Inactivation of the 2‐oxo acid dehydrogenase complexes upon generation of intrinsic radical species

Self‐regulation of the 2‐oxo acid dehydrogenase complexes during catalysis was studied. Radical species as side products of catalysis were detected by spin trapping, lucigenin fluorescence and ferricytochrome  c reduction. Studies of the complexes after converting the bound lipoate or FAD cofactors to nonfunctional derivatives indicated that radicals are generated via FAD. In the presence of oxygen, the 2‐oxo acid, CoA‐dependent production of the superoxide anion radical was detected. In the absence of oxygen, a protein‐bound radical concluded to be the thiyl radical of the complex‐bound dihydrolipoate was trapped by α‐phenyl‐ N ‐ tert ‐butylnitrone. Another, carbon‐centered, radical was trapped in anaerobic reaction of the complex with 2‐oxoglutarate and CoA by 5,5′‐dimethyl‐1‐pyrroline‐ N ‐oxide (DMPO). Generation of radical species was accompanied by the enzyme inactivation. A superoxide scavenger, superoxide dismutase, did not protect the enzyme. However, a thiyl radical scavenger, thioredoxin, prevented the inactivation. It was concluded that the thiyl radical of the complex‐bound dihydrolipoate induces the inactivation by 1e – oxidation of the 2‐oxo acid dehydrogenase catalytic intermediate. A product of this oxidation, the DMPO‐trapped radical fragment of the 2‐oxo acid substrate, inactivates the first component of the complex. The inactivation prevents transformation of the 2‐oxo acids in the absence of terminal substrate, NAD + . The self‐regulation is modulated by thioredoxin which alleviates the adverse effect of the dihydrolipoate intermediate, thus stimulating production of reactive oxygen species by the complexes. The data point to a dual pro‐oxidant action of the complex‐bound dihydrolipoate, propagated through the first and third component enzymes and controlled by thioredoxin and the (NAD +  + NADH) pool.