Okadaic acid was named from the marine sponge Halichondria okadai, from which okadaic acid was isolated for the first time. It has also been isolated from another marine sponge, H. malanodocia, as a cytotoxin. The real producer of okadaic acid is a marine dinoflagellate.
The cytotoxicities of okadaic acid as EC50 against the P388 and L1210 cell lines are 1.7 nanomolar and 17 nanomolar, respectively. Additionally, okadaic acid strongly inhibits protein serine / threonine phosphatase 1, 2A, and 2B. The inhibitory effect of okadaic acid is strongest for 2A, followed by 1, and then 2B. The dissociation constant of the inhibition on protein serine / threonine phosphatase 2A is 30 pico-molar.