Secondary metabolites of Vernonia gigantea roots: Isolation, antimicrobial, chemical and biotransformation studies.

摘要

In a preliminary screening of 34 plants extracts, the roots of Veronia gigantea (Walt.) Trelease (Fam. Compositae) showed good antimicrobial activity against the bacteria Bacillus subtilis and Mycobacterium smegmatis, and the fungi Microsporum gypseum and Trichophyton mentagrophytes.;Bioassay-guided fractionation of the ethanolic extracts of V. gigantea roots afforded the sesquiterpene lactone zaluzanin C ( 35) as the active antifungal principle. Phytochemical investigation of V. gigantea roots afforded also the zaluzanin C related compounds: glucozaluzanin C (36), 6'- O-caffeoyl-glucozaluzanin C (37), 8'-hydroxy-11beta,13-dihydrozaluzanin C (38) and 8alpha-chydroxy-11beta,13-dihydrozaluzanin C-3- O-beta-glucopyranoside (39); and the caffeoylquinic acid derivatives: 3,5-di-O-caffeoylquinic acid (42 ), methyl 3,5-di-O-caffeoyl quinate (43), 1,5-di-O-caffeoylquinic acid (44), methyl 3,4,5-tri- O-caffeoyl quinate (45) and ethyl 3,4,5-tri- O-caffeoyl quinate (46). All these compounds were isolated for the first time from V. gigantea; while 37, 39 and 46 are novel compounds.;Compounds 36--39 have no antifungal activity. Comparison of the structures of these compounds suggested that the alpha-methylene-gamma-lactone group and the nature of the substituent at C-3 are key factors for the presence of antifungal activity.;Microbial transformation studies on zaluzanin C were performed in order to obtain other closely related derivatives that could yield structure-activity relationships.;The bacteria Streptomyces sp. 15077, Streptomyces griseus B 8090; and the fungi Aspergillus ochraceous 1080 and Aspergillus flavipes 11013 were able to biotransform zaluzanin C into: 4beta,15,11beta,13-tetrahydrozaluzanin C, 4beta,15,11beta,13-tetrahydro- epi-zaluzanin C, 4beta,15,11beta,13-tetrahydro-3-dehydrozaluzanin C, 11beta,13-dihydro-10alpha-epoxyzaluzanin C and 4beta,15,11beta,13-tetrahydro-10alpha-epoxyzaluzanin C. All these metabolites have no antimicrobial activity. The microorganisms used in the biotransformation studies reduced the C-11 to C-13 double bond (which is required for activity) suggesting that the microorganisms performed this reaction as a defense mechanism.;Two chemical modifications to the structure of zaluzanin C were performed in order to enhance its antifungal activity. The C-3-OH group was acetylated to yield zaluzanin D; and oxidized with chromium oxide to afford dehydrozaluzanin C.;MIC values for zaluzanin C, zaluzanin D and dehydrozaluzanin C against T. mentagrophytes were 25, 12.5 and 0.78 mug/ml, respectively; compared to 1.56 mug/ml of miconazole nitrate.;A bioassay for ergosterol-sensitive antifungal activity demonstrated that the mechanism of action of dehydrozaluzanin C is different from that of amphotericin B.;It appears that glucozaluzanin C is acting as a "post-inhibitin".