3‘,3“,5‘,5”-Tetraiodophenolsulfonephthalein is a selective inhibitor of Ca(2+)-pumping ATPase in intracellular Ca2+ store

Abstract

3',3",5',5"-Tetraiodophenolsulfonephthalein (I4PSP) stimulated both Ca2+ transient levels and aggregation in response to thrombin in platelets. Ca2+ uptake into skeletal sarcoplasmic reticulum (SR) was inhibited by I4PSP (IC50, 1.8 microM) whereas that of red blood cell ghosts was not affected by it. Furthermore, I4PSP inhibited the SR Ca(2+)-ATPase activity (IC50, 1.1 microM). Kinetic analysis of the inhibitory effects of I4PSP reveals that I4PSP shows an inhibition of noncompetitive and competitive type with respect to low and high concentrations of ATP, respectively. The mode of inhibition by I4PSP is an uncompetitive type with respect to Ca2+. I4PSP decreased the decomposition rate of the phosphorylated enzyme intermediate (EP). The change in the tryptophan fluorescence of SR Ca(2+)-ATPase induced by ATP was reduced by I4PSP indicating inhibition of the EP transition from Ca2E1P to E2P. The concentration-inhibitory response curves for I4PSP in Ca(2+)-ATPase activities and fluorescence changes were closely correlated. These results suggest that I4PSP slows down the structure transition from Ca2E1P to E2P, resulting in inhibition of the Ca(2+)-ATPase activity. On the basis of these results, it is suggested that the stimulation of Ca2+ transient levels in platelets and their aggregation in response to thrombin are due to the inhibition of Ca2+ pump in intracellular Ca2+ stores by I4PSP.