title: Oxygen exchange between Pi in the medium and water during ATP hydrolysis mediated by skinned fibers from rabbit skeletal muscle. Evidence for Pi binding to a force-generating state.
abstract: Oxygen exchange between (18O4)Pi in the medium and water accompanies ATP hydrolysis catalyzed by the calcium-regulated MgATPase of vertebrate skeletal muscle. Exchange was observed in chemically skinned fibers from rabbit psoas muscle held isometrically and activated by 30 microM free Ca2+. The rate of exchange was approximately proportional to Pi concentration (up to 10 mM) and was characterized by an apparent second order rate constant greater than or equal to 475 M-1 S-1 (pH 7.1, ionic strength 0.2 M, 22 degrees C). Much less exchange occurred in the absence of Ca2+ or when ATP was replaced by ADP. It has been inferred from mechanical experiments that Pi can bind to a force-generating ADP-bound state of actomyosin with resultant suppression of force (Hibberd, M. G., Dantzig, J. A., Trentham, D. R., and Goldman, Y. E. (1985) Science 228, 1317-1319). The oxygen exchange results support this inference by providing direct evidence that Pi in the medium binds at the ATPase catalytic site in activated isometric fibers. The inter-relationship of these two effects involving Pi on mechanochemical coupling in muscle is discussed.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D006868:Hydrolysis; D007700:Kinetics; D009119:Muscle Contraction; D009132:Muscles / Q000201:enzymology*; D010100:Oxygen / Q000378:metabolism*; D010103:Oxygen Isotopes; D010710:Phosphates / Q000378:metabolism*; D011817:Rabbits; D014867:Water / Q000378:metabolism
chemical_list: D010103:Oxygen Isotopes; D010710:Phosphates; D014867:Water; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D010100:Oxygen; D002118:Calcium