Isolation of new protein phosphatase inhibitors from two cyanobacteria species, Planktothrix spp.

Two new protein phosphatase inhibitors, oscillamide B (1) and C (2), were isolated from the cyanobacteria Planktothrix (Oscillatoria) agardhii and P. rubescens. The structures of the inhibitors were elucidated by analysis of HRFABMS, 1D and 2D NMR spectra, and chemical degradation. These inhibitors are ureido-containing cyclic peptides and inhibited serine/threonine protein phosphatases PP1 and PP2A. The inhibitory activities were closely related to the Arg and N-Me-Hty residues in the peptides.