The saponin composition of a specimen of black sea cucumber, Holothuria atra collected in the Persian Gulf was studied by a combined approach including LC-MS/MS, Molecular Networking, pure compound isolation, and NMR spectroscopy. The saponin composition of Holothuria atra turned out to be more complex than previously reported. The most abundant saponins in the extract (1⁻4) were isolated and characterized by 1D- and 2D-NMR experiments. Compound 1 was identified as a new triterpene glycoside saponin, holothurin A5. The side chain of the new saponin 1, unprecedented among triterpene glycosides, is characterized by an electrophilic enone function, which can undergo slow water or methanol addition under neutral conditions. The cytotoxic activity of compounds 1⁻4, evaluated on the human cervix carcinoma HeLa cell line, was remarkable, with IC50 values ranging from 1.2 to 2.5 µg/mL.
Keywords: LC-MS/MS; cytotoxic activity; holothurians; molecular networking; saponins; sea cucumbers; triterpene glycosides.