Calcium ions and the regulation of NAD+-linked isocitrate dehydrogenase from the mitochondria of rat heart and other tissues

Abstract

The effects of Ca2+ on the activity of isocitrate dehydrogenase (NAD+) in extracts of rat heart mitochondria were explored in the presence of MgCl2 by using EGTA buffers. In the absence of ADP, Ca2+ (about 30 micrometer) resulted in a slight increase in apparent Km for threo-Ds-isocitrate; in the presence of ADP, Ca2+ (about 25 micrometer) greatly lowered the apparent Km for threo-Ds-isocitrate from 227 micrometer to 53 micrometer without changing the maximum velocity. At 100 micrometer-threo-Ds-isocitrate and 1 mM-ADP, there was an 8-fold activation by Ca2+, with a Km for Ca2+ of 1.2 micrometer. This activation was also observed with Sr2+ (Km 3.1 micrometer), but not with Mn2+ (at concentrations below 2.5 micrometer). Similar effects of Ca2+ were also observed on isocitrate dehydrogenase (NAD+) activity in extracts of mitochondria from liver, kidney, brown adipose tissue and white adipose tissue of the rat. The possible regulatory role of changes in the intramitochondrial concentration of Ca2+ is discussed.

Excitation-contraction coupling and mitochondrial energetics

“…The classical respiratory controlhypothesis of Chance and Williams [37] implies that the rate of respiration is regulated by the availability of ADP to the F 1 /F 0 -ATPase. This is based on the observation that when adding ADP to a suspension of isolated mitochondria, O 2 consumption increases ("state 3 respiration"), and when ADP and/or P i in the assay are consumed, the rate of respiration decreases again ("state 4 respiration" [53,54,56,57,127,128] (Fig. 3A and B), it was proposed that Ca 2+ rather than ADP may regulate respiration in vivo [106].…”

“…This was associated with reduced pyruvate dehydrogenase (PDH) activity, whose K m for Ca 2+ activation is in the range of ∼650 nM to 1 µM ( Fig. 3B; [54,56,57,59,127,128]). Thus, both the reduced amplitude of cytosolic Ca 2+ transients [59] and elevated [Na + ] i [117] may hamper Ca 2+ -activation of TCA cycle enzymes in heart failure.…”

“…The K 0.5 for Ca 2+ activation of the TCA cycle dehydrogenases is in the range of 0.7-1 µM ( Fig. 3B; [54,56,57,127,128]). As reviewed previously [76,77,79,129], Ca 2+ is taken up by mitochondria via a Ca 2+ uniporter (mCU; Fig.…”

On the role of the calcium transport cycle in heart and other mammalian mitochondria

“…It should also be noted that PDH and OGDH are inhibited by their end-products, acetylCoA and succinylCoA, respectively [26,27]. For further details see [9,[11][12][13]16,19,20, 3. remonstration of the ~a*~-sensitivity of the dehydrogenases when located within intact uncoupled mitochondria…”

Evidence that adrenaline activates key oxidative enzymes in rat liver by increasing intramitochondrial [Ca2+]

“…They are pyruvate dehydrogenase (PDH), phosphate phosphatase [l] (whose activation increases amounts of active PDH (PDH. )), NAD+-isocitrate dehydrogenase [2] and oxoglutarate dehydrogenase (OGDH) [3]. Studies on rat heart [4-61, skeletal muscle [7,8] and adipose tissue [9] and pig lymphocytes [lo] have demonstrated that these enzymes can be activated within mitochondria (incubated with physiological concentrations of Mg2+ and Na+) by increases in extramitochondrial [Ca"] within the expected physiological range [l 11.…”