2-Oxindole alkaloids produced by marine-derived Streptomyces sp. B9173

Abstract:

[Background] Marine-derived natural products have become an important source of small molecule drugs in recent years. Genomic analysis showed that Streptomyces sp. B9173 contains a variety of biosynthetic gene clusters of natural products, with the potential to generate multiple new compounds. [Objective] The objective of this paper is to find new compounds with novel structures or unique biological activities from the strain B9173. [Methods] With the method combined with HPLC and LC-MS, we excluded the known compounds produced by B9173 and identified three unknown compounds as isolation targets. The secondary metabolites were isolated and purified using normal or reverse phase column chromatography, gel column chromatography and high performance liquid chromatography (HPLC). The structures of the compounds were elucidated by using mass spectroscopy (MS) and nuclear magnetic resonance (NMR). [Results] Three compounds were purified and identified as tryptanthrin, meisoindigo, and N,N-dimethylisoindigo. All three compounds belong to the 2-oxindole alkaloids. Tryptanthrin has diverse biological activity, including antibacterial activity, anti-inflammatory activity, anti-tumor activity. Tryptanthrin is a potential drug lead and it is the first time to be isolated from bacteria. Meisoindigo is a drug used for clinical treatment of chronic myelogenous leukemia in China. It was isolated for the first time in a microbial fermentation broth. Based on the metabolic background of B9173, we speculated the biosynthetic pathways of three compounds. [Conclusion] Three 2-oxindole alkaloids were purified and structurally elucidated based on their UV-vis spectra and MS data. This study enriches the types of microbially active natural products. The proposed biosynthetic pathways will lay the foundation for further research on the biosynthetic mechanisms of tryptanthrin and meisoindigo. Subsequently biosynthetic pathways for this class of compounds can be reconstructed using synthetic biology techniques, which will provide more convenient and low-cost biosynthesis method for tryptanthrin and meisoindigo.