Okadaic acid

This is a tumor promoter on mouse skin and a potent and specific cell-permeating inhibitor of protein phosphatases (serine phosphatases 2A and 1) produced by the marine black sponge Halichondria okadai. Three other main types of okadaic acid class compounds, Calyculin A, isolated from the marine sponge Discodermia calyx, Microcystin-LR, a cyclic heptapeptide product of the blue-green alga Microcystis aeruginosa, and Tautomycin, found in Streptomyces spirover ticillatus also selectively inhibit protein serine/threonine but not tyrosine phosphatase activity.