Two new protein phosphatase inhibitors, oscillamide B (1) and C (2), were i
solated 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 protei
n phosphatases PP1 and PP2A. The inhibitory activities were closely related
to the Arg and N-Me-Hty residues in the peptides.