Inhibitory effect of a marine-sponge toxin, okadaic acid, on protein phosphatases. Specificity and kinetics

TLDR

Okadaic acid inhibition was tested on type 1, 2A, 2B, 2C, and polycation‑modulated phosphatases. Okadaic acid potently inhibited type 2A phosphatase (ID50 ≈ 1 nM), strongly inhibited type 1 and PCM phosphatases (ID50 0.1–0.5 µM), modestly inhibited type 2B (ID50 4–5 µM), had no effect on type 2C, phosphotyrosyl, or other phosphatases up to 10 µM, and acted as a non‑competitive or mixed inhibitor on the sensitive enzymes.

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.