TOP-DOWN ELASTICITY ANALYSIS AND ITS APPLICATION TO ENERGY-METABOLISMIN ISOLATED-MITOCHONDRIA AND INTACT-CELLS

This paper reviews top-down elasticity analysis, which is a subset of metabolic control analysis. Top-down elasticity analysis provides a sy stematic yet simple experimental method to identify all the primary si tes of action of an effector in complex systems and to distinguish the m from all the secondary, indirect, sites of action. In the top-down a pproach, the complex system (for example, a mitochondrion, cell, organ or organism) is first conceptually divided into a small number of blo cks of reactions interconnected by one or more metabolic intermediates . By changing the concentration of one intermediate when all others ar e held constant and measuring the fluxes through each block of reactio ns, the overall kinetic response of each block to each intermediate ca n be established. The concentrations of intermediates can be changed b y adding new branches to the system or by manipulating the activities of blocks of reactions whose kinetics are not under investigation. To determine how much an effector alters the overall kinetics of a block of reactions, the overall kinetic response of the block to the interme diate is remeasured in the presence of the effector. Blocks that conta in significant primary sites of action will display altered kinetics; blocks that change rate only because of secondary alterations in the c oncentrations of other metabolites will not. If desired, this elastici ty analysis can be repeated with the primary target blocks subdivided into simpler blocks so that the primary sites of action can be defined with more and more precision until, with sufficient subdivision, they are mapped onto individual kinetic steps. Top-down elasticity analysi s has been used to identify the targets of effecters of oxygen consump tion in mitochondria, hepatocytes and thymocytes. Effecters include po isons such as cadmium and hormones such as triiodothyronine. However, the method is more general than this; in principle it can be applied t o any metabolic or other steady-state system.