The mechanism of inhibition of the sarco/endoplasmic reticulum Ca 2+ ATPase by paxilline

Abstract

Paxilline, an indole alkaloid mycotoxin from Penicillium paxilli, is an inhibitor of the sarco/endoplasmic reticulum Ca 2+ ATPase (SERCA). Paxilline inhibited differing isoforms of SERCA with IC 50s between 5 and 50 μM . It inhibited more potently the purified Ca 2+ ATPase activity from skeletal muscle with an IC 50 of 5 μM . Detailed effects of this inhibitor on the Ca 2+ and ATP dependence upon activity indicate that it affects the high-affinity Ca 2+-binding (E1) form of the ATPase. In addition, paxilline is a “competitive” inhibitor with respect to high concentrations of ATP, increasing the regulatory binding site K m , without affecting the catalytic binding site K m . At higher concentrations, paxilline inhibits phosphoenzyme formation from ATP and inorganic phosphate, without affecting nucleotide binding. We therefore suggest that paxilline has two effects on the Ca 2+ ATPase. At lower concentrations ( 5–10 μM ), paxilline inhibits the ATP-dependent acceleration of Ca 2+ release from the phosphoenzyme and/or phosphoenzyme decay. At higher concentrations, paxilline inhibits phosphoenzyme formation.