The interaction of vanadate ions with the Ca-ATPase from sarcoplasmic reticulum.
The interaction of vanadate ions with the Ca-ATPase from sarcoplasmic reticulum vesicles was studied in a native and a fluorescein-labeled ATPase preparation (Pick, U., and Karlish, S. J. D. (1980) Biochim. Biophys. Acta 626, 255-261). Vanadate induced a fluorescence enhancement in a fluorescein-labeled enzyme, indicating that it shifts the equilibrium between the two conformational states of the enzyme by forming a stable E2-Mg-vanadate complex (E2 is the low affinity Ca2+ binding conformational state of the sarcoplasmic reticulum Ca-ATPase). Indications for tight binding of vanadate to the enzyme (K1/2 = 10 microM) in the absence of Ca2+ and for a slow dissociation of vanadate from the enzyme in the presence of Ca2+ are presented. The enzyme-vanadate complex was identified by the appearance of a time lag in the onset of Ca2+ uptake and by a slowing of the fluorescence quenching response to Ca2+. Ca2+ prevented the binding of vanadate to the enzyme. Pyrophosphate (Kd = 2 mM) and ATP (Kd = 25 microM) competitively inhibited the binding of vanadate, indicating that vanadate binds to the low affinity ATP binding site. Binding of vanadate inhibited the high affinity Ca2+ binding to the enzyme at 4 degrees C. Vanadate also inhibited the phosphorylation reaction by inorganic phosphate (Ki = 10 microM) but had no effect on the phosphorylation by ATP. It is suggested that vanadate binds to a special region in the low affinity ATP binding site which is exposed only in the E2 conformation of the enzyme in the absence of Ca2+ and which controls the rate of the conformation transition in the dephosphorylated enzyme. The implications of these results to the role of the low affinity ATP binding sites are discussed.
D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D016650:Fluorescein-5-isothiocyanate; D005452:Fluoresceins / Q000494:pharmacology; D005456:Fluorescent Dyes / Q000494:pharmacology; D007700:Kinetics; D009132:Muscles / Q000201:enzymology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013050:Spectrometry, Fluorescence; D013861:Thiocyanates / Q000494:pharmacology; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology*
D005452:Fluoresceins; D005456:Fluorescent Dyes; D013861:Thiocyanates; D014639:Vanadium; D014638:Vanadates; D000252:Calcium-Transporting ATPases; D016650:Fluorescein-5-isothiocyanate