The mechanism of inhibition of the sarco/endoplasmic reticulum Ca2+ ATPase by paxilline

paxilline, an indole alkaloid mycotoxin from Penicillium paxilli, is an inhibitor of the sarco/endoplasmic reticulum. Ca2+ ATPase (SERCA). Paxilline inhibited differing isoforms of SERCA with IC(50)s between 5 and 50 muM. It inhibited more potently the purified Ca2+ ATPase activity from skeletal muscle with an IC50 of 5 muM. Detailed effects of this inhibitor on the Ca2+ and ATP dependence upon activity indicate that it affects the high-affinity Ca2+-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 Ca2+ ATPase. At lower concentrations (5-10 muM), paxilline inhibits the ATP-dependent acceleration of Ca2+ release from the phosphoenzyme and/or phosphoenzyme decay. At higher concentrations, paxilline inhibits phosphoenzyme formation. (C) 2002 Elsevier Science (USA). All rights reserved.