| Summary: | The saponin composition of a specimen of black sea cucumber, <i>Holothuria atra</i> 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 <i>Holothuria atra</i> turned out to be more complex than previously reported. The most abundant saponins in the extract (<b>1</b>⁻<b>4</b>) were isolated and characterized by 1D- and 2D-NMR experiments. Compound <b>1</b> was identified as a new triterpene glycoside saponin, holothurin A5. The side chain of the new saponin <b>1</b>, 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 <b>1</b>⁻<b>4</b>, evaluated on the human cervix carcinoma HeLa cell line, was remarkable, with IC<sub>50</sub> values ranging from 1.2 to 2.5 µg/mL.
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