Harper M E, Brand M D
Department of Biochemistry, University of Ottawa, Ontario, Canada.
Proc Soc Exp Biol Med. 1995 Mar;208(3):228-37. doi: 10.3181/00379727-208-43852b.
Top-down elasticity analysis is a novel extension of top-down metabolic control analysis. It has provided researchers with a theoretical and practical platform upon which quantitative analyses of the sites of action of hormones and drugs can be based. This approach is easy to apply and involves dividing up the metabolic system in question into two or three blocks of enzyme reactions around an intermediate between the blocks of reactions. The kinetic response of each block to the intermediate is measured in the steady state in situ by determining the flux through the block at different measured intermediate concentrations. The intermediate can be manipulated by titrating the other blocks with suitable inhibitors or activators. Then, to determine which blocks of reactions are quantitatively the most important in terms of any change in the flux rate of the system, a quantitative comparison of the titration curves from the experimental preparations is made with those of the control preparations. In this minireview we will examine, as an example, the use of top-down elasticity analysis for the quantitative identification of the important sites of action of thyroid hormones on oxidative phosphorylation in hepatocytes. The experimental results show that approximately 50% of the change in resting oxygen consumption in hepatocytes from hypothyroid and hyperthyroid rats (compared with euthyroid controls) is attributable to changes in the rate of the mitochondrial proton leak; the remaining 50% is accounted for by changes in nonmitochondrial- and ATP turnover-dependent oxygen consumption in hypothyroid and hyperthyroid hepatocytes, respectively.
自上而下的弹性分析是自上而下的代谢控制分析的一种新扩展。它为研究人员提供了一个理论和实践平台，在此基础上可以对激素和药物的作用位点进行定量分析。这种方法易于应用，包括将所研究的代谢系统围绕反应模块之间的中间体划分为两个或三个酶反应模块。通过在不同测量的中间体浓度下测定通过该模块的通量，在稳态原位测量每个模块对中间体的动力学响应。可以通过用合适的抑制剂或激活剂滴定其他模块来操纵中间体。然后，为了确定就系统通量率的任何变化而言哪些反应模块在数量上最重要，将实验制剂的滴定曲线与对照制剂的滴定曲线进行定量比较。在本综述中，我们将以甲状腺激素对肝细胞氧化磷酸化的重要作用位点的定量鉴定为例，研究自上而下的弹性分析的应用。实验结果表明，甲状腺功能减退和甲状腺功能亢进大鼠肝细胞静息氧消耗的变化（与甲状腺功能正常的对照相比）中，约50% 归因于线粒体质子泄漏率的变化；其余50% 分别由甲状腺功能减退和甲状腺功能亢进肝细胞中与非线粒体和ATP周转相关的氧消耗变化所解释。
