title: A phosphorylated conformational state of the (Ca2+-Mg2+)-ATPase of fast skeletal muscle sarcoplasmic reticulum can mediate rapid Ca2+ release.
abstract: A rapid Ca2+ release from Ca2+-loaded sarcoplasmic reticulum vesicles from fast skeletal muscle can be induced under conditions which permit the formation of a stable phosphorylated intermediate of the (Ca2+-Mg2+)-ATPase. Such a state can be achieved experimentally by phosphorylating the ATPase in the absence of Mg2+ ions, which otherwise would stimulate the dephosphorylation step(s). Also, quercetine stimulates the rapid release of Ca2+ if used in the concentration range which does not produce inhibition of phosphoenzyme formation, but which inhibits phosphoenzyme dephosphorylation. The rapid efflux of Ca2+ ions proceeds as long as the low affinity Ca2+-binding sites facing the lumen of the vesicles are saturated and as long as Ca2+ is removed from the catalytic sites facing the cytosol. A molecular mechanism of the phosphoenzyme-mediated Ca2+ release is proposed. This mechanism is based on a rapid shuttling of the ATPase molecules between an ADP-sensitive and an ADP-insensitive phosphorylated state.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D009132:Muscles / Q000201:enzymology*; D010766:Phosphorylation; D011487:Protein Conformation; D011794:Quercetin / Q000494:pharmacology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000201:enzymology*
chemical_list: D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D011794:Quercetin; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium