Calyculin A. Calyculin A was initially isolated from the marine sponge Discodermia calyx. It is a very potent, cell-permeable PP-1 and PP-2 inhibitor (IC50: 2nM and 0.5 ?1nM respectively) and extremely specific for PP-1 and PP2A among other acidic, alkaline and protein phosphatases. Protein Phosphatase Inhibitors; Protein Phosphatase Inhibitors (PP-1 and PP-2A).