Vanadate inhibition of the Ca-ATPase activity of sarcoplasmic reticulum vesicles

The Ca-dependent ATPase of sarcoplasmic reticulum vesicles was studied with respect to the inhibition by vanadate. The vanadate concentration for half maximal inhibition (Ki) varied with the nucleotide used and with its concentrations in the medium, being 0.7 microM in presence of 5 to 50 microM ATP and increasing progressively up to 7.0 microM when the ATP concentration was raised up to 5 mM. The Ki was about 1 microM in presence of 0.1 to 2.0 mM of either GTP or ITP. The Ki also varied depending on whether the enzyme was preincubated with vanadate in absence of Ca2+ (Ki approximately equal to approximately equal to 1 microM) or in the presence of 0.1 mM Ca2+ (Ki greater than 20 microM). This effect was only observed when the activity was measured 15s after the addition of ATP. For longer incubation intervals the Ki did no longer depend on the presence of Ca2+ in the preincubation medium. Phosphorylation of the enzyme by orthophosphate was competitively inhibited by vanadate. The ATP in equilibrium Pi exchange reaction measured in vesicles which were permeable to Ca2+ was inhibited by vanadate when the Ca2+ concentration was in the range 100-200 microM. However, inhibition of both ATPase activity and ATP in equilibrium Pi. exchange were abolished when the Ca2+ concentrations was raised to the millimolar range.