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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.

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.

An intestinal elastase was purified from Atlantic cod (Gadus morhua) of apparent molecular mass 24.8 kDa as determined by SDS-PAGE and with isoelectric point above pI 9.3. Heat stability and stability towards acidic pH was reduced in the cod enzyme as compared with porcine intestinal elastase. N-terminal amino-acid sequence analysis of cod elastase showed considerable similarity with porcine elastase. The cod enzyme was less sensitive to phenylmethylsulfonyl fluoride inhibition than porcine elastase, but sensitivity towards other inhibitors was similar. Kinetic properties were examined using the substrate Suc-Ala-Ala-Ala-p-nitroanilide and the cod enzyme was found to have more than 2-times turnover rate (kcat) as compared with the porcine enzyme, and slightly higher Km values. Thus, the catalytic efficiency (kcat/Km) of Atlantic cod elastase was about 2-times higher than observed with porcine elastase, which indicates an adaptive response towards the low temperature environmental in which the cod lives. Substrate specificity was studied by digestion of oxidized B-chain of insulin and by using synthetic substrates. Digestion was most rapid at the carbonyl side of alanine residues, but also occurred at valine and leucine residues.