Strontium binding to sarcoplasmic reticulum Ca(2+)-ATPase. Spectroscopic differentiation of the substeps involved

We investigated the consequences of Sr2+ binding to the transport sites of sarcoplasmic reticulum (SR) Ca(2+)-ATPase for two fluorescent conformational probes located in different regions of the ATPase. Using SR vesicles in which Lys-515 in the ATPase had been previously labeled with fluorescein 5'-isothiocyanate (FITC), we found that the Sr(2+)-induced a drop in the fluorescein fluorescence of this FITC-labeled ATPase shifted toward lower Sr2+ concentrations than the Sr(2+)-induced rise in Trp fluorescence for the same FITC-labeled ATPase. The curve describing the Sr(2+)-dependent rise in Trp fluorescence had a characteristic asymmetric shape, and the changes in Trp fluorescence occurred in parallel with the activation by Sr2+ of pNPP hydrolysis by the ATPase. Analysis of these results in terms of the simplest scheme describing the sequential binding of the two Sr2+ ions suggests that under the conditions of these experiments, i.e. at neutral pH in the presence of potassium, the Sr(2+)-induced rise in the Trp fluorescence mainly reflected the formation of ATPase with two ions bound to the transport sites, whereas the binding of a single Sr2+ ion was virtually sufficient to reduce the fluorescence of bound FITC to its minimal level.