Dash Ranjan K, Beard Daniel A
Biotechnology and Bioengineering Center, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226-6509, USA.
J Physiol. 2008 Jul 1;586(13):3267-85. doi: 10.1113/jphysiol.2008.151977. Epub 2008 May 8.
Calcium is a key ion and is known to mediate signalling pathways between cytosol and mitochondria and modulate mitochondrial energy metabolism. To gain a quantitative, biophysical understanding of mitochondrial Ca(2+) regulation, we developed a thermodynamically balanced model of mitochondrial Ca(2+) handling and bioenergetics by integrating kinetic models of mitochondrial Ca(2+) uniporter (CU), Na(+)-Ca(2+) exchanger (NCE), and Na(+)-H(+) exchanger (NHE) into an existing computational model of mitochondrial oxidative phosphorylation. Kinetic flux expressions for the CU, NCE and NHE were developed and individually parameterized based on independent data sets on flux rates measured in purified mitochondria. While available data support a wide range of possible values for the overall activity of the CU in cardiac and liver mitochondria, even at the highest estimated values, the Ca(2+) current through the CU does not have a significant effect on mitochondrial membrane potential. This integrated model was then used to analyse additional data on the dynamics and steady-states of mitochondrial Ca(2+) governed by mitochondrial CU and NCE. Our analysis of the data on the time course of matrix free [Ca(2+)] in respiring mitochondria purified from rabbit heart with addition of different levels of Na(+) to the external buffer medium (with the CU blocked) with two separate models--one with a 2:1 stoichiometry and the other with a 3:1 stoichiometry for the NCE--supports the hypothesis that the NCE is electrogenic with a stoichiometry of 3:1. This hypothesis was further tested by simulating an additional independent data set on the steady-state variations of matrix free [Ca(2+)] with respect to the variations in external free [Ca(2+)] in purified respiring mitochondria from rat heart to show that only the 3:1 stoichiometry model predictions are consistent with the data. Based on these analyses, it is concluded that the mitochondrial NCE is electrogenic with a stoichiometry of 3:1.
钙是一种关键离子，已知其可介导胞质溶胶与线粒体之间的信号通路，并调节线粒体能量代谢。为了对线粒体Ca(2+)调节进行定量的生物物理理解，我们通过将线粒体Ca(2+)单向转运体（CU）、Na(+)-Ca(2+)交换体（NCE）和Na(+)-H(+)交换体（NHE）的动力学模型整合到现有的线粒体氧化磷酸化计算模型中，开发了一种热力学平衡的线粒体Ca(2+)处理和生物能量学模型。基于在纯化线粒体中测量的通量率的独立数据集，开发了CU、NCE和NHE的动力学通量表达式并进行了单独参数化。虽然现有数据支持心脏和肝脏线粒体中CU总体活性的广泛可能值，但即使在最高估计值时，通过CU的Ca(2+)电流对线粒体膜电位也没有显著影响。然后使用这个整合模型来分析由线粒体CU和NCE控制的线粒体Ca(2+)动力学和稳态的其他数据。我们用两个单独的模型分析了从兔心脏纯化的呼吸线粒体中基质游离[Ca(2+)]随时间变化的数据，在外部缓冲介质中添加不同水平的Na(+)（CU被阻断），一个模型中NCE的化学计量比为2:1，另一个模型中为3:1，这支持了NCE是化学计量比为3:1的电生离子的假设。通过模拟关于大鼠心脏纯化呼吸线粒体中基质游离[Ca(2+)]稳态变化相对于外部游离[Ca(2+)]变化的另一个独立数据集，进一步检验了这个假设，结果表明只有化学计量比为3:1的模型预测与数据一致。基于这些分析，得出结论：线粒体NCE是化学计量比为3:1的电生离子。
