Indole alkaloid biosynthesis in Catharanthus roseus: new enzyme activities and identification of cytochrome P450 CYP72A1 as secologanin synthase

The molecular characterization of CYP72A1 from Catharanthus roseus (Madagascar periwinkle) was described nearly a decade ago, but the enzyme function remained unknown. We now show by in situ hybridization and immunohistochemistry that the expression in immature leaves is epidermis-specific. It thus follows the pattern previously established for early enzymes in the pathway to indole alkaloids, suggesting that CYP72A1 may be involved in their biosynthesis. The early reactions in that pathway, i.e. from geraniol to strictosidine, contain several candidates for P450 activities. We investigated in this work two reactions, the conversion of 7-deoxyloganin to loganin (deoxyloganin 7-hydroxylase, DL7H) and the oxidative ring cleavage converting loganin into secologanin (secologanin synthase, SLS). The action of DL7H has not been demonstrated in vitro previously, and SLS has only recently been identified as P450 activity in one other plant. We show for the first time that both enzyme activities are present in microsomes from C. roseus cell cultures. We then tested whether CYP72A1 expressed in E. coli as a translational fusion with the C. roseus P450 reductase (P450Red) has one or both of these activities. The results show that CYP72A1 converts loganin into secologanin.

An alternative methylation pathway in lignin biosynthesis in Zinnia

S-Adenosyl-L-methionine:trans-caffeoyl-coenzyme A 3-O-methyltransferase (CCoAOMT) is implicated in disease resistant response, but whether it is involved in lignin biosynthesis is not known. We isolated a cDNA clone for CCoAOMT in differentiating tracheary elements (TEs) induced from Zinnia-isolated mesophyll cells. RNA gel blot analysis showed that the expression of the CCoAOMT gene was markedly induced during TE differentiation from the isolated mesophyll cells. Tissue print hybridization showed that the expression of the CCoAOMT gene is temporally and spatially regulated and that it is associated with lignification in xylem and in phloem fibers in Zinnia organs. Both CCoAOMT and caffeic acid O-methyltransferase (COMT) activities increased when the isolated Zinnia mesophyll cells were cultured, whereas only CCoAOMT activity was markedly enhanced during lignification in the in vitro-differentiating TEs. The induction pattern of the OMT activity using 5-hydroxyferuloyl CoA as substrate during lignification was the same as that using caffeoyl CoA. Taken together, the results indicate that CCoAOMT is associated with lignification during xylogenesis both in vitro and in the plant, whereas COMT is only involved in a stress response in vitro. We propose that CCoAOMT is involved in an alternative methylation pathway in lignin biosynthesis. In Zinnia in vitro-differentiating TEs, the CCoAOMT mediated methylation pathway is dominant.

Multifunctional flavonoid dioxygenases: flavonol and anthocyanin biosynthesis in Arabidopsis thaliana L

Flavonols and conditionally also anthocyanins, aside from flavonols, are the predominant polyphenols accumulated in various tissues of the model plant Arabidopsis thaliana L. In vitro experiments suggested that the dioxygenases involved in their biosynthesis, flavonol synthase and anthocyanidin synthase, are "multifunctional" enzymes showing distinct side activities. The in vivo relevance of the additional activities attributed to these enzymes, however, has remained obscure. In this review we summarize the most recent results and present final proof of the complementing activities of these synthases for flavonol and anthocyanidin formation in the model plant A. thaliana. The impact of their modification on the biosynthetic pathway and the pattern of flavonoids in different plant tissues are discussed.

Crystal structure of brefeldin A esterase, a bacterial homolog of the mammalian hormone-sensitive lipase

Brefeldin A esterase (BFAE), a detoxifying enzyme isolated from Bacillus subtilis, hydrolyzes and inactivates BFA, a potent fungal inhibitor of intracellular vesicle-dependent secretory transport and poliovirus RNA replication. We have solved the crystal structure of BFAE and we discovered that the previously reported amino acid sequence was in serious error due to frame shifts in the cDNA sequence. The correct sequence, inferred from the experimentally phased electron density map, revealed that BFAE is a homolog of the mammalian hormone sensitive lipase (HSL). It is a canonical alpha/beta hydrolase with two insertions forming the substrate binding pocket. The enzyme contains a lipase-like catalytic triad, Ser 202, Asp 308 and His 338, consistent with mutational studies that implicate the homologous Ser 424, Asp 693 and His 723 in the catalytic triad in human HSL.

Reaction to phytotoxins in a potato population derived from mesophyll protoplasts

Alternaria solani, the causal agent of early blight disease in potato, produces two host-specific, lipidlike toxins in culture. Both compounds are required in the leaf bioassay for the elicitation of typical early blight symptoms, but the compounds are individually inactive. The procedures for the preparation of both compounds are outlined. These compounds can be used effectively to select for toxin-insensitive and sensitive clones of a Russet Burbank potato cultivar that have been regenerated from single mesophyll protoplasts. Furthermore, both sensitivity and insensitivity to the toxins in these clones is well correlated with susceptibility and resistance to A. solani. Potato clones that have been produced by somatic cell regeneration techniques maintain their reaction type to these fungal toxins for at least two generations of vegetative propagation. The genetic basis for this variation among these potato clones remains to be explained.

Differential response of cultured parsley cells to elicitors from two non-pathogenic strains of fungi. 1. Identification of induced products as coumarin derivatives

Dark-grown cell suspension cultures of parsley, Petroselinum hortense, produce furanocoumarins after treatment with elicitor preparations of either Phytophthora megasperma f.sp. glycinea (Pmg elicitor) or Alternaria carthami Chowdhury (Ac elicitor). The linear furanocoumarins, psoralen and xanthotoxin, and the benzodipyrandione, graveolone, are the major products synthesized in response to Pmg elicitor, besides small amounts of the furanocoumarin bergapten. Treatment with Ac elicitor induces predominantly the formation of bergapten and the furanocoumarin isopimpinellin, as well as small amounts of graveolone. While Pmg elicitor leads to cell death within a few days, cell mass increased for at least 6 days after treatment with Ac elicitor. Brefeldin A, a phytotoxin produced by A. carthami, inhibits growth of parsley cell suspension cultures considerably at a concentration of 0.01 mM and growth of the cells ceased at a concentration of 0.1 mM toxin. Concomitantly, furanocoumarin biosynthesis was suppressed in our system by a concentration of brefeldin A within 0.01-0.1 mM.

Characterization and heterologous expression of hydroxycinnamoyl/benzoyl-CoA:anthranilate N-hydroxycinnamoyl/benzoyltransferase from elicited cell cultures of carnation, Dianthus caryophyllus L

Benzoyl-CoA:anthranilate N-benzoyltransferase catalyzes the first committed reaction of phytoalexin biosynthesis in carnation (Dianthus caryophyllus L.), and the product N-benzoylanthranilate is the precursor of several sets of dianthramides. The transferase activity is constitutively expressed in suspension-cultured carnation cells and can be rapidly induced by the addition of yeast extract. The enzyme was purified to homogeneity from yeast-induced carnation cells and shown to consist of a single polypeptide chain of 53 kDa. Roughly 20% of the sequence was identified by micro-sequencing of tryptic peptides, and some of these sequences differed in a few amino acid residues only suggesting the presence of isoenzymes. A specific 0.8 kb cDNA probe was generated by RT-PCR, employing degenerated oligonucleotide primers complementary to two of the tryptic peptides and using poly(A)+ RNA from elicited carnation cells. Five distinct benzoyltransferase clones were isolated from a cDNA library, and three cDNAs, pchcbt1-3, were sequenced and shown to encode full-size N-benzoyltransferases. The translated peptide sequences revealed more than 95% identity among these three clones. The additional two clones harbored insert sequences mostly homologous with pchcbt 1 but differing in the 3'-flanking regions due to variable usage of poly(A) addition sites. The identity of the clones was confirmed by matching the translated polypeptides with the tryptic enzyme sequences as well as by the activity of the benzoyltransferase expressed in Escherichia coli. Therefore, carnation encodes a small family of anthranilate N-benzoyltransferase genes. In vitro, the benzoyltransferases exhibited narrow substrate specificity for anthranilate but accepted a variety of aromatic acyl-CoAs. Catalytic rates with cinnamoyl- or 4-coumaroyl-CoA exceeded those observed with benzoyl-CoA, although the corresponding dianthramides did not accumulate in vivo. Thus the cDNAs described represent also the first hydroxycinnamoyl-transferases cloned from plants, which classifies the enzymes as hydroxycinnamoyl/benzoyltransferases.

Instruments for minimally invasive surgery: principles of ergonomic handles

Although the advantages of minimally invasive surgery (MIS) have been clearly established for the patient, the surgeon must cope with disadvantages caused by unergonomic instrument handles. Pressure areas and persisting nerve lesions have been described in the literature. The shape of the instrument handles has been identified as the reason for these disorders. To prevent these, it is necessary to use ergonomically designed handles for MIS instruments. Anatomic, physiologic, and ergonomic facts as well as the results of the authors' own experiences and tests are presented. On this basis, an ideal ergonomic working posture for the laparoscopic surgeon and an optimal grasp for manipulating the instruments' functional elements are recommended. To enable the surgeon to evaluate ergonomic handles for MIS instruments according to his own needs, 14 criteria for genuine "ergonomic handles" are established. On the basis of these criteria, deficiencies of handles currently available (ring and shank handles at an angle or with axial extension to the instrument shaft, and pistol handles) are discussed. Furthermore, new handles, developed by the authors according to the criteria for genuine ergonomic handles, are presented.

Flavonol synthase from Citrus unshiu is a bifunctional dioxygenase

Flavonol synthase was classified as a 2-oxoglutarate-dependent dioxygenase converting natural (2R,3R)-dihydroflavonols, i.e. dihydrokaempferol, to the corresponding flavonols (kaempferol). Flavonol synthase from Citrus unshiu (Satsuma mandarin), expressed in Escherichia coli and purified to homogeneity, was shown to accept also (2S)-naringenin as a substrate, producing kaempferol in high yield and assigning sequential flavanone 3beta-hydroxylase and flavonol synthase activities to the enzyme. In contrast, dihydrokaempferol was identified as the predominant product from assays performed with the unnatural (2R)-naringenin as substrate. The product which was not converted any further on repeated incubations was identified by 1H NMR and CD spectroscopies as (-)-trans-dihydrokaempferol. The data demonstrate that Citrus flavonol synthase encompasses an additional non-specific activity trans-hydroxylating the flavanones (2S)-naringenin as well as the unnatural (2R)-naringenin at C-3.

Monitor position in laparoscopic surgery

BACKGROUND

One of the key problems in laparoscopy is the ergonomic positioning of the monitor. In this study we tested task performance and muscle strain of subjects in relation to monitor position during laparoscopic surgery.

METHODS

Eighteen subjects simulated laparoscopic suturing by threading tiny pearls with a curved needle. This was repeated in three monitor positions (15 min each): frontal at eye level (A), frontal in height of the operating field (B), and 45 degrees to the right side at eye level (C). Subjects were not allowed to turn their heads during these sessions. After the test they were asked for their preferred monitor position. During all tests the electromyographic (EMG) activity of the main neck muscles was recorded and the number of pearls was counted.

RESULTS

The EMG activity was significantly lower for position A compared to positions C and B (p < 0.05). No significant difference was found between positions B and C. The number of threaded pearls as an indicator for task performance was highest for position B. The difference was statistically significant compared to position C (p = 0.0008) but not between positions A and C (p = 0.0508) or A and B (p = 0.0575). When asked for the preferred monitor position, nine subjects chose two monitors in the frontal positions A and B. No subject preferred the monitor at the side position (C).

CONCLUSION

Regarding EMG data, the monitor positioned frontal at eye level is preferable. Reflecting personal preferences of subjects and task performance, it should be of advantage to place two monitors in front of the surgeon: one in position A for lowest neck strain and the other in position B for difficult tasks with optimal task performance. The monitor position at the side is not advisable.

Cloning of parsley flavone synthase I

A cDNA encoding flavone synthase I was amplified by RT-PCR from leaflets of Petroselinum crispum cv. Italian Giant seedlings and functionally expressed in yeast cells. The identity of the recombinant, 2-oxoglutarate-dependent enzyme was verified in assays converting (2S)-naringenin to apigenin.

Divergent evolution of flavonoid 2-oxoglutarate-dependent dioxygenases in parsley

Flavone synthases (FNSs) catalyze the oxidation of flavanones to flavones, i.e. the formation of apigenin from (2S)-naringenin. While many plants express a microsomal-type FNS II, the soluble FNS I appears to be confined to a few species of the Apiaceae and was cloned recently from parsley plants. FNS I belongs to the Fe(II)/2-oxoglutarate-dependent dioxygenases characterized by short conserved sequence elements for cofactor binding, and its evolutionary context and mode of action are under investigation. Using a homology-based reverse transcription polymerase chain reaction approach, two additional flavonoid-specific dioxygenases were cloned from immature parsley leaflets, which were identified as flavanone 3beta-hydroxylase (FHT) and flavonol synthase (FLS) after expression in yeast cells. Sequence alignments revealed marginal differences among the parsley FNS I and FHT polypeptides of only 6%, while much less identity (about 29%) was observed with the parsley FLS. Analogous to FNS I, FLS oxidizes the flavonoid gamma-pyrone by introducing a C2, C3 double bond, and (2R,3S)-dihydrokaempferol (cis-dihydrokaempferol) was proposed recently as the most likely intermediate in both FNS I and FLS catalysis. Incubation of either FNS I or FLS with cis-dihydrokaempferol exclusively produced kaempferol and confirmed the assumption that flavonol formation occurs via hydroxylation at C3 followed by dehydratation. However, the lack of apigenin in these incubations ruled out cis-dihydrokaempferol as a free intermediate in FNS I catalysis. Furthermore, neither (+)-trans-dihydrokaempferol nor unnatural (-)-trans-dihydrokaempferol and 2-hydroxynaringenin served as a substrate for FNS I. Overall, the data suggest that FNS I has evolved uniquely in some Apiaceae as a paraphyletic gene from FHT, irrespective of the fact that FNS I and FLS catalyze equivalent desaturation reactions.

Molecular evolution of flavonoid dioxygenases in the family Apiaceae

Plant species of the family Apiaceae are known to accumulate flavonoids mainly in the form of flavones and flavonols. Three 2-oxoglutarate-dependent dioxygenases, flavone synthase or flavanone 3 beta-hydroxylase and flavonol synthase are involved in the biosynthesis of these secondary metabolites. The corresponding genes were cloned recently from parsley (Petroselinum crispum) leaves. Flavone synthase I appears to be confined to the Apiaceae, and the unique occurrence as well as its high sequence similarity to flavanone 3beta-hydroxylase laid the basis for evolutionary studies. In order to examine the relationship of these two enzymes throughout the Apiaceae, RT-PCR based cloning and functional identification of flavone synthases I or flavanone 3beta-hydroxylases were accomplished from Ammi majus, Anethum graveolens, Apium graveolens, Pimpinella anisum, Conium maculatum and Daucus carota, yielding three additional synthase and three additional hydroxylase cDNAs. Molecular and phylogenetic analyses of these sequences were compatible with the phylogeny based on morphological characteristics and suggested that flavone synthase I most likely resulted from gene duplication of flavanone 3beta-hydroxylase, and functional diversification at some point during the development of the apiaceae subfamilies. Furthermore, the genomic sequences from Petroselinum crispum and Daucus carota revealed two introns in each of the synthases and a lack of introns in the hydroxylases. These results might be explained by intron losses from the hydroxylases occurring at a later stage of evolution.

Safety, hazards and ergonomics in the operating room

OBJECTIVE

The objective for this study was to address the lack of information regarding the working conditions in the operating room (OR). Safety issues in the OR need to be discussed not only for the sake of patients, but also for personnel, as hazards may occur for all persons within the OR.

METHODS

To evaluate the workplace conditions in the operating room, a survey was conducted among surgeons working in German hospitals. Sixty questions were asked regarding the personal profile, the architectural situation, the devices and instruments as well as working posture and associated pain.

RESULTS

The survey showed elementary ergonomic deficiencies within all fields. Surgeons stated that these deficiencies lead to potential hazards for patients and personnel, potentially on a frequent basis. 97% of the surveyed surgeons see ergonomic improvement in the operating room as necessary.

CONCLUSION

The survey results display a high potential for improvement within all fields. Therefore, industry, surgeons and their professional organizations are asked to work on the optimization of the workplace conditions in the operating room in terms of improvement of quality and efficiency.

S-adenosyl-L-methionine:trans-caffeoyl-coenzyme A 3-O-methyltransferase from elicitor-treated parsley cell suspension cultures

An S-adenosyl-L-methionine:caffeoyl-CoA 3-O-methyltransferase was purified 82-fold from elicitor-induced parsley cell suspension cultures by ammonium sulfate fractionation, anionic exchange and hydrophobic interaction chromatographies, and chromatofocusing. The enzyme has an apparent pI of 5.7 and a molecular weight of approx 48,000 determined by gel filtration chromatography. Maximal activity was observed at pH 7.5 in 50 mM phosphate or Tris-HCl buffers and the additional presence of 0.5 M NaCl. The methyltransferase activity was dependent on Mg2+, whereas EDTA, Mn2+, and Ca2+ inhibited the reaction. The partially purified enzyme efficiently catalyzed the methylation of caffeoyl-CoA, but also accepted with low affinity various other caffeic esters as substrates. Dark-grown parsley cells contained considerable methyltransferase activity which was nevertheless increased approx threefold within 12 h following the addition of a crude fungal elicitor to the cell suspensions. We propose that the O-methyltransferase activity is an important component in the rapid resistance response of the cells, which depends on the formation of cell wall-bound ferulic polymers.

Molecular cloning and functional characterization of psoralen synthase, the first committed monooxygenase of furanocoumarin biosynthesis

Ammi majus L. accumulates linear furanocoumarins by cytochrome P450 (CYP)-dependent conversion of 6-prenylumbelliferone via (+)-marmesin to psoralen. Relevant activities, i.e. psoralen synthase, are induced rapidly from negligible background levels upon elicitation of A. majus cultures with transient maxima at 9-10 h and were recovered in labile microsomes. Expressed sequence tags were cloned from elicited Ammi cells by a nested DD-RT-PCR strategy with CYP-specific primers, and full-size cDNAs were generated from those fragments correlated in abundance with the induction profile of furanocoumarin-specific activities. One of these cDNAs representing a transcript of maximal abundance at 4 h of elicitation was assigned CYP71AJ1. Functional expression in Escherichia coli or yeast cells initially failed but was accomplished eventually in yeast cells after swapping the N-terminal membrane anchor domain with that of CYP73A1. The recombinant enzyme was identified as psoralen synthase with narrow substrate specificity for (+)-marmesin. Psoralen synthase catalyzes a unique carbon-chain cleavage reaction concomitantly releasing acetone by syn-elimination. Related plants, i.e. Heracleum mantegazzianum, are known to produce both linear and angular furanocoumarins by analogous conversion of 8-prenylumbelliferone via (+)-columbianetin to angelicin, and it was suggested that angelicin synthase has evolved from psoralen synthase. However, (+)-columbianetin failed as substrate but competitively inhibited psoralen synthase activity. Analogy modeling and docked solutions defined the conditions for high affinity substrate binding and predicted the minimal requirements to accommodate (+)-columbianetin in the active site cavity. The studies suggested that several point mutations are necessary to pave the road toward angelicin synthase evolution.

Arabidopsis thaliana expresses a second functional flavonol synthase

Arabidopsis thaliana L. produces flavonoid pigments, i.e. flavonols, anthocyanidins and proanthocyanidins, from dihydroflavonol substrates. A small family of putative flavonol synthase (FLS) genes had been recognized in Arabidopsis, and functional activity was attributed only to FLS1. Nevertheless, other FLS activities must be present, because A. thalianafls1 mutants still accumulate significant amounts of flavonols. The recombinant FLSs and leucoanthocyanidin dioxygenase (LDOX) proteins were therefore examined for their enzyme activities, which led to the identification of FLS3 as a second active FLS. This enzyme is therefore likely responsible for the formation of flavonols in the ldox/fls1-2 double mutant. These double mutant and biochemical data demonstrate for the first time that LDOX is capable of catalyzing the in planta formation of flavonols.

Plant cell wall reinforcement in the disease-resistance response: molecular composition and regulation

The disease-resistance response of plant cells is composed of a multitude of biochemical events, and the activation of one of these, the phenylpropanoid metabolism, is pivotal for the survival of cells under stress conditions. The basic features of this facet of the disease-resistance response are beginning to be unraveled in model plant cell culture systems. These studies revealed a novel, alternative pathway for the synthesis of cell wall bound hydroxycinnamoyl esters and lignin. The investigations have, therefore, set the stage for a detailed analysis of the induction process that includes fast, posttranslational activation mechanisms as well as de novo enzyme synthesis. The biosynthesis of phenolic compounds destined for the cell wall is considered to reach far beyond the mere physical strengthening of the cells and includes additional functions, e.g., the release of antimycotic hydroxybenzaldehydes, which are vital for stress compensation. Key words: elicitor-induced phenylpropanoids, cell wall reinforcement, hydroxycinnamoyl esters, lignin, caffeoyl-CoA-specific 3-O-methyltransferase, disease resistance response, parsley (Petroselinum crispum) cell cultures.

Evolution of flavone synthase I from parsley flavanone 3beta-hydroxylase by site-directed mutagenesis

Flavanone 3beta-hydroxylase (FHT) and flavone synthase I (FNS I) are 2-oxoglutarate-dependent dioxygenases with 80% sequence identity, which catalyze distinct reactions in flavonoid biosynthesis. However, FNS I has been reported exclusively from a few Apiaceae species, whereas FHTs are more abundant. Domain-swapping experiments joining the N terminus of parsley (Petroselinum crispum) FHT with the C terminus of parsley FNS I and vice versa revealed that the C-terminal portion is not essential for FNS I activity. Sequence alignments identified 26 amino acid substitutions conserved in FHT versus FNS I genes. Homology modeling, based on the related anthocyanidin synthase structure, assigned seven of these amino acids (FHT/FNS I, M106T, I115T, V116I, I131F, D195E, V200I, L215V, and K216R) to the active site. Accordingly, FHT was modified by site-directed mutagenesis, creating mutants encoding from one to seven substitutions, which were expressed in yeast (Saccharomyces cerevisiae) for FNS I and FHT assays. The exchange I131F in combination with either M106T and D195E or L215V and K216R replacements was sufficient to confer some FNS I side activity. Introduction of all seven FNS I substitutions into the FHT sequence, however, caused a nearly complete change in enzyme activity from FHT to FNS I. Both FHT and FNS I were proposed to initially withdraw the beta-face-configured hydrogen from carbon-3 of the naringenin substrate. Our results suggest that the 7-fold substitution affects the orientation of the substrate in the active-site pocket such that this is followed by syn-elimination of hydrogen from carbon-2 (FNS I reaction) rather than the rebound hydroxylation of carbon-3 (FHT reaction).

Differential expression of chitinases in Vitis vinifera L. responding to systemic acquired resistance activators or fungal challenge

The concept of systemic acquired resistance (SAR) enables a novel approach to crop protection, and particular pathogenesis-related proteins, i.e. an acidic chitinase, have been classified as markers of the SAR response. Basic class I (VCHIT1b) and a class III (VCH3) chitinase cDNAs were cloned from cultured Vitis vinifera L. cv Pinot Noir cells and used to probe the induction response of grapevine cells to salicylic acid or yeast elicitor. Furthermore, the cells were treated with the commercial SAR activators 2,6-dichloroiso-nicotinic acid or benzo(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester. Elicitor or salicylic acid induced both VCHIT1b and VCH3 transcript abundances, whereas 2,6-dichloroiso-nicotinic acid or benzo(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester enhanced exclusively the expression of VCH3. To assess the systemic sensation of chitinase expression, single leaves of Vitis vinifera L. cv Pinot Noir or Vitis rupestris plants were inoculated with Plasmopara viticola spore suspensions, and the VCH3 and VCHIT1b mRNA amounts in the infected versus the adjacent, healthy leaf were monitored. Two VCH3 mRNA maxima were observed 2 and 6 d postinoculation in the infected, susceptible V. vinifera tissue, whereas in the healthy leaf the transcript increased from low levels d 2 postinoculation to prominent levels d 6 to 8 postinoculation. The level of VCH3 mRNA increased also over 4 d in the inoculated, resistant V. rupestris tissue. However, necrotic spots rapidly limited the infection, and the VCH3 transcript was undetectable in the upper-stage, healthy leaf. The expression of VCHIT1b remained negligible under either experimental condition. Overall, the results suggest that the selective expression of VCH3 might be a reliable indicator of the SAR response in V. vinifera L.

Anthocyanidin synthase fromGerbera hybridacatalyzes the conversion of (+)-catechin to cyanidin and a novel procyanidin

Anthocyanidins were proposed to derive from (+)-naringenin via (2R,3R)-dihydroflavonol(s) and (2R,3S,4S)-leucocyanidin(s) which are eventually oxidized by anthocyanidin synthase (ANS). Recently, the role of ANS has been put into question, because the recombinant enzyme from Arabidopsis exhibited primarily flavonol synthase (FLS) activity with negligible ANS activity. This and other studies led to the proposal that ANS as well as FLS may select for dihydroflavonoid substrates carrying a "beta-face" C-3 hydroxyl group and initially form the 3-geminal diol by "alpha-face" hydroxylation. Assays with recombinant ANS from Gerbera hybrida fully supported the proposal and were extended to catechin and epicatechin isomers as potential substrates to delineate the enzyme specificity. Gerbera ANS converted (+)-catechin to two major and one minor product, whereas ent(-)-catechin (2S,3R-trans-catechin), (-)-epicatechin, ent(+)-epicatechin (2S,3S-cis-epicatechin) and (-)-gallocatechin were not accepted. The K(m) value for (+)-catechin was determined at 175 microM, and the products were identified by LC-MS(n) and NMR as the 4,4-dimer of oxidized (+)-catechin (93%), cyanidin (7%) and quercetin (trace). When these incubations were repeated in the presence of UDP-glucose:flavonoid 3-O-glucosyltransferase from Fragariaxananassa (FaGT1), the product ratio shifted to cyanidin 3-O-glucoside (60%), cyanidin (14%) and dimeric oxidized (+)-catechin (26%) at an overall equivalent rate of conversion. The data appear to identify (+)-catechin as another substrate of ANS in vivo and shed new light on the mechanism of its catalysis. Moreover, the enzymatic dimerization of catechin monomers is reported for the first time suggesting a role for ANS beyond the oxidation of leucocyanidins.

Functional expression and mutational analysis of flavonol synthase from Citrus unshiu

Flavonols are produced by the desaturation of flavanols catalyzed by flavonol synthase. The enzyme belongs to the class of intermolecular dioxygenases which depend on molecular oxygen and FeII/2-oxoglutarate for activity, and have been in focus of structural studies recently. Flavonol synthase cDNAs were cloned from six plant species, but none of the enzymes had been studied in detail. Therefore, a cDNA from Citrus unshiu (Satsuma mandarin) designated as flavonol synthase was expressed in Escherichia coli, and the purified recombinant enzyme was subjected to kinetic and mutational chacterizations. The integrity of the recombinant synthase was revealed by a molecular ion from MALDI-TOF mass spectrometry at m/z 37888 +/- 40 (as compared to 37899 Da calculated for the translated polypeptide), and by partial N-terminal sequencing. Maximal flavonol synthase activity was observed in the range of pH 5-6 with dihydroquercetin as substrate and a temperature optimum at about 37 degrees C. Km values of 272, 11 and 36 micro m were determined for dihydroquercetin, FeII and 2-oxoglutarate, respectively, with a sixfold higher affinity to dihydrokaempferol (Km 45 micro m). Flavonol synthase polypeptides share an overall sequence similarity of 85% (47% identity), whereas only 30-60% similarity were apparent with other dioxygenases. Like the other dioxygenases of this class, Citrus flavonol synthase cDNA encodes eight strictly conserved amino-acid residues which include two histidines (His221, His277) and one acidic amino acid (Asp223) residue for FeII-coordination, an arginine (Arg287) proposed to bind 2-oxoglutarate, and four amino acids (Gly68, His75, Gly261, Pro207) with no obvious functionality. Replacements of Gly68 and Gly261 by alanine reduced the catalytic activity by 95%, while the exchange of these Gly residues for proline completely abolished the enzyme activity. Alternatively, the substitution of Pro207 by glycine hardly affected the activity. The data suggest that Gly68 and Gly261, at least, are required for proper folding of the flavonol synthase polypeptide.

Elicitor-induced biosynthesis of psoralens in Ammi majus L. suspension cultures. Microsomal conversion of demethylsuberosin into (+)marmesin and psoralen

Suspension cultures of Ammi majus L. cells produce various linear furanocoumarins in response to treatment with elicitor preparations from either Alternaria carthami Chowdhury or Phytophthora megasperma f.sp. glycinea. Microsomes which were isolated from these cells 14 h after addition of the elicitor efficiently catalyzed the conversion of demethyl [3-14C]suberosin into labelled (+)marmesin in the presence of NADPH and oxygen. In contrast to the chemical cyclization of demethylsuberosin by m-chloroperoxybenzoic acid, the reaction catalyzed by the marmesin synthase proceeded rapidly and no intermediate demethylsuberosin epoxide could be recovered. Significant blue-light-reversible inhibition by carbon monoxide and inhibition by various chemicals known to inhibit reactions dependent on cytochrome P450 suggested that the marmesin synthase is a cytochrome-P450-dependent monooxygenase. Upon prolonged incubation, a subsequent major labelled product originated from (+)marmesin, which was identified as psoralen. The psoralen synthase was also characterized as a cytochrome-P450-dependent monooxygenase. Both the marmesin synthase and the psoralen synthase, as well as enzymes catalyzing the formation of demethylsuberosin and O-prenylumbelliferone from umbelliferone and dimethylallyl diphosphate, were associated with the endoplasmic reticulum in Ammi majus cells and their activities were concomitantly induced by elicitor treatment of the cells. We propose that in vivo these enzymes are active in the lumen of the endoplasmic reticulum from where the furanocoumarin phytoalexins are excreted into the cell culture fluid.