Quinone reductases of higher plants

A review of the ethnobotany, contemporary uses, chemistry and pharmacology of the genus Thesium (Santalaceae)

ETHNOPHARMACOLOGICAL RELEVANCE

Many plant species representing the hemi-parasitic genus Thesium play important roles in communities around the globe as evidenced by the numerous ethnobotanical and contemporary uses, and pharmacological activities. However, no attempt has been made to amalgamate and analyze all of the available information. A comprehensive survey is needed to highlight knowledge gaps, as well as to determine the economic importance and commercial potential of the genus.

AIMS OF THIS REVIEW

To provide a comprehensive report on the species diversity, geographical distribution, ethnobotany, contemporary uses, chemistry, pharmacology and toxicology of the genus Thesium, as well as to give insights into possible future research opportunities.

MATERIALS AND METHODS

Literature on the ethnobotany, contemporary uses, chemistry and pharmacology of Thesium was gathered from standard search engines (Google, Google Scholar, PubMed, SciFinder and Scopus) using the phrase Thesium, as well as generic synonyms. Additional information came from relevant books, theses, patents and label information from herbarium specimens in the National Herbarium in Pretoria, South Africa. Information on geographical distributions was compiled from regional floras, regional revisions, original species descriptions and databases (GBIF, IPNI, Plants of the World online and The Plant List). Scientific names and synonyms were validated through www.plantsoftheworldonline.org.

RESULTS

A total of 23 Thesium species, 17 from Africa and six from Asia, were found to have traditional and contemporary uses. Despite the near cosmopolitan distribution of the genus, no uses were recorded for Europe, Australia and North America. Thesium plants are most commonly used as medicines (18 species), functional foods and beverages (seven species), charms (six species) and crafts (three species), but also have several other minor uses. Charm uses were restricted to southern and East Africa, while several contemporary uses such as functional feeds and fodders, growth mediums and fertilizers and veterinary medicine were unique to Asia. Thesium chinense is by far the most utilized and versatile species with a total of 173 uses in nine use categories, followed by T. longifolium with 39 uses in six use categories. No specific trends were observed in the plant parts used. As a medicine Thesium is used to treat 137 ailments, predominantly reproductive and breast (22), respiratory tract (18), degenerative (11), digestive (11) and urinary (10) ailments. Chemical analyses are available for only eight species, with flavonoids, fatty acids and alkaloids as the main compounds. The potential influence of the host plants on the chemistry and pharmacology remains unexplored. The pharmacological activities of two species, T. chinense and T. viride, have been studied, while three other species are reported to be poisonous. Thesium chinense has analgesic, anti-inflammatory, anti-oxidation, chemopreventive, cytotoxic and other general therapeutic properties, and T. viride anti-bacterial activity.

CONCLUSION

This study has highlighted the ethnobotanical, contemporary and pharmacological importance of Thesium and informed possible future research opportunities. While ample information is available on the traditional uses of the richly diverse African Thesium species, the few Asian species dominate the literature on contemporary uses and pharmacology, while relevant literature on species in the rest of the world is altogether lacking. In light of the popularity of Asian species as ingredients in contemporary medicines and products, further research is needed into African species and their uses, including potential commercial uses. As an important medicinal hemi-parasite in both Africa and Asia, the identities of ethnobotanically relevant species and their phytochemistry, pharmacology and toxicology remain underexplored and require more research attention.

Nitrogen stress-induced alterations in the leaf proteome of two wheat varieties grown at different nitrogen levels

Inorganic nitrogen (N) is a key limiting factor of the agricultural productivity. Nitrogen utilization efficiency has significant impact on crop growth and yield as well as on the reduction in production cost. The excessive nitrogen application is accompanied with severe negative impact on environment. Thus to reduce the environmental contamination, improving NUE is need of an hour. In our study we have deployed comparative proteome analysis using 2-DE to investigate the effect of the nitrogen nutrition on differential expression pattern of leaf proteins in low-N sensitive and low-N tolerant wheat (Triticum aestivum L.) varieties. Results showed a comprehensive picture of the post-transcriptional response to different nitrogen regimes administered which would be expected to serve as a basic platform for further characterization of gene function and regulation. We detected proteins related to photosynthesis, glycolysis, nitrogen metabolism, sulphur metabolism and defence. Our results provide new insights towards the altered protein pattern in response to N stress. Through this study we suggest that genes functioning in many physiological events coordinate the response to availability of nitrogen and also for the improvement of NUE of crops.

Comparative mapping of a QTL controlling black point formation in barley

The dark discoloration of the embryo end of barley grain (known as black point) is a physiological disorder and the discovery of a quantitative trait locus (QTL) on 2H confirms this trait is controlled genetically. The mechanisms underlying black point tolerance can now be dissected through identification of candidate genes. Comparisons between the QTL identified on chromosomes 2H of barley and 2B of wheat suggest that they are in similar positions near the centromere. In silico analysis, using rice, identified genes residing on two comparative chromosomes (4 and 7) of the rice genome. Analysis of the 12.6 Mb region revealed 1928 unique annotations classified into 11 functional categories. Expressed sequence tags (ESTs) with high sequence similarity to enzymes proposed to be involved in black point formation were used to develop restriction fragment length polymorphisms (RFLPs). To ensure an even coverage of markers across the QTL, RFLP markers were also developed from other ESTs. Mapping of these markers has reduced the QTL region from 28 to 18 cM. This study has identified candidate genes for the control of black point formation and paves the way for future research to develop black point resistant barley cultivars.

Naphthoquinone-dependent generation of superoxide radicals by quinone reductase isolated from the plasma membrane of soybean

Using a tetrazolium-based assay, a NAD(P)H oxidoreductase was purified from plasma membranes prepared from soybean (Glycine max) hypocotyls. The enzyme, a tetramer of 85 kD, produces O2(.-) by a reaction that depended on menadione or several other 1,4-naphthoquinones, in apparent agreement with a classification as a one-electron-transferring flavoenzyme producing semiquinone radicals. However, the enzyme displayed catalytic and molecular properties of obligatory two-electron-transferring quinone reductases of the DT-diaphorase type, including insensitivity to inhibition by diphenyleneiodonium. This apparent discrepancy was clarified by investigating the pH-dependent reactivity of menadionehydroquinone toward O2 and identifying the protein by mass spectrometry and immunological techniques. The enzyme turned out to be a classical NAD(P)H:quinone-acceptor oxidoreductase (EC 1.6.5.2, formerly 1.6.99.2) that reduces menadione to menadionehydroquinone and subsequently undergoes autoxidation at pH > or = 6.5. Autoxidation involves the production of the semiquinone as an intermediate, creating the conditions for one-electron reduction of O2. The possible function of this enzyme in the generation of O2(.-) and H2O2 at the plasma membrane of plants in vivo is discussed.

A novel NADPH:diamide oxidoreductase activity in arabidopsis thaliana P1 zeta-crystallin

The zeta-crystallin (ZCr) gene P1 of Arabidopsis thaliana, known to confer tolerance toward the oxidizing drug 1,1'-azobis(N, N-dimethylformamide) (diamide) to yeast [Babiychuk, E., Kushnir, S., Belles-Boix, E., Van Montagu, M. & Inzé, D. (1995) J. Biol. Chem. 270, 26224], was expressed in Escherichia coli to characterize biochemical properties of the P1-zeta-crystallin (P1-ZCr). Recombinant P1-ZCr, a noncovalent dimer, showed NADPH:quinone oxidoreductase activity with specificity to quinones similar to that of guinea-pig ZCr. P1-ZCr also catalyzed the divalent reduction of diamide to 1,2-bis(N,N-dimethylcarbamoyl)hydrazine, with a kcat comparable with that for quinones. Two other azodicarbonyl compounds also served as substrates of P1-ZCr. Guinea-pig ZCr, however, did not catalyze the azodicarbonyl reduction. Hence, plant ZCr is distinct from mammalian ZCr, and can be referred to as NADPH:azodicarbonyl/quinone reductase. The quinone-reducing reaction was accompanied by radical chain reactions to produce superoxide radicals, while the azodicarbonyl-reducing reaction was not. Specificity to NADPH, as judged by kcat/Km, was > 1000-fold higher than that to NADH both for quinones and diamide. N-Ethylmaleimide and p-chloromercuribenzoic acid inhibited both quinone-reducing and diamide-reducing activities. Both NADPH and NADP+ suppressed the inhibition, but NADH did not, suggesting that sulfhydryl groups reside in the binding site for the phosphate group on the adenosine moiety of NADPH. The diamide-reducing activity of P1-ZCr accounts for the tolerance of P1-overexpressing yeast to diamide. Other possible physiological functions of P1-ZCr in plants are discussed.

Molecular characterization of monodehydroascorbate radical reductase from cucumber highly expressed in Escherichia coli

Monodehydroascorbate radical (MDA) reductase, an FAD-enzyme, is the first enzyme to be identified whose substrate is an organic radical and catalyzes the reduction of MDA to ascorbate by NAD(P)H. Its cDNA has been cloned from cucumber seedlings (Sano, S., and Asada, K. (1994) Plant Cell Physiol. 35, 425-437), and a plasmid was constructed in the present study that allowed a high level expression in Escherichia coli of the cDNA-encoding MDA reductase using the T7 RNA polymerase expression system. The recombinant MDA reductase was purified to a crystalline state, with a yield of over 20 mg/liter of culture, and it exhibited spectroscopic properties of the FAD similar to those of the enzyme purified from cucumber fruits during redox reactions with NADH and MDA. The red semiquinone of the FAD of MDA reductase was generated by photoreduction. p-Chloromercuribenzoate inhibited the reduction of the enzyme-FAD by NADH, and dicumarol suppressed electron transfer from the reduced enzyme to MDA. The specificity of electron acceptors of the recombinant enzyme appeared to be similar to that of MDA reductase, even though the amino acid sequence encoded by the cDNA was somewhat different from that of the enzyme purified from cucumber fruits. The Km values for NADH and NADPH of the recombinant enzyme indicated a high affinity of the enzyme for NADH. The reaction catalyzed by the enzyme did not exhibit saturation kinetics with MDA up to 3 microM. A second order rate constant for the reduction of the enzyme-FAD with NADH was 1.25 x 10(8) M-1 s-1, as determined by a stopped-flow method, and its value decreased with increases in ionic strength, an indication of the enhanced electrostatic guidance of NADH to the enzyme-FAD.

Ketopantoyl lactone reductase is a conjugated polyketone reductase

Ketopantoyl lactone reductase (EC 1.1.1.168) of Saccharomyces cerevisiae was found to catalyze the reduction of a variety of natural and unnatural conjugated polyketone compounds and quinones, such as isatin, ninhydrin, camphorquinone and beta-naphthoquinone in the presence of NADPH. 5-Bromoisatin is the best substrate for the enzyme (Km = 3.1 mM; Vmax = 650 mumol/min/mg). The enzyme is inhibited by quercetin, and several polyketones. These results suggest that ketopantoyl lactone reductase is a carbonyl reductase which specifically catalyzes the reduction of conjugated polyketones.

Ketopantoyl-lactone reductase from Candida parapsilosis: purification and characterization as a conjugated polyketone reductase

Ketopantoyl-lactone reductase (2-dehydropantoyl-lactone reductase, EC 1.1.1.168) was purified and crystallized from cells of Candida parapsilosis IFO 0708. The enzyme was found to be homogeneous on ultracentrifugation, high-performance gel-permeation liquid chromatography and SDS-polyacrylamide gel electrophoresis. The relative molecular mass of the native and SDS-treated enzyme is approximately 40,000. The isoelectric point of the enzyme is 6.3. The enzyme was found to catalyze specifically the reduction of a variety of natural and unnatural polyketones and quinones other than ketopantoyl lactone in the presence of NADPH. Isatin and 5-methylisatin are rapidly reduced by the enzyme, the Km and Vmax values for isatin being 14 microM and 306 mumol/min per mg protein, respectively. Ketopantoyl lactone is also a good substrate (Km = 333 microM and Vmax = 481 mumol/min per mg protein). Reverse reaction was not detected with pantoyl lactone and NADP+. The enzyme is inhibited by quercetin, several polyketones and SH-reagents. 3,4-Dihydroxy-3-cyclobutene-1,2-dione, cyclohexenediol-1,2,3,4-tetraone and parabanic acid are uncompetitive inhibitors for the enzyme, the Ki values being 1.4, 0.2 and 3140 microM, respectively, with isatin as substrate. Comparison of the enzyme with the conjugated polyketone reductase of Mucor ambiguus (S. Shimizu, H. Hattori, H. Hata and H. Yamada (1988) Eur. J. Biochem. 174, 37-44) and ketopantoyl-lactone reductase of Saccharomyces cerevisiae suggested that ketopantoyl-lactone reductase is a kind of conjugated polyketone reductase.

A novel fungal enzyme, NADPH-dependent carbonyl reductase, showing high specificity to conjugated polyketones. Purification and characterization

A novel enzyme which specifically catalyzes the reduction of conjugated polyketones was purified to homogeneity from cells of Mucor ambiguus AKU 3006. The enzyme has a strict requirement for NADPH and irreversibly reduces a number of quinones such as p-benzoquinone, alpha-naphthoquinone and acenaphthenequione. The enzyme also reduces polyketones such as isatin and ketopantoyl lactone, and their derivatives. The apparent Km values for isatin and ketopantoyl lactone are 49.9 microM and 714 microM, respectively. The reduction of ketopantoyl lactone proceeds stereospecifically to yield L-(+)-pantoyl lactone. The pro-S (A) hydrogen at C-4 of NADPH is transferred to the substrate. The enzyme is not a flavoprotein and consists of two polypeptide chains with an identical relative molecular mass of 27,500. Quercetin, dicoumarol and some SH reagents inhibit the enzyme activity. 3-Methyl-1,2-cyclopentanedione and 1,3-cyclohexanedione are uncompetitive inhibitors with Ki values of 80.9 microM and 64.5 microM, respectively, to ketopantoyl lactone.

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 α-naphthoquinone, dihydro-α-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.