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J. Biol. Chem., Vol. 276, Issue 46, 42793-42800, November 16, 2001

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Correlation between Uncoupled ATP Hydrolysis and Heat Production by the Sarcoplasmic Reticulum Ca²⁺-ATPase
COUPLING EFFECT OF FLUORIDE^*

Marcelo Reis, Mariana Farage, Angela Cristina L. de Souza, and Leopoldo de Meis

From the Departamento de Bioquímica Médica, Instituto de Ciências Biomédicas, Centro de Ciências da Saúde, Universidade Federal do Rio de Janeiro, Cidade Universitária, RJ, 21941 590, Brasil

The sarcoplasmic reticulum Ca²⁺-ATPase transports Ca²⁺ using the chemical energy derived from ATP hydrolysis. Part of the chemical energy is used to translocate Ca²⁺ through the membrane (work) and part is dissipated as heat. The amount of heat produced during catalysis increases after formation of the Ca²⁺ gradient across the vesicle membrane. In the absence of gradient (leaky vesicles) the amount of heat produced/mol of ATP cleaved is half of that measured in the presence of the gradient. After formation of the gradient, part of the ATPase activity is not coupled to Ca²⁺ transport. We now show that NaF can impair the uncoupled ATPase activity with discrete effect on the ATPase activity coupled to Ca²⁺ transport. For the control vesicles not treated with NaF, after formation of the gradient only 20% of the ATP cleaved is coupled to Ca²⁺ transport, and the caloric yield of the total ATPase activity (coupled plus uncoupled) is 22.8 kcal released/mol of ATP cleaved. In contrast, the vesicles treated with NaF consume only the ATP needed to maintain the gradient, and the caloric yield of ATP hydrolysis is 3.1 kcal/mol of ATP. The slow ATPase activity measured in vesicles treated with NaF has the same Ca²⁺ dependence as the control vesicles. This demonstrates unambiguously that the uncoupled activity is an actual pathway of the Ca²⁺-ATPase rather than a contaminating phosphatase. We conclude that when ATP hydrolysis occurs without coupled biological work most of the chemical energy is dissipated as heat. Thus, uncoupled ATPase activity appears to be the mechanistic feature underlying the ability of the Ca²⁺-ATPase to modulated heat production.

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