CHARACTERIZATION OF NATURAL TOXINS WITH INHIBITORY ACTIVITY AGAINST SERINE THREONINE PROTEIN PHOSPHATASES

Recent studies suggest that the ability to inhibit the activity of cer tain serine/threonine protein phosphatases underlies the toxicity of s everal natural compounds including: okadaic acid, microcystin-LR, nodu larin, calyculin A and tautomycin. To characterize further the actions of these toxins, this study compares the inhibitory effects of okadai c acid, chemical derivatives of okadaic acid, microcystin-LR, microcys tin-LA, nodularin, calyculin A and tautomycin on the activity of serin e/threonine protein phosphatases types 1 (PP1), 2A (PP2A) and a recent ly identified protein phosphatase purified from bovine brain (PP3). Th is study shows that, like PP1 and PP2A, the activity of PP3 is potentl y inhibited by okadaic acid, both microcystins, nodularin, calyculin A and tautomycin. Further characterization of the toxins employing the purified catalytic subunits of PP1, PP2A and PP3 under identical exper imental conditions indicates that: (a) okadaic acid, microcystin-LR, a nd microcystin-LA inhibit PP2A and PP3 more potently than PP1 (order o f potency PP2A > PP3 > PP1); (b) nodularin inhibits PP1 and PP3 at a s imilar concentration that is slightly higher than that which affects P P2A, and (c) both calyculin A and tautomycin show little selectivity a mong the phosphatases tested. This study also shows that the chemical modification of the (C1) carboxyl group of okadaic acid can have a pro found influence on the inhibitory activity of this toxin. Esterificati on of okadaic acid, producing methyl okadaate, or reduction, producing okadaol, greatly decreases the inhibitory effects against all three e nzymes tested. Further reduction, producing 1-nor-okadaone, or acetyla tion, producing okadaic acid tetraacetate, results in compounds with n o inhibitory activity. In contrast, the substitution of alanine (-LA) for arginine (-LR) in microcystin has no apparent effect on the inhibi tory activity against PP1, PP2A or PP3.