title: A kinetic study of the interaction of vanadate with the Ca2+ + Mg2+-dependent ATPase from sarcoplasmic reticulum.
abstract: Ca2+ + Mg2+-dependent ATPase from sarcoplasmic reticulum was inhibited by preincubation with vanadate. When the inhibited enzyme was preincubated in the presence of vanadate and assayed in its absence, a slow reactivation process was observed. This slow, hysteretic, process was exploited to study the influence of Ca2+ and ATP on the dissociation of vanadate. Ca2+ alone slowly displaced vanadate from the inhibited enzyme, and a rate constant of 0.1 min-1, at 25 degrees C, was calculated for this re-activation process. However, ATP re-activated with an apparent constant that hyperbolically depended on ATP concentration, and from it a rate constant for vanadate dissociation induced by ATP of 0.5 min-1 was calculated. It is deduced from the kinetic studies that ATP binds to the enzyme-vanadate complex, forming a ternary complex, with a dissociation constant of 4 microM, and that this binding accelerates vanadate dissociation. Binding experiments with [14C]ATP showed that ATP binds to the enzyme-vanadate complex with a dissociation constant of 12 microM, i.e. the affinities calculated with the isotope technique and the kinetic procedure are of the same order of magnitude.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D004789:Enzyme Activation / Q000187:drug effects; D007700:Kinetics; D046911:Macromolecular Substances; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology*
chemical_list: D046911:Macromolecular Substances; D014639:Vanadium; D014638:Vanadates; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium