The effect of calcium overload on mitochondrial NAD(P)-dependent dehydrogenases

Abstract

The response of Krebs cycle dehydrogenases to Ca2+ in physiological concentration range is well established, but little is known about Ca2+ overload induced changes in the activity of essential mitochondrial dehydrogenases. We examined whether very fast decrease in the amount of mitochondrial NAD(P)H under condition of Ca2+ overload may be caused by inhibition of Ca2+ sensitive NAD(P)-dependent dehydrogenases. Ca2+ overload (increase in extramitochondrial Ca2+ concentration from 1 lM to 10 and 30 lM) did not change the activity of mitochondrial pyruvate and 2-oxoglutarate dehydrogenases. Ca2+ inhibited NADP-dependent isocitrate dehydrogenase (ICDH) by 12% at 10 lM and 20% at 30 lM Ca2+ at subsaturating concentration of substrate. NAD-dependent ICDH was activated (21%) by increase of Ca2+ concentration from 1 lM to 10 lM. At saturating substrate concentration Ca2+ had not effect on the activity of NADP-dependent ICDH but NAD-dependent ICDH was activated: activity was higher by 43% at 10 lM Ca2+ in comparison to that at 5 nM; further increase in Ca2+ concentration did not affect the activity. The activity of NADP-ICDH was 100–350- fold higher than NAD-ICDH and increase in Ca2+ concentration substantially decreased the ratio of NADP-ICDH vs. NADICDH activity. The fact that supra-physiological Ca2+ levels moderately inhibit only NADP-ICDH cannot provide explanation for Ca2+ overload induced NAD(P)H depletion and inhibition of respiration in heart mitochondria.