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      Differential requirements of calcium for oxoglutarate dehydrogenase and mitochondrial nitric-oxide synthase under hypoxia: impact on the regulation of mitochondrial oxygen consumption.

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          Abstract

          Studies with isolated mitochondria are performed at artificially high pO(2) (220 to 250 microM oxygen), although this condition is hyperoxic for these organelles. It was the aim of this study to evaluate the effect of hypoxia (20-30 microM) on the calcium-dependent activation of 2-oxoglutarate dehydrogenase (or 2-ketoglutarate dehydrogenase; OGDH) and mitochondrial nitric-oxide synthase (mtNOS). Mitochondria had a P/O value 15% higher in hypoxia than that in normoxia, indicating that oxidative phosphorylation and electron transfer were more efficiently coupled, whereas the intramitochondrial free calcium concentrations were higher (2-3-fold) at lower pO(2). These increases were abrogated by ruthenium red indicating that the higher uptake via the calcium uniporter was involved in this process. Mitochondria at high calcium concentration microdomains may produce nitric oxide, given the K(0.5) of calcium for OGDH (0.16 microM) and mtNOS (approximately 1 microM). Nitric oxide, by binding to cytochrome oxidase in competition with oxygen, decreases the rate of oxygen consumption. This condition is highly beneficial for the following reasons: i, these mitochondria are still able to produce ATP and support calcium clearance; ii, it prevents the accumulation of ROS by slowing the rate of oxygen consumption (hence ROS production); iii, the onset of anoxia is delayed, allowing oxygen to diffuse back to these sites, thereby ameliorating the oxygen gradient between regions of high and low calcium concentration. In this way, oxygen depletion at the latter sites is prevented. This, in turn, assures continued aerobic metabolism which may involve the activated dehydrogenases.

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          Author and article information

          Journal
          Comp. Biochem. Physiol., Part A Mol. Integr. Physiol.
          Comparative biochemistry and physiology. Part A, Molecular & integrative physiology
          Elsevier BV
          1095-6433
          1095-6433
          Oct 2005
          : 142
          : 2
          Affiliations
          [1 ] Department of Chemistry, University of Minnesota, Duluth, MN 55812, USA.
          Article
          S1095-6433(05)00108-X
          10.1016/j.cbpb.2005.05.004
          15972265
          2d94ffde-2493-484c-9293-11c4a92f3803
          History

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