[Study of the rotational mobility of E1- and E2- conformers of Ca-ATPase in sarcoplasmic reticulum membranes using a...

Membrane preparations of sarcoplasmic reticulum Ca-ATPase from rabbit skeletal muscles were covalently labelled with eosin-5'-isothiocyanate at Lys 515 residue in a putative active site and with 4-(iodoacetamido)-eosin (presumably, at Cys 670 and Cys 674 residues). These preparations were used to measure the laser flash induced phosphorescence anisotropy in a microsecond time scale. An analysis of effects of diethyl ether, glycerol, the nonionic detergent C12E9 and Ca-ATPase ligands stabilizing the enzyme in the E2 or E1 conformeric state, on the anisotropy parameters showed that Ca-ATPase of sarcoplasmic reticulum membranes is present in both monomeric and oligomeric states. The enzyme transition from the E1 to the E2 conformation is due to the increase in the content of oligomeric complexes and their average size. These data support the hypothesis that the oligomeric state of Ca-ATPase can be changed during enzyme reaction cycle.