| Marine microorganisms have been proved to be a rich source of secondary metabolites because of the special environment of the ocean. Many of these compounds are structurally unique and biologically active, which are generally good candidates in drug discovery. Furthermore, the biosynthesis of these bioactive natural products has attracted great attention in the research community.This thesis can be divided into three parts. The first part reported the screening of representative endophytes isolated from the mangrove in Guangxi, China. 21 strains of endophytes have been isolated among the 522 isolates collected. Two of them were selected by screening of chemistry diversity and potential bioactivity; they are Phoma sp. SK3RW1M and Alternaria sp. SK12JN3L. In the subsequent chemical studies, six new compounds (3-1, 3-2, 3-3, 4-1, 4-2, and 4-3) were isolated and identified by using IR, UV, MS, and NMR techniques.The second part of the thesis focused on the bioactivities of the known compounds fusaric acid and its metal complexes and actinomycin V. Antimycobacterial assays showed that Cd(II) and Cu(II) complexes exhibited potent inhibitory activity against M. bovis BCG (MIC = 4μg/ml) and M. tuberculosis H37Rv (MIC = 10μg/ml) respectively. Actinomycin V showed significant inhibitory activity against breast cancer cell lines.The third part reported our attempts to explore the biosynthesis pathways of the copper complex of fusaric acid (FA) in the mangrove endophyte Fusarium oxysporum ZZF51. The results collected so far allowed us to hypothesize that Cu2+ may act as a"signaling molecule"to awaken the silent FA biosynthetic genes in ZZF51, inducing intracellular production of FA followed by chelation with Cu2+. This signaling network was triggered specifically by Cu2+ and may be interfered by other metal ions.In the first chapter, the challenges and opportunities of natural products used in drug discovery have been fully discussed. Various efficient and effective applications of natural products that have been used to improve the drug discovery process have been reported. The most recent developments in the triggering and regulation of the cryptic fungal gene clusters and the biosynthesis of secondary metabolites have also been discussed.The second chapter reported the screening of representative endophytes isolated from the mangrove in Guangxi, China. 21 strains of endophytes have been isolated among the 522 isolates collected. Two of them were selected by screening of chemistry diversity and potential bioactivity, both of which were subjected to chemical investigations in the subsequent chapters.The third chapter reported the isolation and identification of the secondary metabolites from the endophyte Phoma sp. SK3RW1M collected from the Guangxi mangrove, China. Two closely related xanthones, 1-hydroxy-8-hydroxymethyl-6-methoxy-3-methyl-xanthen-9-one (3-1), 1-hydroxy-8-hydroxymethyl-3-methoxy-6-methyl-xanthen-9-one (3-2), and a new lactone, 1,8-dihydroxy-10-methoxy-3-methyl-dibenzo[b,e]oxepine-6,11-dione (3-3),were isolated. This is the first report of xanthone derivatives isolated as secondary metabolites from Phoma species. Their structures were elucidated by using spectroscopic methods, mainly 1D and 2D NMR techniques and the structure of compound 3-1 was confirmed by X-ray crystallography. Cytotoxicity assays showed that all of the three new compounds were inactive against KB and KBv200 cells (IC50 > 50μg/mL).The fourth chapter reported the isolation and identification of the secondary metabolites from the endophyte Alternaria sp. SK12JN3L collected from the Guangxi mangrove, China. Three closely related anthraquinones, 1,7-dihydroxy-3-methoxy-5-methyl-anthraquinone (4-1), 1-hydroxy-3,7-dimethoxy-5-methyl-anthraquinone (4-2) and 1,3,7-trimethoxy-5-methyl-anthraquinone (4-3), together with four known compounds, alternariol, alternariol monomethyl ether, dactylariol and tetrahydroaltersolanol B, were isolated. The structures of all compounds were elucidated by using spectroscopic methods, mainly 1D and 2D NMR techniques. The crystallographic data of tetrahydroaltersolanol B (4-8) was reported for the first time. Cytotoxicity assays showed that all the three new compounds were inactive against KB and KBv200 cells (IC50 > 50μg/mL).As the prevalence of multidrug-resistant Mycobacterium tuberculosis increases, there is an urgent need for new anti-tuberculosis drugs with novel mechanisms of action. As part of our ongoing search for antimycobacterial metabolites from mangrove endophyte, the fifth chapter of this thesis reported the antimycobacterial activities of the metal complexes of fusaric acid. Results showed that Cd(II) and Cu(II) complexes exhibited potent inhibitory activity against M. bovis BCG (MIC = 4μg/ml) and M. tuberculosis H37Rv (MIC = 10μg/ml) respectively. This is the first report of antimycobacterial activity of mangrove Fusarium metabolite and its coordination metal complexes.The sixth chapter reported the significant cytotoxicity of actinomycin V against the breast cancer cell lines. Results showed that the primary mode of cell death in these assays appeared to be via apoptosis, during which some key molecules caspase-8, -9, and -3 were activated, resulting in the break down of the PARP protein.The last chapter fully discussed our first attempt to explore the biosynthesis of the copper complex of fusaric acid in the mangrove endophte Fusarium oxysporum ZZF51. Cu-fusaric acid complex (Cu-FA) was previously isolated from ZZF51 by our group. From another endophyte Fusarium sp. B2 collected from the mangrove in the same sea area, only FA but no Cu-FA was isolated in the same culture condition. This phenomenon attracted our attention and great interest. By the parallel experiments with ZZF51 and B2, we explored how Cu-FA and FA were produced in these strains. We also tested if the production of FA-metal complex was specifically stimulated by Cu2+, or if other metal ions would function in the same way. The results collected so far allowed us to hypothesize that Cu2+ may act as a"signaling molecule"to awaken the silent FA biosynthetic genes in ZZF51, inducing intracellular production of FA followed by chelation with Cu2+. This signaling network was triggered specifically by Cu2+ and may be interfered by other metal ions.
|
PDF Full Text Request