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.

The regulatory subunits of fission yeast protein phosphatase 2A (PP2A) affect cell morphogenesis, cell wall synthesis and cytokinesis

“…1993). This is consistent with findings by Felix et al (1990) that okadaic acid, a potent PP2A inhibitor (Biolajan & Takai 1988), leads to activation of cdc2 kinase in Xenopus egg extracts. Inhibition of PP2A activity leads to premature mitotic activation.…”

Ba2+ replaces Ca2+/calmodulin in the activation of protein phosphatases and in exocytosis of all major transmitters

“…In addition, because the phosphatases type 1 and type 2A appear to be involved in stimulus-evoked and Ca2+-dependent dephosphorylation in nerve terminals (Sim et al, 1993), the effects of okadaic acid, an inhibitor of these phosphatases, was also tested (see Bialojan and Takai, 1988;Cohen et al, 1990). Okadaic acid potently inhibited both Ba 2+-and K+/Ca2+-evoked transmitter release ( Fig.…”

Substrate specificities of catalytic fragments of protein tyrosine phosphatases (HPTPβ, LAR, and CD45) toward phosphotyrosylpeptide substrates and thiophosphotyrosylated peptides as inhibitors

“…More importantly the pY(S)-PLCy 1 1-mer peptide inhibits the HPTPO catalytic fragment 73-fold and 110-fold more potently than the LAR-Dl and CD45 catalytic domains. Whereas okadaic acid (Bialojan & Takai, 1988;Cohen et al, 1989) and microcystin (Honkanen et al, 1990) on the one hand, and FK506 and cyclosporin (presented by their respective immunophilin FKBP and cyclophilin) on the other hand (Liu et al, 1991), are potent and selective inhibitors of PSPases, PP 1, PP2A, and PP2B, there are no PTPaseselective or potent inhibitors yet reported. The inhibition of HPTPO with the pY(S)-PLCy 1 1-mer peptide is so far unprecedented in its potency (3 pM of K I ) and selectivity (73-110-fold lower KI), and this observation raises the possibility of the use of pY(S)-PLCy peptide or analogs as PTPase-selective inhibitors for the study of the physiological role of particular PTPases.…”