THE STABLE, INACTIVE BUT REACTIVATABLE, UNFOLDING INTERMEDIATE OF RATMUSCLE SARCOPLASMIC-RETICULUM CA2-ATPASE DIFFERS FROM THE AGE-MODIFIED FORM OF THIS PROTEIN()

Ca2+-ATPase from rat skeletal-muscle sarcoplasmic-reticulum has been r eported to be less stable in old animals. These changes were suggested to be due to age-related modifications in the membrane. In the presen t study, the guanidinium-chloride (GuHCl)-induced inactivation, and re activation by dilution of the denaturant, of this enzyme have been inv estigated. The cooperativity of the inactivation-transition was found to increase when the membrane was dissolved in the non-ionic detergent polyoxyethylene 10 laurylether. There is an inactive unfolding interm ediate in equilibrium with the native state at low concentrations of G uHCl. This intermediate can be reactivated, even after as long a time as 19 h. The stability of this intermediate is only slightly affected by detergent solubilization of the enzyme. Hence, the membrane probabl y only plays a minor role in stabilizing the intermediate.