title: Cooperative effects of Ca2+ and Sr2+ on sarcoplasmic reticulum adenosine triphosphatase.
abstract: The intrinsic fluorescence of purified Ca-ATPase from skeletal sarcoplasmic reticulum was measured in the presence of various concentrations of Ca2+, Sr2+, and Ba2+. Ca2+ and Sr2+ induce positive cooperative fluorescence enhancement, whereas Ba2+ does not change the fluorescence of ATPase. ATP does not seem to modify the kinetic parameters of Ca2+ and Sr2+ binding to ATPase. Nevertheless, p-nitrophenylphosphate hydrolysis, activated by Ca2+ or Sr2+ at various pHs, changes the affinity and the cooperative behavior for both cations and two components appear in the Hill plots. For Ca2+, nH of 1.6 to 3.5 were obtained, and 1.06 to 1.83 for Sr2+; nH changes of the second component seem to be pH dependent. Differences in the ratio between rates of Ca2+ transport and substrate hydrolysis by sarcoplasmic reticulum were found, i.e., two for ATP and one for p-nitrophenylphosphate. For Sr2+ this ratio was one for either ATP or p-nitrophenylphosphate.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001464:Barium / Q000378:metabolism; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004789:Enzyme Activation; D066298:In Vitro Techniques; D009596:Nitrophenols / Q000378:metabolism; D009943:Organophosphorus Compounds / Q000378:metabolism; D010744:Phosphoric Monoester Hydrolases / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence; D013324:Strontium / Q000378:metabolism*
chemical_list: D009596:Nitrophenols; D009943:Organophosphorus Compounds; D001464:Barium; C008644:nitrophenylphosphate; D000255:Adenosine Triphosphate; D010744:Phosphoric Monoester Hydrolases; D000252:Calcium-Transporting ATPases; D002118:Calcium; D013324:Strontium