Research on Metabolites of Nitrofurans SAS10 from Endophytic Fungi in Mangrove Forests
Mangrove forests live in harsh environments such as high temperature, high salinity, low oxygen, waterlogged soil, tidal movements, strong winds, and waves, providing unique habitats for fungi, bacteria, and other microorganisms. Fungi derived from mangroves produce novel and highly active secondary metabolites due to their unique living environment, and have made important contributions to the discovery of lead compounds such as various anti-inflammatory drugs, novel antibiotics, antifungal drugs, and anticancer drugs. Aspergillus fumigatus is a saprophytic nutritional fungus that mainly inhabits soil. It is the most common pathogenic bacterium of invasive aspergillosis, and its spores and some metabolic components are easily transmitted through the air to cause human lung disease infections, such as fungal allergic asthma, variant reactive bronchopulmonary aspergillosis, and invasive pulmonary aspergillosis. The genus Aspergillus is distributed around the world, with high survival adaptability, strong reproductive ability, and diverse metabolites. According to literature reports, more than 220 secondary metabolites with good activity were isolated from Aspergillus niger from 2006 to 2016. The main metabolites include polyketide compounds, diketopiperazine derivatives, alkaloids, aflatoxins, and quinoline derivatives. Among them, dimeric naphthopyranone compounds have moderate antibacterial activity against various pathogenic bacteria, exhibit certain inhibitory activity against various cancer cells, and also have mutagenic, antioxidant, and related enzyme inhibition activities. Our research group’s preliminary studies have shown that the endophytic fungus Aspergillus fumigatus SAS10 from the mangrove plants in Dongzhaigang Nature Reserve, Hainan Province, is rich in metabolites during rice fermentation and cultivation. Previous studies have obtained 5 pyranone compounds and 2 ester compounds from this strain. On the basis of previous work, this article continues to delve into the chemical composition and antibacterial activity of rice fermentation metabolites, in order to discover lead compounds with unique structures and significant antibacterial activity.

 

Our research group studied the chemical composition of secondary metabolites from the endophytic fungus Aspergillus fumigatus SAS10 in mangroves. Using various separation, purification, and identification techniques, we extracted and isolated seven dimeric naphthopyranone compounds and one naphthopyranone angular monomer from the fermentation broth. All compounds were newly reported secondary metabolites of the endophytic fungus Aspergillus fumigatus in mangroves in the South China Sea, providing a new source for the acquisition of dimeric naphthopyranone compounds. Based on the structure of the compounds and related research, we speculate that their biosynthetic pathway is the acetic acid malonic acid pathway (see Figure 2). Seven molecules of acetyl CoA can undergo a series of reactions to obtain linear naphthopyranone parent nuclei (such as compounds 8 and 11) and angular naphthopyranone parent nuclei (such as compounds 9 and 10). Then, under the action of Gip1 related enzymes, one electron on the hydroxyl group of the naphthopyranone monomer is removed to form a bound oxygen free radical. After electron rearrangement in the conjugated system, the carbon on the monomer is activated, and the two carbon activated monomers are coupled by themselves or to each other to form dimers 1-7.

According to literature reports, this type of compound exhibits good activity in antibacterial and anti-tumor aspects. Compounds 1 and 4 showed certain inhibitory activity on human pancreatic cancer cell PANC-1, breast cancer cell MDA-MB-231, human colon cancer cell Caco-2 and ovarian cancer cell SK-OV-3. Among them, compounds 1 showed strong cytotoxicity to PANC-1 cells, with an IC50 value of 8.25 ± 2.20 μ M; 2. 6 and 8 have good antibacterial (Bacillus subtilis, Escherichia coli, and Pseudomonas fluorescens) and antifungal (Pseudomonas aeruginosa and Candida albicans) activities, with 2 pairs of Bacillus subtilis and Pseudomonas fluorescens having MICs of 1.9 μ g/mL (positive controls were penicillin: 0.78 μ g/mL), and 6 pairs of Pseudomonas fluorescens and Trichophyton rubrum having MICs of 7.8 μ g/mL (positive controls were penicillin: 0.78 μ g/mL and ketoconazole: 3.9 μ g/mL); 1. 4, 6, and 7 showed weak antibacterial effects against Candida albicans, Escherichia coli, and Staphylococcus aureus (the positive control for the former was amphotericin B, while the positive control for the latter two was ampicillin sodium). Our antibacterial activity indicates that compounds 1-8 have varying degrees of inhibitory activity against Staphylococcus aureus, Escherichia coli, and two types of methicillin-resistant Staphylococcus aureus. Among them, compound 6 has an IC50 of 32.42 μ g/mL against methicillin-resistant Staphylococcus aureus (MRSA N315). In addition, the inhibitory activity of dimeric naphthopyranone against Staphylococcus aureus (S. aureus ATCC 29213) and two methicillin-resistant Staphylococcus aureus (MRSA N315 and MRSA NCTC 10442) is generally stronger than that of linear naphthopyranone monomers. The antibacterial activity of dimeric naphthopyranone compounds with different configurations (linear and angular) also varies to some extent. In the future, we will conduct in-depth research on the antibacterial structure-activity relationship of dimeric naphthopyranone compounds, laying the foundation for the development of this new type of antibacterial agent.