ERYTHRINS, NEW TOXIC METABOLITES FROM THE MARINE CILIATE PSEUDOKERONOPSIS ERYTHRINA USED AS CHEMICAL DEFENSE AGAINST PREDATORS

Marine protozoa are known for their ability to produce a vast and chemically diverse array of secondary metabolites that are involved in different ecological functions. Morphospecies belonging to genus Euplotes have been extensively studied for their ability to produce chemically diverse secondary metabolites and, interestingly, it was found that strains belonging to same genetic clades were characterized by a different profile of bioactive compounds [1]. From the genus Pseudokeronopsis only two classes of pigments have been so far isolated, keronopsins as defensive molecules of Pseudokeronopsis rubra [2] and, more recently, keronopsamides from cell culture of the marine ciliate Pseudokeronopsis riccii [3]. We report here on the characterization of new secondary metabolites, erythrins, produced by cell cultures of Pseudokeronopsis erythrina (Ciliophora, Hypotricha). Their structure have been elucidated by extensive NMR and high resolution MS measurements and are characterized by a central 4-hydroxyunsaturated delta-lactone ring bearing an alkyl saturated chain at C(2) and a butyl -benzenoid group at C(5). The simultaneous presence of the corresponding 4-sulphate analogues has also been ascertained and a reasonable proposal of their biosynthesis will be reported. Cold-shock treatment has been performed to induce the discharge of these metabolites from cell pigment granules. The analysis of cytotoxic activity on a panel of free-living ciliates and micro-invertebrates, together with some observation on the defensive behavior by P. erythrina, indicated that erythrins are very effective for its chemical defence.