Identification of Novel Gymnodimines and Spirolides from the Marine Dinoflagellate Alexandrium ostenfeldii.

Cyclic imine toxins are neurotoxic, macrocyclic compounds produced by marine dinoflagellates. Mass spectrometric screenings of extracts from natural plankton assemblages revealed a high chemical diversity among this toxin class, yet only few toxins are structurally known. Here we report the structural characterization of four novel cyclic-imine toxins (two gymnodimines (GYMs) and two spirolides (SPXs)) from cultures of Alexandrium ostenfeldii. A GYM with m/z 510 (1) was identified as 16-desmethylGYM D. A GYM with m/z 526 was identified as the hydroxylated degradation product of (1) with an exocyclic methylene at C-17 and an allylic hydroxyl group at C-18. This compound was named GYM E (2). We further identified a SPX with m/z 694 as 20-hydroxy-13,19-didesmethylSPX C (10) and a SPX with m/z 696 as 20-hydroxy-13,19-didesmethylSPX D (11). This is the first report of GYMs without a methyl group at ring D and SPXs with hydroxyl groups at position C-20. These compounds can be conceived as derivatives of the same nascent polyketide chain, supporting the hypothesis that GYMs and SPXs are produced through common biosynthetic genes. Both novel GYMs 1 and 2 were detected in significant amounts in extracts from natural plankton assemblages (1: 447 pg; 2: 1250 pg; 11: 40 pg per mL filtered seawater respectively).

Structure and toxicity of AZA-59, an azaspiracid shellfish poisoning toxin produced by Azadinium poporum (Dinophyceae).

To date, the putative shellfish toxin azaspiracid 59 (AZA-59) produced by Azadinium poporum (Dinophyceae) has been the only AZA found in isolates from the Pacific Northwest coast of the USA (Northeast Pacific Ocean). Anecdotal reports of sporadic diarrhetic shellfish poisoning-like illness, with the absence of DSP toxin or Vibrio contamination, led to efforts to look for other potential toxins, such as AZAs, in water and shellfish from the region. A. poporum was found in Puget Sound and the outer coast of Washington State, USA, and a novel AZA (putative AZA-59) was detected in low quantities in SPATT resins and shellfish. Here, an A. poporum strain from Puget Sound was mass-cultured and AZA-59 was subsequently purified and structurally characterized. In vitro cytotoxicity of AZA-59 towards Jurkat T lymphocytes and acute intraperitoneal toxicity in mice in comparison to AZA-1 allowed the derivation of a provisional toxicity equivalency factor of 0.8 for AZA-59. Quantification of AZA-59 using ELISA and LC-MS/MS yielded reasonable quantitative results when AZA-1 was used as an external reference standard. This study assesses the toxic potency of AZA-59 and will inform guidelines for its potential monitoring in case of increasing toxin levels in edible shellfish.

In Silico Modeling of Spirolides and Gymnodimines: Determination of S Configuration at Butenolide Ring Carbon C-4.

Only few naturally occurring cyclic imines have been fully structurally elucidated or synthesized to date. The configuration at the C-4 carbon plays a pivotal role in the neurotoxicity of many of these metabolites, for example, gymnodomines (GYMs) and spirolides (SPXs). However, the stereochemistry at this position is not accessible by nuclear Overhauser effect-nuclear magnetic resonance spectroscopy (NOE-NMR) due to unconstrained rotation of the single carbon bond between C-4 and C-5. Consequently, the relative configuration of GYMs and SPXs at C-4 and its role in protein binding remains elusive. Here, we determined the stereochemical configuration at carbon C-4 in the butenolide ring of spirolide- and gymnodimine-phycotoxins by comparison of measured 13C NMR shifts with values obtained in silico using force field, semiempirical and density functional theory methods. This comparison demonstrated that modeled data support S configuration at C-4 for all studied SPXs and GYMs, suggesting a biosynthetically conserved relative configuration at carbon C-4 among these toxins.

Gymnodimine A and 13-desMethyl Spirolide C Alter Intracellular Calcium Levels via Acetylcholine Receptors.

Gymnodimines and spirolides are cyclic imine phycotoxins and known antagonists of nicotinic acetylcholine receptors (nAChRs). We investigated the effect of gymnodimine A (GYM A) and 13-desmethyl spirolide C (SPX 1) from Alexandrium ostenfeldii on rat pheochromocytoma (PC12) cells by monitoring intracellular calcium levels ([Ca]i). Using whole cells, the presence of 0.5 µM of GYM A or SPX 1 induced an increase in [Ca]i mediated by acetylcholine receptors (AChRs) and inhibited further activation of AChRs by acetylcholine (ACh). To differentiate the effects of GYM A or SPX 1, the toxins were applied to cells with pharmacologically isolated nAChRs and muscarinic AChRs (mAChRs) as mediated by the addition of atropine and tubocurarine, respectively. GYM A and SPX 1 activated nAChRs and inhibited the further activation of nAChRs by ACh, indicating that both toxins mimicked the activity of ACh. Regarding mAChRs, a differential response was observed between the two toxins. Only GYM A activated mAChRs, resulting in elevated [Ca]i, but both toxins prevented a subsequent activation by ACh. The absence of the triketal ring system in GYM A may provide the basis for a selective activation of mAChRs. GYM A and SPX 1 induced no changes in [Ca]i when nAChRs and mAChRs were inhibited simultaneously, indicating that both toxins target AChRs.