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J. Biol. Chem., Vol. 264, Issue 23, 13430-13439, 08, 1989
B Wan, KF LaNoue, JY Cheung and RC Scaduto Jr
The relationship of extramitochondrial Ca2+ to intramitochondrial Ca2+ and
the influence of intramitochondrial free Ca2+ concentrations on various
steps of the citric acid cycle were evaluated. Ca2+ was measured using the
Ca2+ sensitive fluorescent dye fura-2 trapped inside the rat heart
mitochondria. The rate of utilization of specific substrates and the rate
of accumulation of citric acid cycle intermediates were measured at matrix
free Ca2+ ranging from 0 to 1.2 microM. A change in matrix free Ca2+ from 0
to 0.3 microM caused a 135% increase in ADP stimulated oxidation of 0.6 mM
alpha-ketoglutarate (K0.5 = 0.15 microM). In the absence of ADP and the
presence of 0.6 mM alpha-ketoglutarate, Ca2+ (0.3 microM) increased NAD(H)
reduction from 0 to 40%. On the other hand, when pyruvate (10 microM to 5
mM) was substrate, pyruvate dehydrogenase flux was insensitive to Ca2+ and
isocitrate dehydrogenase was sensitive to Ca2+ only in the presence of
added ADP. In separate experiments pyruvate dehydrogenase activation
(dephosphorylation) was measured. Under the conditions of the present
study, pyruvate dehydrogenase was found to be almost 100% activated at all
levels of Ca2+, thus explaining the Ca2+ insensitivity of the flux
measurements. However, if the mitochondria were incubated in the absence of
pyruvate, with excess alpha-ketoglutarate and excess ATP, the pyruvate
dehydrogenase complex was only 20% active in the absence of added Ca2+ and
activity increased to 100% at 2 microM Ca2+. Activation by Ca2+ required
more Ca2+ (K0.5 = 1 microM) than for alpha- ketoglutarate dehydrogenase.
The data suggest that in heart mitochondria alpha-ketoglutarate
dehydrogenase may be a more physiologically relevant target of Ca2+ action
than pyruvate dehydrogenase.
Regulation of citric acid cycle by calcium
Department of Physiology, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey 17033.
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