THERMAL UNFOLDING OF MONOMERIC CA(II),MG(II)-ATPASE FROM SARCOPLASMIC-RETICULUM OF RABBIT SKELETAL-MUSCLE

The thermal unfolding of monomeric and delipidated Ca2+-ATPase, solubi lized in C(12)E(8), can be appropriately described as a non-two-state irreversible denaturation, with only one endothermic peak. In the Ca2 concentration range (0-0.5 mM) which stimulates the ATPase activity o f solubilized monomeric ATPase, Ca2+ shifts the critical temperature m idpoint of the denaturation process (T-m) from 42 to 50 degrees C with out segregation of the endothermic peak into two separate components. Because 20 mM Mg2+ only shifts the T-m from 42 to 44 degrees C, we con clude that the effect of Ca2+ upon the T-m is likely to be due to bind ing to the high affinity Ca2+ sites in the ATPase. The effect of Ca2upon the enthalpy of denaturation is biphasic, suggesting the presence of low affinity Ca2+ sites (K-0.5 in the millimolar range) in monomer ic and solubilized ATPase.