Activation of partially folded mitochondrial malate dehydrogenase by thioredoxin

CD spectroscopy reveals that mitochondrial malate dehydrogenase in 3M guanidinium chloride shows little residual secondary structure. Refolding of the denatured protein by dilution with buffer of pH 7.5 does not restore the CD spectrum of the native enzyme. A partially folded intermediate, possessing 25 % of the cc-helix content of the native enzyme, is formed upon dilution. The partially folded intermediate binds the extrinsic probe 1-anilinonaphtalene-8-sulfonate, and the increase in fluorescence (tenfold) is accompanied by a blue shift in the band position of the emission spectrum. Partially folded malate dehydrogenase is devoid of catalytic activity. In vitro, refolding of the denatured protein takes place in the presence of dithiotreitol and thioredoxin. In the presence of micromolar concentrations of thioredoxin, a recovery of approximately 70% of the catalytic activity was observed. Emission-anisotropy titrations of oxidized thioredoxin, tagged with a fluorescent probe, revealed that the oxidoreductase recognizes partially folded intermediates of malate dehydrogenase with a dissociation constant of 6 mu M. Moreover, a covalently linked complex formed by thioredoxin and monomeric malate dehydrogenase was detected by SDS/PAGE. A general mechanism is postulated for the reactivation of denatured proteins by thioredoxin.