We have studied the fluorescence of the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum labelled with fluorescein isothiocyanate. The change in intensity of fluorescein fluorescence caused by addition of Ca2+ to the labelled ATPase can be interpreted in terms of a two-conformation model for the ATPase, one conformation (E1) having a high affinity for Ca2+, the other (E2) a low affinity. Effects of Ca2+ as a function of pH allow an estimate of the effect of pH on the E1/E2 ratio, consistent with kinetic studies. A model is presented for binding of Ca2+ to the ATPase as a function of pH that is consistent both with the data on the E1/E2 equilibrium and with literature data on Ca2+ binding.

We have shown that changes in fluorescence intensity for the Ca2+ + Mg2+-activated ATPase of sarcoplasmic reticulum labelled with fluorescein isothiocyanate following the addition of Ca2+ can give the ratio of the two conformations (E1 and E2) of the ATPase. We show that the fluorescence response to Ca2+ is unaffected by Mg2+, phosphate or K+, implying that these ions bind equally well to the E1 and E2 conformations. A model is presented for phosphorylation of the ATPase by phosphate as a function of pH, Mg2+, K+ and Ca2+.