Top-down control analysis of ATP turnover, glycolysis and oxidative phosphorylation in rat hepatocytes.

Abstract

Control analysis was used to analyse the internal control of rat hepatocyte metabolism. The reactions of the cell were grouped into nine metabolic blocks linked by five key intermediates. The blocks were glycogen breakdown, glucose release, glycolysis, lactate production, NADH oxidation, pyruvate oxidation, mitochondrial proton leak, mitochondrial phosphorylation and ATP consumption. The linking intermediates were intracellular glucose-6-phosphate, pyruvate and ATP levels, cytoplasmic NADH/NAD ratio and mitochondrial membrane potential. The steady-state fluxes through the blocks and the levels of the intermediates were measured in the absence and presence of specific effectors of hepatocyte metabolism. Application of the multiple modulation approach gave the kinetic responses of each block to each intermediate (the elasticities). These were then used to calculate all of the control coefficients, which describe the degree of control each block had over the level of each intermediate, and over the rate of each process. Within this full description of control, many different interactions could be identified. One key finding was that the processes that consumed ATP had only 35% of the control over the rate of ATP consumption. Instead, the reactions that produced ATP exerted the most control over ATP consumption rate; particularly important were mitochondrial phosphorylation (30% of control) and glycolysis (19%). The rate of glycolysis was positively controlled by the glycolytic enzymes themselves (66% of control) and by ATP consumption (47%). Mitochondrial production of ATP, including oxidative, proton leak and phosphorylation processes, had negative control over glycolysis (-26%; the Pasteur effect). In contrast, glycolysis had little control over the rate of ATP production by the mitochondria (-10%; the Crabtree effect). Control over the flux through the mitochondrial phosphorylation block was shared between pyruvate oxidation (23%), ATP consumption (28%) and the mitochondrial phosphorylation block itself (64%).