Three new 2,5-diketopiperazines from the fish intestinal Streptomyces sp. MNU FJ-36

Abstract The gut actinobacteria of marine-inhabited fish is one of the most important reservoirs of novel natural products. Currently, the Streptomyces sp. MNU FJ-36 was isolated from the intestinal fabric of Katsuwonus sp. and determined by 16S rRNA analysis. From the cultures of the S. sp. MNU FJ-36, three new 2,5-diketopiperazines (2,5-DKPs) were discovered and identified as 3-(3-hydroxy-4-methoxybenzyl)-6-isobutyl-2,5-diketopiperazine (1), 3-(1,3-benzodioxol-5-ylmethyl)-6-isobutyl-2,5-diketopiperazine (2) and 3-(1,3-benzodioxol-5-ylmethyl)-6-isopropyl-2,5-diketopiperazine (3). Their structures were elucidated on the basis of spectroscopic data analysis. All the compounds were also evaluated for their inhibitory activity against P388, A-549 and HCT-116 cell lines with the MTT assay. Graphical abstract


Introduction
Marine microorganisms are an important resource for the discovery of the various bioactive natural compounds (Haefner 2003;Zhang et al. 2005;Marris 2006;Gerwick & Moore 2012;Sun et al. 2014). Remarkably, the mutualistic symbiosis relationship between fish and its intestinal microorganisms (and plant and its endophytes) has been established for a long evolutionary time (Yan et al. 2012;Zhou, Yang, Peng, et al. 2013;Zhang et al. 2014). The microorganisms of animal guts are known to play a significant role in regulating the physiology, nutrition and immune system of their hosts. The gut contains 10 7 -10 11 bacteria per gram in the content, including proteobacteria, firmicutes, actinobacteria and so on (Sanchez et al. 2012). It is noteworthy that the intestinal fabric from the ocean-originated fishes is important but remains an unexplored resource for discovery of actinomycetes producing new metabolites. Therefore, chemical investigations on the intestinal microbes of the ocean-originated fish might afford novel chemical structures and bioactive lead compounds for drug discovery.

Results and discussion
The cultures of S. sp. MNU FJ-36 were carried out on 8 L ISP2 solid medium at 30 °C for 7 days, and the cultures were extracted at room temperature. The crude extract was purified by successive column chromatography on silica gel and Sephadex LH-20 to afford compounds 1-3. Their structures were assigned by spectroscopic approaches.
All three new compounds were assayed for their cytotoxic effects on the P388, A-549 and HCT-116 cell lines by the MTT method with doxorubicin as the positive control. The three compounds displayed weak cytotoxicity against A-549 cell line, and compounds 2 and 3 also showed weak cytotoxicity against HCT-116 cell line (Table 1).

Conclusions
The chemical investigation of cultures of the S. sp. MNU FJ-36 resulted in the isolation of three new 2,5-DKPs 1-3. All of the compounds displayed weak cytotoxicity against A-549 cell line, and compounds 2 and 3 also exhibited weak inhibitory activity against HCT-116 cell line.

Supplementary material
Supplementary material relating to this article is available online, alongside the experimental part, Table S1-S3.

Disclosure statement
No potential conflict of interest was reported by the authors.