Planktocyclin, a cyclooctapeptide protease inhibitor produced by the freshwater cyanobacterium Planktothrix rubescens

The freshwater cyanobacterium Planktothrix rubescens produces the cyclooctapeptide cyclo(Pro-Gly-Leu-Val-Met-Phe-Gly-Val). The chemical structure is new. This homodetic cyclic octapeptide was named planktocyclin (1). It consists solely of proteinogenic L-amino acids and is a strong inhibitor of mammalian trypsin and cc-chymotrypsin and a moderately active inhibitor of human recombinant caspase-8. Mass spectrometric and 2D-NMR spectroscopic data allowed the determination of its structure. Synthetic planktocyclin was identical to the natural product.