Inhibition of protein phosphatases‐1 and ‐2A with acanthifolicin

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Abstract
Acanthifolicin (9,10‐epithio‐okadaic acid from Pandoras acanthifolium) inhibited protein phosphatase‐1 (PP1) similarly to okadaic acid (IC50 = 20 nM and 19 nM, respectively) but was slightly less active against protein phosphatase‐2A (PP2A) (IC50 1 nM and 0.2 nM, respectively). Methyl esterification of acanthifolicin sharply reduced its activity. PP2A was inhibited with an IC50 = 5.0 μM, whilst PP1 was inhibited < 10% at 250 μM toxin. Okadaic acid methyl ester was similarly inactive whereas dinophysistoxin‐1 (35‐methyl okadaic acid) inhibited PP1/2A almost as potently as okadaic acid. Pure acanthifolicin/okadaic acid methyl ester may be useful as specific inhibitors of PP2A at 1–10 μM concentrations in vitro and perhaps in vivo. The data also indicate that a region on these toxins important for PP1/2A inhibition comprises the single carboxyl group.