Stoichiometric traps in the tricarboxylic acid cycle. I. Self-inhibition and triggering phenomena.

The steady-state oxidation of 2 mM pyruvate in pigeon and rat heart mitochondria in the presence of ADP-glucose-hexokinase load can be strongly inhibited by excess (10-40 mM) of pyruvate or beta-hydroxybutyrate. This inhibition is accompanied by the accumulation of alpha-ketoglutarate and a decrease of malate. The mechanism of such substrate inhibition may be associated with the limitation of the tricarboxylic acid cycle flux by low levels of oxaloacetate and free CoA due to their being trapped as alpha-ketoglutarate and acetyl-CoA. Contrary to pyruvate, the ketone bodies in the absence of other substrates produce self-inhibition of their oxidation at as low concentrations as 0.5-1 mM. At 10-15 mM of acetoacetate, a complete suppression of respiration may develop. At a high load (preset by ADP or the uncoupler CCCP), the suppression is characterised by the accumulation of malate and a decrease of alpha-ketoglutarate. At low loads, the reverse distribution of the intermediates takes place. It is concluded that the system of ketone body oxidation in heart mitochondria is an example of biochemical triggers (systems with two alternative stable states).