ACTIVATION OF MITOCHONDRIAL 2-OXOACID DEHYDROGENASES BY THIOREDOXIN

The regulation of mitochondrial dehydrogenases of 2-oxoacids by thiore doxin is established. It is found that at low NAD(+) and saturating co ncentrations of 2-oxoacids and CoA, inactivation of 2-oxoacid dehydrog enase complexes takes place, preventing NAD(+) reduction under such co nditions, However, addition of oxidized E. coli thioredoxin to the rea ction medium without dithiothreitol allows effective NAD(+) reduction at this substrate ratio. Product accumulation curves show that thiored oxin activates the complexes by protecting them from the inactivation observed in the conditions when the complex-bound dihydrolipoate is ac cumulated. Disappearance of the activatory effect of thioredoxin after its treatment with SH-specific reagents indicates the involvement of the redox-active cysteine couple of thioredoxin in its activation of 2 -oxoacid dehydrogenase complexes, The redox-inactive thioredoxin not o nly shows no activation, but in fact exerts an inhibitory effect. The inhibition manifests the complex formation between SH-modified thiored oxin and dehydrogenase systems, involving amino acid residues of thior edoxin other than cysteine. High efficiency of thioredoxin from E. col i as compared to chloroplast thioredoxin f and glutathione disulfide i s revealed. This indicates the importance of specific protein structur e also for the influence of the redox-active thioredoxin upon the 2-ox oacid dehydrogenase complexes, The results obtained suggest that these key enzyme systems of mitochondrial metabolism represent previously u nidentified targets for the action of mitochondrial thioredoxin, which is known to resemble the E. coli counterpart studied in this work.