Activation of Pyruvate Dehydrogenase Complex and Inhibition of Krebs Cycle and Mitochondrial Respiration by Excess Pyruvate

Experiments on isolated rat liver mitochondria showed that pyruvate (10–30 mM) in the presence of L-glutamate causes concentration-dependent inhibition of ADP-activated respiration. Respiration is reactivated by 3 mM L-malate. Both effects are reproduced in the presence of D,L-carnitine (AcCar), indicating the important role of acetylCoA (AcCoA) in the regulation of the Krebs cycle reactions. During pyruvate oxidation, the respiration rate decreases within a few hundred seconds. The effect is also reproduced in the presence of dichloroacetate (DCA), an inhibitor of pyruvate dehydrogenase kinase (PDK), and is not observed with excess AcCar, indicating dephosphorylation of pyruvate dehydrogenase (PDH) upon inhibition of PDK by pyruvate (+ADHP) or DCA. The effects of pyruvate and AcCar depend on the duration of the preliminary incubation of mitochondria in state 2. Experiments on frozen/thwarts mitochondrils show that preincubation of mitochondria with pyruvate excess restores PDH activity and suppresses α-ketoglutarate dehydrogenase (α-KGDH) activity, as detected by NADH fluorescence. Thus, a possible mechanism of inhibition of respiration by pyruvate may be a combined mechanism that includes (1) allosteric inhibition of citrate synthase by excess AcCoA at low concentrations of oxaloacetate and α-KGDH with possible participation of acetoacetylCoA; (2) slow acetylation of α-KGDH and other enzymes of the cycle by excess AcCoA with slow reactivation of PDH by pyruvate.