THE INHIBITORS THAPSIGARGIN AND 2,5-DI(TERT-BUTYL)-1,4-BENZOHYDROQUINONE FAVOR THE E2 FORM OF THE CA-2+,MG-2+-ATPASE

2,5-Di(tert-butyl)-1,4-benzohydroquinone has been shown to inhibit the Ca2+,Mg2+-ATPase of sarcoplasmic reticulum with an affinity of 0.4-mu-M. It has been shown to shift the E2-E1 equilibrium for the ATPase towards E2, as shown previously for the inhibitor thapsigargin. The shift towards E2 results in a decrease in affinity for Ca2+, as also observed for thapsigargin. A marked decrease in the rate of the E2-E1 transition is observed for both BHQ and thapsigargin. A decrease in the equilibrium level of phosphorylation by P(i) and of the steady-state level of phosphorylation by ATP are consistent with a decrease in the equilibrium constant for phosphorylation by P(i) and an increase in the rate of dephosphorylation.