Bioactive cyclo-(S-Pro-R-Leu) from Aspergillus flavus, the marine endophytic fungus from brown alga, Dictyota kunthi

The marine environment is regarded as a rich source of bioactive secondary metabolites, and some of these compounds have been shown to be useful for novel drug discovery. In recent years, research on the chemistry of endophytic fungi from marine algae has shown them to be an important source of secondary metabolites with a wide range of biological activities. The objective of this study was to describe the isolation and identification of bioactive compounds obtained from Aspergillus flavus, hosted in the brown alga, Dictyota kunthi, together with fl-glucuronidase bioassay of them. This fungal strain was cultivated on a large scale and extracted with ethyl acetate (EtOAc). Using various column chromatographic and spectroscopic techniques, we isolated and characterized two metabolites, cyclo-(S-Pro-R-Leu), 1 and genistein 2. Cyclo-(S-Pro-R-Leu) inhibited fl-glucuronidase with an IC50 value of 83.9 +/- 0.1 mu M when compared to glucosaccharo-1,4-lactone (IC50 = 48.4 +/- 1.3 mu M). This appears to be the first report of the isolation of cyclo-(S-Pro-R-Leu) from this entophyte. To the best of our knowledge, this is the first report of significant fl-glucuronidase inhibitory activity of this compound. From these findings, we can confirm that the endophyte from the brown alga, Dictyota kunthi, produces a fl-glucuronidase inhibitory metabolite.