Inhibitors of the Detoxifying Enzyme of the Phytoalexin Brassinin Based on Quinoline and Isoquinoline Scaffolds.

The detoxification of the phytoalexin brassinin to indole-3-carboxaldehyde and S-methyl dithiocarbamate is catalyzed by brassinin oxidase (BOLm), an inducible fungal enzyme produced by the plant pathogen Leptosphaeria maculans. Twenty-six substituted quinolines and isoquinolines are synthesized and evaluated for antifungal activity against L. maculans and inhibition of BOLm. Eleven compounds that inhibit BOLm activity are reported, of which 3-ethyl-6-phenylquinoline displays the highest inhibitory effect. In general, substituted 3-phenylquinolines show significantly higher inhibitory activities than the corresponding 2-phenylquinolines. Overall, these results indicate that the quinoline scaffold is a good lead to design paldoxins (phytoalexin detoxification inhibitors) that inhibit the detoxification of brassinin by L. maculans.

Metabolism and metabolites of dithiocarbamates in the plant pathogenic fungus Leptosphaeria maculans.

Synthetic compounds containing a dithiocarbamate group are known to have a variety of biological effects and applications including antifungal, herbicidal, and insecticidal application. Leptosphaeria maculans is a fungal pathogen of crucifers able to detoxify efficiently the only plant natural product containing a dithiocarbamate group, the phytoalexin brassinin. To evaluate the effects of dithiocarbamates on L. maculans, a number of structurally diverse S-methyl dithiocarbamates containing indolyl, biphenyl, and benzimidazolyl moieties were synthesized, and their antifungal activities and metabolism by L. maculans were investigated. All dithiocarbamates were transformed by L. maculans through hydrolysis to the corresponding amines, which were less antifungal than the parent compounds. Two dithiocarbonates were shown to be much less antifungal than the corresponding dithiocarbamates. Results of this investigation indicate that S-methyl dithiocarbamates are not useful inhibitors of L. maculans and that their rates of transformation by L. maculans did not correlate with the antifungal activity of the particular compound.

The cruciferous phytoalexins rapalexin A, brussalexin A and erucalexin: chemistry and metabolism in Leptosphaeria maculans.

The interactions of the cruciferous phytoalexins rapalexin A (1), brussalexin A (2) and erucalexin (3) with the fungal plant pathogen Leptosphaeria maculans were analyzed and their inhibitory activities against this pathogen were determined. The reaction of L. maculans to N-methyl S-(indolyl-3-methyl)carbamodithioate, an analogue of brussalexin A, was also investigated. Rapalexin A was resistant to metabolism and was the most inhibitory of all compounds tested, suggesting that increasing concentrations of rapalexin A in Brassica species would improve their disease resistance to L. maculans. By contrast, erucalexin was quickly detoxified by reduction to yield 3-dihydroerucalexins. The relative configurations of the diastereomeric mixture of dihydroerucalexins were established by 1D (1)H nuclear Overhauser enhancement spectroscopy (NOE). Brussalexin A was chemically unstable decomposing mainly to indolyl-3-methanol, a product with anti-cancer properties. For this reason, brussalexin A might be of interest to use as a prodrug.

Brassinin oxidase mediated transformation of the phytoalexin brassinin: structure of the elusive co-product, deuterium isotope effect and stereoselectivity.

Brassinin oxidase, a fungal detoxifying enzyme that mediates the conversion of the phytoalexin brassinin into indole-3-carboxaldehyde, is the first enzyme described to date that catalyzes the transformation of a dithiocarbamate group into an aldehyde equivalent. Brassinin is an essential phytoalexin due to its antifungal activity and its role as biosynthetic precursor of other phytoalexins produced in plants of the family Brassicaceae (common name crucifer). In this report, the isolation, structure determination and synthesis of the elusive co-product of brassinin transformation by brassinin oxidase, S-methyl dithiocarbamate, the syntheses of dideuterated and (R) and (S) monodeuterated brassinins, kinetic analyses of isotope effects and chemical modifications of brassinin oxidase are described. The reaction of [1'-(2)H(2)]brassinin was found to be slowed by a kinetic isotope effect of 5.3 on the value of k(cat)/K(m). This result indicates that the hydride/hydrogen transfer step preceding brassinin transformation is rate determining in the overall reaction. In addition, the use of (R) and (S)-[1'-(2)H]brassinins as substrates indicated that the hydride/hydrogen transfer step is ca. 88% stereoselective for the pro-R hydrogen. A detailed chemical mechanism of the enzymatic transformation of brassinin is proposed.

Discovery of inhibitors of brassinin oxidase based on the scaffolds of the phytoalexins brassilexin and wasalexin.

Inhibitors of brassinin oxidase (BOLm), a unique phytoalexin detoxifying enzyme produced by the plant pathogenic fungus Leptosphaeria maculans (asexual stage Phoma lingam), were designed based on scaffolds of the phytoalexins brassilexin and wasalexin. Evaluation of these compounds using purified BOLm established that the inhibitory effect of brassilexin and derivatives decreased as follows: 6-chlorobrassilexin approximately 6-bromobrassilexin>5-bromobrassilexin approximately 5-chlorobrassilexin approximately 6-fluorobrassilexin>8-methylbrassilexin>brassilexin approximately 5-fluorobrassilexin. 6-Chlorobrassilexin was determined to be the best competitive inhibitor of BOLm discovered to date, with a K(i)=31microM. Importantly, brassilexin and derivatives did not appear to induce BOLm in fungal cultures. Overall, these results suggest that the brassilexin scaffold is a good lead for further development of paldoxins against L. maculans, as it inhibits competitively BOLm without apparent induction.

Substrate specificity and inhibition of brassinin hydrolases, detoxifying enzymes from the plant pathogens Leptosphaeria maculans and Alternaria brassicicola.

Blackleg (Leptosphaeria maculans and Leptosphaeria biglobosa) and black spot (Alternaria brassicicola) fungi are devastating plant pathogens known to detoxify the plant defence metabolite, brassinin. The significant roles of brassinin as a crucifer phytoalexin and as a biosynthetic precursor of several other plant defences make it important in plant fitness. Brassinin detoxifying enzymes produced by L. maculans and A. brassicicola catalyse the detoxification of brassinin by hydrolysis of its dithiocarbamate group to indolyl-3-methanamine. The purification and characterization of brassinin hydrolases produced by L. maculans (BHLmL2) and A. brassicicola (BHAb) were accomplished: native BHLmL2 was found to be a tetrameric protein with a molecular mass of 220 kDa, whereas native BHAb was found to be a dimeric protein of 120 kDa. Protein characterization using LC-MS/MS and sequence alignment analyses suggested that both enzymes belong to the family of amidases with the catalytic Ser/Ser/Lys triad. Furthermore, chemical modification of BHLmL2 and BHAb with selective reagents suggested that the amino acid serine was involved in the catalytic activity of both enzymes. The overall results indicated that BHs have new substrate specificities with a new catalytic activity that can be designated as dithiocarbamate hydrolase. Investigation of the effect of various phytoalexins on the activities of BHLmL2 and BHAb indicated that cyclobrassinin was a competitive inhibitor of both enzymes. On the basis of pH dependence, sequence analyses, chemical modifications of amino acid residues and identification of headspace volatiles, a chemical mechanism for hydrolysis of the dithiocarbamate group of brassinin catalysed by BHLmL2 and BHAb is proposed. The current information should facilitate the design of specific synthetic inhibitors of these enzymes for plant treatments against blackleg and black spot fungal infections.

Synthetic inhibitors of the fungal detoxifying enzyme brassinin oxidase based on the phytoalexin camalexin scaffold.

Brassinin (1) is an essential phytoalexin produced in plants of the family Brassicaceae (common name crucifer) due to its role as a biosynthetic precursor of other phytoalexins and antimicrobial activity. The dithiocarbamate group of brassinin (1) is the toxophore responsible for its fairly broad antifungal activity. To the detriment of many agriculturally important crops, several pathogenic fungi of crucifers are able to overcome brassinin by detoxification. In this work, inhibitors of brassinin oxidase, a phytoalexin detoxifying enzyme produced by the plant pathogenic fungus Leptosphaeria maculans (asexual stage Phoma lingam ), were synthesized and evaluated. The camalexin scaffold was used for the design of brassinin oxidase inhibitors (i.e., paldoxins, phytoalexin detoxification inhibitors) because camalexin is a phytoalexin not produced by the Brassica species and L. maculans is unable to metabolize it. The inhibitory effect of camalexin and derivatives decreased as follows: 5-methoxycamalexin > 5-fluorocamalexin = 6-methoxycamalexin > camalexin > 6-fluorocamalexin; 5-methoxycamalexin was determined to be the best inhibitor of brassinin oxidase discovered to date. In addition, the results suggested that camalexin might induce fungal pathways protecting L. maculans against oxidative stress (induction of superoxide dismutase) as well as brassinin toxicity (induction of brassinin oxidase). Overall, these results revealed additional biological effects of camalexin and its natural derivatives and emphasized that different phytoalexins could have positive or negative impacts on plant resistance to different fungal pathogens.

Detoxification of the phytoalexin brassinin by isolates of Leptosphaeria maculans pathogenic on brown mustard involves an inducible hydrolase.

Brassinin is a phytoalexin produced by plants from the family Brassicaceae that displays antifungal activity against a number of pathogens of Brassica species, including Leptosphaeria maculans (Desm.) Ces. et de Not. [asexual stage Phoma lingam (Tode ex Fr.) Desm.] and L. biglobosa. The interaction of a group of isolates of L. maculans virulent on brown mustard (Brassica juncea) with brassinin was investigated. The metabolic pathway for degradation of brassinin, the substrate selectivity of the putative detoxifying hydrolase, as well as the antifungal activity of metabolites and analogs of brassinin are reported. Brassinin hydrolase activity was detectable only in cell-free homogenates resulting from cultures induced with brassinin, N'-methylbrassinin, or camalexin. The phytoalexin camalexin was a substantially stronger inhibitor of these isolates than brassinin, causing complete growth inhibition at 0.5mM.