Stellettamide-A, a novel inhibitor of calmodulin, isolated from a marine sponge

1 Stellettamide A (ST-A), a novel marine toxin isolated from a marine sponge, inhibited high K+(72.7 mM)-induced contraction in the smooth muscle of guinea-pig taenia coli with an IC50 of 88 mu M. 2 In the taenia permeabilized with Triton X-100, ST-A inhibited Ca2+ (3 and 10 mu M)-induced contractions with an IC50 of 46 mu M for 3 mu M Ca2+ and 105 mu M for 10 mu M Ca2+. In the permeabilized taenia, calyculin-A (300 nM), a potent inhibitor of type-1 and type-2A phosphatases, induced sustained contraction in the absence of Ca2+. ST-A had no effect on this contraction. 3 ST-A inhibited Mg2+-ATPase activity in native actomyosin prepared from chicken gizzard with an IC50 of 25 mu M. 4 In a reconstituted smooth muscle contractile system containing calmodulin, myosin light chain (MLC) and MLC kinase, ST-A inhibited MLC phosphorylation with an IC50 of 152 mu M. The inhibitory effect of ST-A was antagonized by increasing the concentration of calmodulin. 5 ST-A inhibited calmodulin activity, assessed by Ca2+/calmodulin-dependent enzymes, (Ca2+-Mg2+)-ATPase of erythrocyte membrane, with an IC50 of 100 mu M and phosphodiesterase prepared from bovine cardiac muscle with an IC50 of 52 mu M. The inhibitory effect on phosphodiesterase activity was antagonized by increasing the calmodulin concentration. 6 Interaction between ST-A and calmodulin was demonstrated by instantaneous quenching of the intrinsic tyrosine fluorescence of calmodulin by ST-A (3-300 mu M). Similar results were obtained in the presence or absence of Ca2+ suggesting that ST-A binds to calmodulin and that Ca2+ is not essential for the binding of ST-A to calmodulin. 7 These results suggest that ST-A, isolated from marine metabolites, is a novel inhibitor of calmodulin.