Chemical Studies on the Bioactive Secondary Metabolites, Cerebrosides, Polyketides, and Miscellaneous Compounds from the Marine-Derived Fungi: Structure Elucidation and Microbial Transformation

Abstract

Marine-derived microorganisms continue to attract attention as a rich source of structurally novel bioactive metabolites that are potential lead compounds for the development of new drugs. More than 99% of naturally occurring microorganism remains uncultured since methods for isolation and cultivation of microorganisms from natural environments still remain to be established. In our investigation for bioactive natural products, the marine-derived fungi were selected as our targets for chemical research. The secondary metabolites were obtained from combining the modern knowledge of microorganisms and chemical techniques in culture, extraction, isolation, and purification. Our investigations also focused on the biotransformation of some bioactive compounds by marine-derived microorganisms. Our recent investigation was concentrated on the induced production of the bioactive compounds, so called feeding experiment, by addition of some chemicals (NaBr, NaF, NaI, CaBr2...) to the culture medium. The total structure of secondary metabolites and their biological activities have been investigated, and the results are as follows: 1. Two new antibacterial cerebroside derivatives, flavusides A and B, have been isolated from the extract of a marine isolate of the fungus Aspergillus flavus (MFA500). The structure and absolute stereochemistry of two cerebrosides were assigned on the basis of NMR and Tandem FAB？VMS/MS experiments. Flavusides A and B exhibited a mild antibacterial activity against Staphylococcus aureus, methicillin-resistant S. aureus, and multidrug-resistant S. aureus, with the minimum inhibitory concentration (MIC) values of 15.6 ？？g/mL for S. aureus and 31.2 ？？g/mL for methicillin-resistant S. aureus and multidrug-resistant S. aureus. 2. Bioassay-guided fractionation of the secondary metabolites from unidentified marine-derived fungus (MFA503) led to the isolation of 4,15-diacetoxyscirpenol as well as cyclonerodiol from the culture broth. The structures of two compounds were elucidated on the basis of spectroscopic analyses. 4,15-Diacetoxyscirpenol is a potent inhibitor of HIF (Hypoxia Inducible Factor) function. 3. Polyketides, naphtho[1,2-b]pyran-4-one, naphtho[2,3-b]pyran-4-one, and 2,3-dihydronaphto[2,3-b]pyran-4-one, were isolated from mycelium extract of unidentified marine-derived fungus (MFA773). The structures of the compounds were assigned on the basis of spectroscopic analysis. Naphtho[2,3-b]pyran-4-one and 2,3-dihydronaphto[2,3-b]pyran-4-one exhibited significant radical-scavenging activity against 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) with IC50 values of 0.3 ？gM and 0.02 ？gM, respectively, which were more active than the positive control, L-ascorbic acid (IC50, 20.0 ？gM). 4. p-Hydroxyphenethyl alcohol and 5-methyl-2,4(1H,3H)-pyrimidinedione have been isolated from the culture broth of the marine-derived fungus Fusarium sp. (MFA474). The structures of the compounds were established on the basis of the spectroscopic method. 5. The biological transformation of the biologically active chlorogentisyl alcohol, isolated from the marine-derived fungus Aspergillus sp., was studied. Preparative scale fermentation of chlorogentisyl alcohol with marine-derived fungus Chrysosporium synchronum (BAac049) resulted in the isolation of a new glycosidic metabolite, 1-O-(？？-D-mannopyranosyl)chlorogentisyl alcohol. The stereostructure of the new metabolite obtained was assigned on the basis of detailed spectroscopic data analyses, chemical reaction, and chemical synthesis. The new metabolite exhibited significant radical-scavenging activity against 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) with IC50 values of 4.7 ？gM, which was more active than the positive control, L-ascorbic acid (IC50, 20.0 ？gM). 6. The biotransformation study was conducted on the anthranilic acid. Preparative scale fermentation of anthranilic acid with Fusarium sp. (MFA474) has resulted in the isolation of a 2-(acetylamino)-3-hydroxybenzoic acid. The structure of the metabolite was elucidated on the basis of the spectroscopic data analyses. 7. The addition of NaBr and CaBr2 (each 50 mM) to the fermentation medium of an unidentified fungal strain (MFB151) resulted in induced production of two new metabolites, 2-(2,5-dibromo-3,4-dihydroxyphenyl)acetic acid methyl ester and 2-(6-bromo-3,4-dihydroxyphenyl)acetic acid methyl ester. The structures of two new metabolites were assigned through the combination of spectroscopic data analyses. 2-(2,5-Dibromo-3,4-dihydroxyphenyl)acetic acid methyl ester and 2-(6-bromo-3,4-dihydroxyphenyl)acetic acid methyl ester exhibited free radical-scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) with IC50 values of 12.1 ？gM and 14.2 ？gM, respectively, which were more active than the positive control, L-ascorbic acid (IC50, 20.0 ？gM). 8. The biological transformation of 4-amino-3-hydroxy-1-naphthalenesulfonic acid was investigated. Various strains of the marine-derived microorganisms were investigated for their ability to transform 4-amino-3-hydroxy-1-naphthalenesulfonic acid. Only Fusarium sp. (MFA474) was capable of transforming 4-amino-3-hydroxy-1-naphthalenesulfonic acid to its metabolite, 2-hydroxy-1,4-naphthalenedione. 9. The microbial transformation of the anthranilic acid by Penicilllium sp. (MFAac049) has resulted in the isolation of (Ac049+AA)B2.3.2rH2. The structure of the metabolite was based primarily on 1D and 2D NMR analyses. The structure elucidation of the product is now under investigation.