthoquinone,2)、2,5-二羟基-1,4-萘醌(2,5-dihydroxy-1,4-naphthoquinone,3)、3,5-二羟基-1,4-萘醌(2,5-dihydroxy-1,4-naphthoquinone,4)、5,8-二羟基-1,4-萘醌(2,5-dihydroxy-1,4-naphthoquinone,5)、5-甲氧基-1,4-萘醌(5-methoxy-1,4-naphthoquinone,6)、5,7-二羟基色原酮(5,7-dihydroxychromone,7)、异香草酸(isovanillic acid,8)、没食子酸(gallic acid,9)、β-谷甾醇(β-sitosterol,10)和β-胡萝卜苷(β-daucosterol,11)。化合物3、4、6～8为首次从该属植物中分离得到,化合物1为首次从该植物中分离得到。细胞毒活性测试结果表明,化合物3和4对HepG2细胞表现出强的抑制作用,IC50值分别为5.0±0.6μmol/L和7.0±0.5μmol/L;2和5能显著抑制HL-60细胞的增殖,IC50值分别为9.3±1.2μmol/L和2.3±0.2μmol/L。     

Structures of Naphthoquinones Produced by the Fungus,Fusarium sp., and Their Biological Activity toward Pollen Germination

From an unidentified Fusarium sp., eight substances possessing a 1,4-naphthoquinone skeleton were isolated (F2~F9) and their structures were elucidated from the physical and chemical evidence. The effect of these compounds together with 1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone and 5,8-dihydroxy-1,4-naphthoquinone on the pollen germination of Pinus thunbergii Pari was examined. 5,8-Dihydroxy-1,4-naphthoquinone showed marked inhibitory activity.     

Aminonaphthoquinone induces oxidative stress in Staphylococcus aureus.

The biological activity of 5-amino-8-hydroxy-1,4-naphthoquinone (ANQ) on Staphylococcus aureus was investigated in comparison with the unsubstituted 1,4-naphthoquinone (NQ). Complete inhibition of microbial growth was observed with ANQ and NQ at 50 and 10 microg/mL, respectively. The antibacterial effect of naphthoquinones decreased in the presence of sodium ascorbate, but the superoxide scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron) was able to protect S. aureus only from the harmful effect of ANQ. Naphthoquinones blocked oxygen uptake and induced cyanide-insensitive oxygen consumption. When combining rotenone or salicylhydroxamic acid with ANQ or NQ, a slight decrease in respiratory activity was observed. Assays in the presence of naphthoquinones induced an increase of lipid peroxidation in S. aureus, as determined by thiobarbituric acid reactive substances. These results showed that 1,4-naphthoquinones effectively act as electron acceptors and induce an increase in reactive oxygen species that are toxic to S. aureus cells.     

Synthesis of fluoro- and hydroxy-derivatives of vitamin K as substrates or inhibitors of the liver microsomal vitamin K-dependent carboxylase.

The rat liver microsomal vitamin K-dependent carboxylase catalyzes the carboxylation of glutamyl to gamma-carboxyglutamyl residues in the presence of reduced vitamin K, O2 and CO2. The specificity of the enzyme for the vitamin substrate has been probed by the synthesis of a series of fluoro- hydroxy- and methoxy-analogs. 2-Fluoro-methyl-3-phytyl-1,4-naphthoquinone and 2-methyl-3-(1'-fluorodecyl)-1,4-naphthoquinone were synthesized but found to be unstable under enzyme assay conditions. The reduced (naphthohydroquinone) forms of 2-hydroxy-methyl-3-phytyl-1,4-naphthoquinone, 2-methoxymethyl-3-phytyl-1,4-naphthoquinone and 2-methyl-3-(1'-hydroxy-decyl)-1,4-naphthoquinone were inactive as substrates, but inhibitors of the enzyme. The two hydroxy analogs were shown to be low Ki (less than 10 microM) inhibitors of the reduced 2-methyl-3-phytyl-1,4-naphthoquinone-dependent activity of the enzyme. The oxidized forms of these compounds did not inhibit the enzyme and they had no activity as in vivo anticoagulants.     

A New Group of Aromatic Prenyltransferases in Fungi,Catalyzing a 2,7-Dihydroxynaphthalene 3-Dimethylallyl-transferase Reaction

Five fungal genomes from the Ascomycota (sac fungi) were found to contain a gene with sequence similarity to a recently discovered small group of bacterial prenyltransferases that catalyze the C-prenylation of aromatic substrates in secondary metabolism. The genes from Aspergillus terreus NIH2624, Botryotinia fuckeliana B05.10 and Sclerotinia sclerotiorum 1980 were expressed in Escherichia coli, and the resulting His₈-tagged proteins were purified and investigated biochemically. Their substrate specificity was found to be different from that of any other prenyltransferase investigated previously. Using 2,7-dihydroxynaphthalene (2,7-DHN) and dimethylallyl diphosphate as substrates, they catalyzed a regiospecific Friedel-Crafts alkylation of 2,7-DHN at position 3. Using the enzyme of A. terreus, the K_m values for 2,7-DHN and dimethylallyl diphosphate were determined as 324 ± 25 μm and 325 ± 35 μm, respectively, and k_cat as 0.026 ± 0.001 s⁻¹. A significantly lower level of prenylation activity was found using dihydrophenazine-1-carboxylic acid as aromatic substrate, and only traces of products were detected with aspulvinone E, flaviolin, or 4-hydroxybenzoic acid. No product was formed with l-tryptophan, l-tyrosine, or 4-hydroxyphenylpyruvate. The genes for these fungal prenyltransferases are not located within recognizable secondary metabolic gene clusters. Their physiological function is yet unknown.     

Studies on Effects of Certain Quinones: II. Photosynthetic Incorporation of CO(2) by Chlorella

The effects of various quinone herbicides and fungicides on the photosynthetic ¹⁴CO₂ fixation and the incorporation of ¹⁴C among the products of photosynthesis in Chlorella pyrenoidosa was investigated. Addition of 30 μm 2,3-dichloro-1,4-naphthoquinone (dichlone), 2-amino-3-chloro-1,4-naphthoquinone (06K-quinone), or 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) inhibited CO₂ fixation, whereas 1,4-benzoquinone had no effect. Treatment with 3 μm or higher concentrations of dichlone, 06K-quinone or 1,4-benzoquinone also produced marked changes in the pattern of ¹⁴C distribution. A noticeable effect was an increase in the proportion of ¹⁴C in sucrose and glycine accompanied by a reduction in ¹⁴C lipids and glutamic acid. These changes appear to occur as a result of shifts in the flow of carbon along various biosynthetic pathways of photosynthetic CO₂ fixation. It is suggested that inactivation of coenzyme A and shortage of reduced triphosphopyridine nucleotide in the quinone-treated cells inhibited the synthesis of lipids and glutamic acid, thereby diverting more carbon into sucrose and glycine.     

An unusual reduction on the quinonoid ring of 5-amino-8-hydroxy-1,4-naphthoquinone by Staphylococcus aureus

AIMS: The objective of this study was to investigate the interactions between 5-amino-8-hydroxy-1,4-naphthoquinone (ANQ) and Staphylococcus aureus. METHODS AND RESULTS: The compound ANQ display antimicrobial activity against S. aureus. During incubation with 50 microg ml(-1) of ANQ, an unusual reduction reaction takes place and leads to the isolation of 2,3-dihydro-5-amino-8-hydroxy-1,4-naphthoquinone (ANQ-H(2)), fully characterized by means of (13)C-NMR and (1)H-NMR, plus infrared, UV-visible and mass spectroscopy. Oxygen uptake by S. aureus cells was inhibited by ANQ, but in a significantly minor extent by ANQ-H(2). CONCLUSIONS: The ability of S. aureus to reduce the double bond at C2-C3 of the ANQ is a unusual behaviour for biological transformation of naphthoquinones. SIGNIFICANCE AND IMPACT OF THE STUDY: This uncommon reaction may provide valuable understanding of the S. aureus regarding to the antimicrobial effect and the acquisition of resistance to naphthoquinones.     

Isolation of Natural Naphthoquinones from <Emphasis Type="Italic">Juglans regia</Emphasis> and In Vitro Antioxidant and Cytotoxic Studies of Naphthoquinones and the Synthetic Naphthofuran Derivatives

The naphthoquinones and their derivatives containing hydroxyl group exhibit wide range of pharmacological activities, such as antioxidant, antibacterial, antiviral, anticancer, antimalarial, and antifungal activities. In particular, the antioxidant and anticancer behaviors of these compounds continue to draw attention of researchers. In the present communication, three natural naphthoquinones—juglone, lawsone, and plumbagin—isolated from the chloroform extract of nutshells of Juglans regia Linn. and two 1,4-naphthoquinone derivatives—ethyl-5-hydroxynaphtho[ 1,2-b]furan-3-carboxylate and diethylnaphtho[1,2-b:4,3-b′]difuran-3,4-dicarboxylate—and three 5-hydroxy- 1,4-naphthoquinone derivatives—diethyl-7-hydroxynaphtho[1,2-b:4,3-b']difuran-3,4-dicarboxylate,4-ethoxycarbonyl- 7-hydroxynaphtho[1,2-b:4,3-b']difuran-3-carboxylic acid, and 7-hydroxynaphtho[1,2-b:4,3-b']difuran-3,4- dicarboxylic acid were synthesized and examined for their in vitro antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid (ABTS) bioassays. In addition, the cytotoxicity test using human hepatocellular liver carcinoma cell line (HepG2) was carried out for all the compounds. The 5-hydroxy-1,4-naphthoquinone derivatives displayed almost equivalent scavenging activity in DPPH assay and higher activity in ABTS assay relative to ascorbic acid. On the other hand, naphthoquinones Juglone and Plumbagin showed lesser antioxidant activity, but higher cytotoxic activity than naphthofurans except for diethyl naphtho[1,2-b:4,3-b′]difuran-3,4-dicarboxylate, which showed excellent cytotoxic activity.