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Extracts of the Floridian marine cyanobacterium Lyngbya cf. confervoides were found to deter feeding by reef fish and sea urchins (Diadema antillarum). This antifeedant activity may be a reflection of the secondary metabolite content, known to be comprised of many serine protease inhibitors. Further chemical and NMR spectroscopic investigation led us to isolate and structurally characterize a new serine protease inhibitor 1 that is formally derived from an intramolecular condensation of largamide D (2). The cyclization resulted in diminished activity, but to different extents against two serine proteases tested. This finding suggests that cyanobacteria can endogenously modulate the activity of their protease inhibitors.

Two cyclodepsipeptides named kempopeptins A (1) and B (2) were isolated from a collection of a Floridian marine cyanobacterium, Lyngbya sp., that had previously afforded the structurally related potent elastase inhibitors lyngbyastatin 7 and somamide B. The structures of 1 and 2 were elucidated mainly by 1D and 2D NMR spectroscopy, and the absolute configuration was established by chiral HPLC and Marfey's analysis of the degradation products. Kempopeptin A (1) exhibited an IC50 against elastase of 0.32 microM and against chymotrypsin of 2.6 microM, while kempopeptin B (2) inhibited trypsin with an IC50 of 8.4 microM.

Three new analogues of dolastatin 13, termed lyngbyastatins 5-7 ( 1- 3), were isolated from two different collections of marine cyanobacteria, Lyngbya spp., from South Florida. Their planar structures were deduced by a combination of NMR techniques, and the absolute configurations were established by modified Marfey's analysis of the acid hydrolyzates. The related cyclodepsipeptide somamide B ( 4), previously reported from a Fijian cyanobacterium, has also been found in one of the extracts, and its absolute stereochemistry was unambiguously assigned for the first time. Compounds 1- 4 were found to selectively inhibit elastase over several other serine proteases, with IC50 values for porcine pancreatic elastase ranging from 3 to 10 nM.

Lyngbyastatin 4 (1), a new depsipeptide containing the unusual amino acid homotyrosine and a 3-amino-6-hydroxy-2-piperidone (Ahp) residue, was isolated from a collection of the marine cyanobacterium Lyngbya confervoides off the Florida Atlantic coast. Its gross structure was determined by NMR spectroscopy, and the configurations of asymmetric centers were assigned after chiral HPLC analysis of hydrolysis products. Lyngbyastatin 4 (1) is an analogue of the sea hare isolate dolastatin 13 and several marine cyanobacterial metabolites, further supporting the notion that many of the dolastatins are of cyanobacterial origin. Lyngbyastatin 4 (1) selectively inhibits elastase and chymotrypsin in vitro over other serine proteases with IC50 values of 0.03 and 0.30 microM, respectively.