Interactions of vanadate oligomers with sarcoplasmic reticulum Ca 2+-ATPase

Abstract

Upon addition of sarcoplasmic reticulum (SR), the line width of tetrameric vanadate signal of 51V-NMR spectra narrowed in the presence of ATP and Ca 2+, whereas monomeric vanadate line widths were broadened. Thus, ATP decreases the affinity of the enzyme for tetravanadate whereas it induces the interaction with monomeric vanadate. In the presence of Ca 2+ it was observed that tetrameric and decameric vanadate bind to SR ATPase whereas monomeric vanadate only binds to SR when ATP is present. However, decameric vanadate clearly differs from vanadate oligomers present in monovanadate solutions in preventing the accumulation of Ca 2+ by sarcoplasmic reticulum (SR) vesicles coupled to ATP hydrolysis. Mg 2+ increased the inhibitory effect promoted by decavanadate whereas a slight enhancement of Ca 2+ uptake was observed in the presence of monovanadate. For 5 mM Mg 2+, a nominal 2 mM vanadium ‘decavanadate’ solution containing about 190 to 200 μM decameric and less than 100 μM monomeric species depressed the rate of Ca 2+ uptake by 50% whereas a nominal 2 mM monovanadate solution containing about 662 μM monomeric, 143 μM dimeric and 252 μM tetrameric species had no effect on the rate of Ca 2+ accumulation. However, 2 mM ‘decavanadate’ inhibits by 75% the SR Ca 2+-ATPase activity whereas the presence of 2 mM ‘monovanadate’ produces an inhibitory effect below 50%. Therefore, the Ca:ATP stoichiometry of Ca 2+ transport is enhanced by monovanadate. In the presence of oxalate, inhibition of SR Ca 2+-ATPase activity by these solutions is enhanced to 97% and 86% whereas in the presence of the ionophore lasalocid, the inhibitory values were 87% and 19% for 2 mM decavanadate and 2 mM monovanadate solutions, respectively. Apparently, the increase of vesicular Ca 2+ concentration counteracts monovanadate inhibition of SR Ca 2+-ATPase activity but it does not significantly affect decavanadate inhibition.