The Secondary Metabolites And Chemical Defensiveness Of Six Corals From The South China Sea

Marine invertebrates from coral reef ecosystem, rich in novel bioactivesecondary metabolites, have become the most important bioresources for screeningand discovery of marine drugs and marine natural products. Despite lack of efficientphysical protection in the highly competitive and hostile environment, the marineinvertebrates including corals and sponges can survive mainly relying on theirchemical defensive system by a series of secondary metabolites accumulating in theirbodies or releasing to their surroundings. The chemical defensive functions of thesesecondary metabolites were found to serve as antifeedant, antifouling agents,allelopathic substances as well as ichthyotoxin. In recent years, South China Sea hasbecome the focus of marine natural products research area. In the course ofinvestigation on new chemical defensive secondary metabolites and preliminary studyfor their structure-activity relationships, six corals and one sponge endogenous fungus,collected from the South China Sea, have been chemically investigated. The researchstrategy and methodology of chemical ecology provided foundations for the researchand development of bioactivity marine natural products and marine medicinalbioresource.Six invertebrates including gorgonian corals Dichotella gemmacea, Anthogorgiaocbracea, Verrucella umbraculum, Villogorgia sp., Menella sp., soft coralSarcophyton sp., and one sponge endogenous fungus Aspergillus sp., collected fromthe South China Sea, were investigated for their bioactive secondary metabolites. Thecompounds were isolated by column chromatography on silica gel, Sephadex LH-20and preparative reversed phase HPLC, and identified by spectroscopy of NMR, MS,IR, UV, as well as chemical methods. From these organisms,147compounds were obtained, including18new compounds. More than8structural types were found,including20sesquiterpenoids,38diterpenoids,50steroids,5tricresol trimers,2dipeptides,29lipids,2alkaloids and1aromatic compound.The isolated compounds were evaluated their chemecology activities byhigh-throughput screening programs on various of screening modles includingantifouling, ichthyotoxic, microalgal growth-inhibition, lethality and so on. A varietyof bioactive compounds had been found, including56antifouling, lethality activity,ichthyotoxic, and microalgal growth-inhibition activity compounds. Among thebioactive compounds,18antifouling activity compounds (25,26,27,28,30,31,32,35,36,37,38,39,44,62,72,74,75, and139),15lethality activity compounds (2,7,13,14,34,38,39,40,51,53,62,72,75,116, and124),9ichthyotoxic compounds(72,73,74,118,124,125,127,128, and129),14microalgal growth-inhibitionactivity compounds (26,34,37,51,53,55,61,62,63,116,117,118,123,124)，andone insecticide activity compound (133) showed potential values for further study.Pharmacological activities, such as anticancer, antibacterial, and antiviral activity,were also evaluated for the isolated compounds at the same time. The result showedthat42compounds exhibited cytotoxic and antibacterial activity, especially, sixcytotoxic activity compounds (38,39,40,51,61, and62)possessed potential valuesfor further study.The structure-activity relationships were studied for the bioactive compounds.The furan ring is an important active group for briarane diterpenoids. The hydroxyl atC-3and24-ketal are significant active group for polyhydroxylated steroids, especially,the disappearance (acetylation) of hydroxyl at C-3probably weakened the cytotoxicactivity. Bisabolane-type sesquiterpenoid dimeric compounds116and118, possessingthe S and S configurations at C-7and C-7′, displayed better potent cytotoxicity thancompound117, with the S and R configurations at C-7and C-7′. These resultsindicated that the cytotoxic activity might be weakened due to mesomeric effect.The secondary metabolites of gorgonian coral Dichotella gemmacea collected from different time and areas were compared and analyzed. It indicated that the samegorgonian collected from different time and areas shared similarities and differences,which supplied foundation for further investigation for gorgonian corals.The relationships of biological origin between bisabolane-type sesquiterpenoidand tricresol trimers were analysed preliminarily. The bisabolane sesquiterpenoidskeleton might be derived from a mevalonic acid pathway. The phenol precursorsmight come from shikimic acid pathway of gallic acid or moss acid pathway. Theanalysis may provide foundation for investigation of their biogenetic pathway.Innovations for this study are as follows. Under the guidance of chemicalecology activity,18new compounds were obtained. In addition,56compoundsexhibited significant strong ecology activity,6compounds played strong antitumouractivity, and6compounds showed strong ecology activity and antitumour activity atthe same time. The bioactive marine secondary metabolites from corals and spongeendogenous fungus provided diversity of compounds for the discovery of bioactivesecondary metabolites and exploration of new medicinal bioresources.