Highly Cytotoxic Metabolites from the Culture Supernatant of the Temperate Dinoflagellate Protoceratium cf. reticulatum

In the course of our search for antitumor compounds in dinoflagellates, the culture broths of two strains of Protoceratium cf. reticulatum showed extremely potent cytotoxicity against human tumor cell lines. The four equally active principles, named protoceratins I (1), II (2), III (3), and IV (4), were purified and their structures were studied. The major principle, protoceratin I (1), proved to be identical with 2-homoyessotoxin, a well-known shellfish toxin. Protoceratins II (2), III (3), and IV (4) were determined to be di-, mono-, and triarabinoside of 1, respectively. They are the first examples of glycosides of dinoflagellate polyethers.