Quantitative Relationship Between the Protein Secondary Structure in Cardiac Sarcolemma and the Activity of the Membrane-bound Ca 2+ -ATPase

Abstract. In the absence of ATP, increasing concentrations of calcium within a range between 0.1-8.0 mmol. I" 1 gradually lowered the a-helix content of proteins in rat heart sarcolemma requiring no energy supply. In the presence of ATP, similar concentrations of calcium stepwise activated the sarcolemmal low-affinity Ca 2+ -ATPase. A mathematical analysis of the data obtained revealed a quantitative relationship between calcium-induced stimulation of the Ca 2+ -ATPase activity and a diminution of the a-helix contents of membrane proteins in cardiac sarcolemma. The cooperation between changes in protein conformation and energy consumption in relation to the supposed role of low-affinity Ca 2+ -ATPase in gating the calcium channel are discussed