title: Inhibitory effect of a marine-sponge toxin, okadaic acid, on protein phosphatases. Specificity and kinetics.
abstract: The inhibitory effect of a marine-sponge toxin, okadaic acid, was examined on type 1, type 2A, type 2B and type 2C protein phosphatases as well as on a polycation-modulated (PCM) phosphatase. Of the protein phosphatases examined, the catalytic subunit of type 2A phosphatase from rabbit skeletal muscle was most potently inhibited. For the phosphorylated myosin light-chain (PMLC) phosphatase activity of the enzyme, the concentration of okadaic acid required to obtain 50% inhibition (ID50) was about 1 nM. The PMLC phosphatase activities of type 1 and PCM phosphatase were also strongly inhibited (ID50 0.1-0.5 microM). The PMCL phosphatase activity of type 2B phosphatase (calcineurin) was inhibited to a lesser extent (ID50 4-5 microM). Similar results were obtained for the phosphorylase a phosphatase activity of type 1 and PCM phosphatases and for the p-nitrophenyl phosphate phosphatase activity of calcineurin. The following phosphatases were not affected by up to 10 microM-okadaic acid: type 2C phosphatase, phosphotyrosyl phosphatase, inositol 1,4,5-trisphosphate phosphatase, acid phosphatases and alkaline phosphatases. Thus okadaic acid had a relatively high specificity for type 2A, type 1 and PCM phosphatases. Kinetic studies showed that okadaic acid acts as a non-competitive or mixed inhibitor on the okadaic acid-sensitive enzymes.
authors.affiliation: II. Physiologisches Institut, Universität Heidelberg, Federal Republic of Germany.
authors.forename: C
authors.initials: C
authors.lastname: Bialojan
authors.forename: A
authors.initials: A
authors.lastname: Takai
mesh_terms: D000818:Animals; D019703:Calcineurin; D002148:Calmodulin-Binding Proteins / Q000037:antagonists & inhibitors; D004305:Dose-Response Relationship, Drug; D004988:Ethers, Cyclic / Q000494:pharmacology*; D007700:Kinetics; D008387:Marine Toxins / Q000494:pharmacology*; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D011073:Polyamines; D000071228:Polyelectrolytes; D011108:Polymers / Q000494:pharmacology; D011161:Porifera / Q000032:analysis*
chemical_list: D019703:Calcineurin; D002148:Calmodulin-Binding Proteins; D004988:Ethers, Cyclic; D008387:Marine Toxins; D019319:Okadaic Acid; D010749:Phosphoprotein Phosphatases; D011073:Polyamines; D000071228:Polyelectrolytes; D011108:Polymers