CANTHARIDIN, ANOTHER NATURAL TOXIN THAT INHIBITS THE ACTIVITY OF SERINE THREONINE PROTEIN PHOSPHATASES TYPE-1 AND TYPE-2A

Cantharidin, a natural toxicant of blister beetles, is a strong inhibi tor of protein phosphatases types 1(PPI) and 2A (PP2A). Like okadaic a cid, cantharidin inhibits the activity of the purified catalytic subun it of PP2A (IC50 = 0.16 muM) at a lower concentration than that of PP1 (IC50 = 1.7 muM) and only inhibits the activity of protein phosphatas e type 2B (PP2B) at high concentrations. Dose-inhibition studies condu cted with whole cell homogenates indicate that cantharidin also inhibi ts the native forms of these enzymes. Thus, cantharidin, which is econ omical and readily available, may be useful as an additional probe for studying the functions of serine/threonine protein phosphatases.