Bioprospecting of the Balinese marine sponges and nudibranchs

Abstract

Secondary metabolites play an invaluable role as a starting point in drug discovery process. Among marine organisms, sponges and nudibranch mollusc (Mollusca: Gastropoda: Nudibranchia) have been the prolific sources of bioactive secondary metabolites with diverse chemical structures and promising bioactivities. This report presents the isolation and structure elucidation of bioactive natural products from the Balinese marine sponge Aplysinella strongylata and the nudibranchs Hypselodoris infucata and Glossodoris hikuerensis. The samples were collected from Tulamben beach Bali, an unexplored site but rich in marine biodiversity. The separation and purification of the metabolites were achieved by mean of NP-flash column chromatography and HPLC. Structure elucidations of the isolated compounds were completed by analysis of spectroscopic dataset including 1H and 2D NMR. A series of bromotyrosine-derived alkaloid possessing a spirooxepinisoxazoline moiety were identified from the extract of A. strongylata. One of the metabolites, 19-hydroxypsammaplysin E (1), showed the best antimalarial activity, with an IC50 value of 6.4 μM. The absolute configuration of psammaplysin series compounds has been assigned as (6R, 7R) using experimental and calculated electronic circular dichroism (ECD) data and NMR analysis of MPA esters prepared from the acetamide derivative of psammaplysin A. A furanosesquiterpen, (−)-furodysinin (2), was identified for the first time from the nudibranch H. infucata. The compound inhibited the growth of HeLa cell line at IC50 102.7 µg/mL. Two new scalarane sesterterpenes (3-4) were characterised from an organic extract of a single specimen of the nudibranch G. hikuerensis.