Nitrogen metabolism in the isolated perfused rat liver. Nitrogen balance, redox state and rates of proteolysis

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Abstract
Nitrogen balances were measured in isolated perfused rat livers in the presence and absence of nitrogen donors. In all instances the balance apparently was incomplete. The expression [alanine][α-oxoglutarate]/[pyruvate][glutamate] remained fairly constant under the metabolic conditions studied, indicating that it may be at near-equilibrium. The source of the extra nitrogen seems to be derived from increased hepatic proteolysis. The addition of a nitrogen donor to the perfusate arrested proteolysis, as did the addition of pyruvate. The free mitochondrial [NAD+]/[NADH] ratio, calculated from the glutamate dehydrogenase and β-hydroxybutyrate dehydrogenase reactants, showed similar values and exhibited parallel changes under most metabolic situations studied. These results suggest that, under the reported experimental conditions, both dehydrogenases share a common mitochondrial NAD pool. Glutamate dehydrogenase plays an important role in hepatic nitrogen metabolism in vivo.