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J. Biol. Chem., Vol. 276, Issue 46, 42793-42800, November 16, 2001
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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.
Correlation between Uncoupled ATP Hydrolysis and Heat Production
by the Sarcoplasmic Reticulum Ca2+-ATPase
COUPLING EFFECT OF FLUORIDE*
*
This work was supported by grants from PRONEX:
Financiadora de Estudos e Projetos (FINEP), Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq), and
Fundação de Amparo à Pesquisa do Estado do Rio de
Janeiro (FAPERJ).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed. Tel.: 55-21-2270-1635;
Fax: 55-21-2270-8647; E-mail: demeis@biqmed.ufrj.br.
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