Characterization of (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum by laser-excited europium luminescence.

The molecular environment of Ca2+ translocating sites of skeletal muscle sarcoplasmic reticulum (SR) (Ca2+ + Mg2+)-ATPase has been studied by pulsed-laser excited luminescence of Eu3+ used as a Ca2+ analogue. Interaction of Eu3+ with SR was characterized by investigating its effect on partial reactions of the Ca2+ transport cycle. In native SR vesicles, Eu3+ was found to inhibit Ca2+ binding, phosphoenzyme formation, ATP hydrolysis activity and Ca2+ uptake in parallel fashion. The non-specific binding of Eu3+ to acidic phospholipids associated with the enzyme was prevented by purifying (Ca2+ + Mg2+)-ATPase and exchanging the endogenous lipids with a neutral phospholipid, dioleoylglycerophosphocholine. The results demonstrate that the observed inhibition of Ca2+ transport by Eu3+ is due to its binding to Ca2+ translocating sites. The 7F0----5D0 transition of Eu3+ bound to these sites was monitored. The non-Lorentzian nature of the excitation profile and a double-exponential fluorescence decay revealed the heterogeneity of the two sites. Measurement of fluorescence decay rates in H2O/D2O mixture buffers further distinguished the sites. The number of water molecules in the first co-ordination sphere of Eu3+ bound at transport sites were found to be 4 and 1.5. Addition of ATP reduced these numbers to zero and 0.6. These data show that the calcium ions in translocating sites are well enclosed by protein ligands and are further occluded down to zero or one water molecule of solvation during the transport process.