The relatioship between the reductive amination and carboxylation of 2-oxoglutarate in rat liver mitochondria

Abstract

• Two NADPH-linked reductive reactions of 2-oxoglutarate, the amination to glutamate and the carboxylation to isocitrate were studied in isolated rat liver mitochondria. The amination was measured as the production of glutamate plus aspartate and the carboxylation as the bicarbonate-dependent production of citrate plus isocitrate. • The rate of the amination was higher and more dependent on 2-oxoglutarate concentration in State 3 than in State 4. The reaction was correlated with the concentration of NH 4 + and the intramitochondrial content of ADP. Incontrast with the amination the carboxylation was higher in State 4 than in State 3, indicating that the reaction was primarily dependent on the degree of NADP reductino. The amination, but not the carboxylation, could attain a rate high enough to increases the levle of NADP + in mitochondria. • Octanoate stimulated both reactions in State 3, and only the amination in State 4. The stimulation was correlated with an increase of NADP reduction. • During the intense synthesis of glutamate at 10 mM NH 4 +, the formation of isocitrate from 2-oxoglutarate was strongly suppressed. However, at 1 mM NH 4 +, the concentration approaching that in the liver cell, 2-oxoglutarate was reduced more quickly to isocitrate than to glutamate in State 4 in the absence of octanoate. In the presence of octanoate the amination was stimulated and its rate exceeded that of the carboxylation only at low concentration of 2-oxoglutarate. Transition to State 3 shifted that balance between these reactions in favour of glutamate synthesis. • The physiological implications of both processes are discussed.