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J Biol Chem, Vol. 275, Issue 17, 12521-12529, April 28, 2000
From the Departamento de Bioquimica y Biologia Molecular A,
Edificio de Veterinaria, Universidad de Murcia en Espinardo,
30071 Murcia, Spain
Ca2+ transport and UTP
hydrolysis catalyzed by sarcoplasmic reticulum Ca2+-ATPase
from skeletal muscle was studied. A passive Ca2+ load
inside microsomal vesicles clearly decreased the net uptake rate and
the final accumulation of Ca2+ but not the UTP hydrolysis
rate, causing energy uncoupling. In the absence of passive leak, the
Ca2+/Pi coupling ratio was 0.7-0.8. UTP
hydrolysis did not maintain a rapid component of Ca2+
exchange between the cytoplasmic and lumenal compartments as occurs
with ATP. The uncoupling process in the presence of UTP is associated
with: (i) the absence of a steady state accumulation of ADP-insensitive
phosphoenzyme; (ii) the cytoplasmic dissociation of Ca2+
bound to the ADP-sensitive phosphoenzyme; and (iii) the absence of
enzyme inhibition by cyclopiazonic acid. All these characteristics confirm the lack of enzyme conformations with low Ca2+
affinity and point to the existence of an uncoupling mechanism mediated
by a phosphorylated form of the enzyme. Suboptimal coupling values can
be explained in molecular terms by the proposed functional model.
Insight into the Uncoupling Mechanism of Sarcoplasmic Reticulum
ATPase Using the Phosphorylating Substrate UTP*
*
This work was supported by Spanish Ministerio de Educacion y
Cultura Grant PB97-1039.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: Departamento de
Bioquimica y Biologia Molecular A, Edificio de Veterinaria, Universidad de Murcia, Campus de Espinardo, 30071 Murcia, Spain. Fax:
34-968-364-147; E-mail: fbelda@fcu.um.es.
Copyright © 2000 by The American Society for Biochemistry and Molecular Biology, Inc.
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