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

Experiments on isolated rat liver mitochondria have shown that pyruvate (10–30 mM) in the presence of L-glutamate causes concentration-dependent inhibition of respiration activated by ADP. Respiration is reactivated by 3 mM of L-malate. Both effects are reproduced in the presence of D, L-acetylcarnitine (AcCar), which indicates the important role of acetylCoA (AcCoA) in the regulation of Krebs cycle reactions. When pyruvate is oxidized, the respiration rate decreases within a few hundred seconds. The effect is 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) when PDK is inhibited by pyruvate (+ADP) or DCA. The effects of pyruvate and AcCar depend on the preincubation duration of mitochondria in state 2. Experiments on frozen/thawed mitochondria show that preincubation of mitochondria with pyruvate restores PDH activity and suppresses the activity of α-ketoglutarate dehydrogenase (α-KGDH) detected by NADH fluorescence. Thus, a possible mechanism of the respiration inhibition by pyruvate is a mechanism combining (1) allosteric inhibition of citrate synthase by the excess of AcCoA at low concentrations of oxaloacetate and α-KGDH with the possible participation of acetoacetylCoA and (2) slow acetylation of α-KGDH and other cycle enzymes by the excess of AcCoA during slow reactivation of PDH by pyruvate.