Isolation and structure determination of malevamide E, a dolastatin 14 analogue, from the marine cyanobacterium Symploca laete-viridis

A new depsipeptide, malevamide E (1), was isolated from field-collected colonies of the filamentous cyanobacterium Symploca laete-viridis. The gross structure of 1 was determined by spectroscopic analyses, including one- and two-dimensional NMR and accurately measured MS/MS. Chiral HPLC analyses of an acid hydrolysate of 1 allowed the stereochemical assignments of its amino acid residues, which include N-methyl-L-alanine, alpha-N,gamma-N-dimethyl-L-asparagine, N-methyl-L-phenylalanine, L-proline, D-valine, and N-methyl-L-valine. LC-MS/MS analysis of S. laete-viridis fractions established the co-occurrence of malevamide E (1) and its homologue dolastatin 14 (2), which was previously reported in low yield from the sea hare Dolabella auricularia. Malevamide E (1) demonstrated a dose-dependent (2-45 mu M) inhibition of store-operated Ca2+ entry in thapsigargin-treated human embryonic kidney (HEK) cells, indicating an inhibitory effect on Ca2+ release-activated Ca2+ (CRAC) channels.