Control of oxidative phosphorylation, gluconeogenesis, ureagenesis and ATP turnover in isolated perfused rat liver analyzed by top-down metabolic control analysis

We have analyzed the control exerted by the pathways of oxidative phosphorylation, gluconeogenesis, ureagenesis, and maintenance ATP consumption over each other's rates in isolated, perfused rat liver using top-down metabolic control analysis. The livers from fasted rats were perfused with 3-hydroxybutyrate as respiratory substrate, lactate as substrate for gluconeogenesis, and ammonium as substrate for urea synthesis, in conditions where these pathways were only linked by their common intermediates: ATP, ADP, and Pi. The rates of oxygen consumption, glucose and urea synthesis were measured continuously. The pathways were perturbed either by adding specific inhibitors or by adding new pathways that consumed ATP, and the relative changes in pathway rates were used to calculate the flux control coefficients of each pathway over all pathway rates. When the liver was in a relatively inactive metabolic state, where ATP was only being used by the maintenance ATP-consuming pathways, then essentially all the control over ATP production and consumption was located in the maintenance ATP consumers with ATP production having no control. Whereas, when the liver was in a highly active state using extra ATP for both glucose and urea synthesis, then ATP production (from oxidative phosphorylation) had strong control over its own rate and the rates of glucose and urea synthesis, but gluconeogenesis and ureagenesis still had strong control over their own rates and negative control over each others rates, i.e. they competed for the limited ATP supply. The rate of the maintenance ATP consumers is remarkably insensitive to changes in ATP production and consumption, but exerts considerable control over all other pathways. These results indicate that the general assumption that the rates of ATP production and consumption are controlled exclusively by ATP consumers is false under conditions where a significant amount of ATP is used for biosynthetic processes, such as glucose and urea synthesis, and indicate that the latter processes may be partly controlled by regulators of ATP production and by other ATP-consuming pathways.