AFFINITY OF OKADAIC ACID TO TYPE-1 AND TYPE-2A PROTEIN PHOSPHATASES IS MARKEDLY REDUCED BY OXIDATION OF ITS 27-HYDROXYL GROUP

Okadaic acid (OA), a potent inhibitor of type-1 and type-2A protein phosphatases (PP1 and PP2A), has four hydroxyl groups at 2, 7, 24 and 27 positions (see Figure 1). By chemical treatment of OA we synthesized a derivative, in which the 27-hydroxyl group was specifically oxidized (27-dehydro-OA). The inhibitory effect of this OA derivative was examined on the activities of PPI and PP2A, which were inhibited by intact OA with dissociation constants (K-i) of 150 nM and 32 pM respectively. We found that the affinity of OA was decreased 40-fold (K-i = 6 mu M) with PP1 and 230-fold (K-i = 7.3 nM) with PP2A after oxidation of the 27-hydroxyl group. According to the model of the three-dimensional conformation of OA on the basis of X-ray analyses, the 27-hydroxyl group appears to be present in a position relatively free from intramolecular bonding formation, in comparison with the other three hydroxyl groups. The marked increases in the K-i values for PP1 and PP2A, which indicate the reduction of the absolute values of the free energy of binding by 9 kJ/mol and 14 kJ/mol respectively, may imply that the 27-hydroxyl group serves as a binding site with the phosphatase molecules.