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Role of Calcium in the Thermal Inactivation of Calcium Transport Proteins¹

Biophysics Laboratory, Physics Department, Texas Tech University, Lubbock, Texas 79409

Using purified sarcoplasmic reticulum membranes as model systems, the role of calcium ion in the thermal inactivation of membrane calcium transport was investigated. Hyperthermia induces calcium release from the heavy fraction of sarcoplasmic reticulum. This calcium channel related calcium release was inhibited by the presence of glycerol and enhanced by the presence of ethanol. Calcium was found to protect the thermal-induced calcium transport inactivation of CaATPase in the light fraction of sarcoplasmic reticulum. Both glycerol and cholesterol protect the thermal inactivation of CaATPase. Yet their effects on the calcium-induced protection kinetics were rather different, i.e., the glycerol inserts its protection effect by increasing the degree of cooperativity of calcium binding, while cholesterol increases the calcium-binding affinity. The calcium protection effect was attributed to the ability of calcium to enhance the thermal stability of the protein. This was demonstrated by an upshift (30–39°C) of the transition temperature of the rotational parameter of the native tryptophans of CaATPase in the presence of calcium.

¹ This work was supported by the National Cancer Institute of the NIH (CA47610), an Institutional Biomedical Research Support Grant (RR 05648-19), and the Robert A. Welch Research Foundation (D-1158).

Received 5/ 2/89. Revised 9/ 1/89. Accepted 9/14/89.