Pyruvate metabolism in rat liver mitochondria

Abstract

Mitochondria are generally regarded as the powerhouse of the cell having the main task of supplying energy in the form of ATP produced during oxidative phosphorylation from ADP and P i . Furthermore, it is well known that mitochondria take up Ca 2+ ions in an energy-dependent manner. Other important reactions are the Krebs cycle, oxidation of fatty acids and the urea cycle [1,2]. One may ask, which of these reactions are optimized and play the most important role? There are probably no dominant tasks in liver mitochondria and we are therefore faced with multiple optimizations, depending on the cellular demands.Mitochondria from liver and kidney contain pyruvate carboxylase and are actively involved in gluconeogenesis [3]. The aim of this study was to discover the importance of this first step in gluconeogenenesis, and special conditions were therefore chosen to allow us to study the carboxylation of pyruvate, the Krebs cycle and ketone body production by ignoring the many other reactions also present in these organelles. In this sense, the model and experiments of mitochondrial pyruvate metabolism are biased and do not consider the typical intracellular environment of hepatocytes.Genomics has led to construction of the stoichiometry matrices of several simple organisms such as Escherichia coli and Saccharomyces cerevisiae. Applying linear programming methods, researchers have analysed the optimizations of different goals, the most prominent being the maximization of cellular growth [4][5][6][7]. It goes without saying that this cannot be the major task of liver mitochondria. Therefore, we investigated the optimization of metabolic functions. In A representative model of mitochondrial pyruvate metabolism was broken down into its extremal independent currents and compared with experimental data obtained from liver mitochondria incubated with pyruvate as a substrate but in the absence of added adenosine diphosphate. Assuming no regulation of enzymatic activities, the free-flow prediction for the output of the model shows large discrepancies with the experimental data. To study the objective of the incubated mitochondria, we calculate the conversion cone of the model, which describes the possible input ⁄ output behaviour of the network. We demonstrate the consistency of the experimental data with the model because all measured data are within this cone. Because they are close to the boundary of the cone, we deduce that pyruvate is converted very efficiently (93%) to produce the measured extramitochondrial metabolites. We find that the main function of the incubated mitochondria is the production of malate and citrate, supporting the anaplerotic pathways in the cytosol, notably gluconeogenesis and fatty acid synthesis. Finally, we show that the major flow through the enzymatic steps of the mitochondrial pyruvate metabolism can be reliably predicted based on the stoichiometric model plus the measured extramitochondrial products. A major advantage of this method is that neither kinetic simulations nor radioactive trace...

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