Uncoupled ATPase activity and heat production by the sarcoplasmic reticulum Ca2+-ATPase. Regulation by ADP.

By L. Meis. 2001

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Abstract Sarcoplasmic reticulum vesicles of rabbit skeletal muscle accumulate Ca2+ at the expense of ATP hydrolysis. The heat released during the hydrolysis of each ATP molecule varies depending on whether or not a Ca2+ gradient is formed across the vesicle membrane. After Ca2+accumulation, a part of the Ca2+-ATPase activity is not coupled with Ca2+ transport (Yu, X., and Inesi, G. (1995)J. Biol. Chem. 270, 4361–4367). I now show that both the heat produced during substrate hydrolysis and the uncoupled ATPase activity vary depending on the ADP/ATP ratio in the medium. With a low ratio, the Ca2+ transport is exothermic, and the formation of the gradient increases the amount of heat produced during the hydrolysis of each ATP molecule cleaved. With a high ADP/ATP ratio, the Ca2+ transport is endothermic, and formation of a gradient increased the amount of heat absorbed from the medium. Heat is absorbed from the medium when the Ca2+ efflux is coupled with the synthesis of ATP (5.7 kcal/mol of ATP). When there is no ATP synthesis, the Ca2+ efflux is exothermic (14–16 kcal/Ca2+mol). It is concluded that in the presence of a low ADP concentration the uncoupled ATPase activity is the dominant route of heat production. With a high ADP/ATP ratio, the uncoupled ATPase activity is abolished, and the Ca2+ transport is endothermic. The possible correlation of these findings with thermogenesis and anoxia is discussed.

Subfields: Molecular Biology

View Paper: https://www.semanticscholar.org/paper/a75fccb54a1bad551345111bb5349d2f54f74738

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