Stimulation of alanine metabolism by ammonia in the perfused rat liver. Quantitative analysis by means of a mathematical model

Abstract

The effect of ammonia on the catabolism of alanine was studied in the perfused rat liver. Addition of 0.5 mM NH4Cl to the perfusion medium containing 5 mM alanine plus 0.1 mM octanoate produced drastic changes in the metabolite concentrations in the efflux medium. Not only the rate of ureogenesis was activated, but also the formation of glucose, lactate and pyruvate. Additionally, respiration was stimulated, the output of ketone bodies decreased, and the redox ratios lactate/pyruvate as well as 3-hydroxybutyrate/acetoacetate became more oxidized. To interpret the causes of these metabolic changes, a mathematical model was developed. It contains kinetic equations by which fluxes through essential pathways of alanine catabolism, gluconeogenesis and energy metabolism were related to the intracellular concentrations of pyruvate, oxaloacetate and ammonia, as well as to the redox ratios lactate/pyruvate and 3-hydroxybutyrate/acetoacetate. Using a nonlinear regression procedure, the model was suitable to be fitted to the data found in the experiments. The consistency of the model and experiment allowed the changes caused by ammonia to be explained. Primarily, ammonia stimulated ureogenesis hence accelerating the deamination of alanine which led to the increased formation of pyruvate, lactate and glucose. The enhanced energetic load resulting from ureogenesis and gluconeogenesis shifted the mitochondrial and cytosolic NAD systems towards more oxidized states which additionally modified the flux rates. The results demonstrate that there is a high degree of cooperativity between the metabolic pathways.