Thermally induced disintegration of the Bacillus stearothermophilus dihydrolipoamide dehydrogenase

Upon heat treatment of the pyruvate dehydrogenase complex from Bacillus ste arothermophilus, the most thermostable component is a dihydrolipoamide dehy drogenase (E3c). To understand this stability, the thermal disintegration o f E3 dissociated from the complex (E3d) was examined, comparing with that o f E3c. Judging from residual activity and inactivation rate, E3d was less t hermostable than E3c; E3d and E3c lost half of their original activities up on incubations for 30 min at 79 degrees C and 90 degrees C, respectively. H eat treatment of E3d raised the fluorescence intensities of Trp residue, in trinsic FAD, and extrinsic 8-anilinonaphthalene-1-sulfonate. E3d lost FAD, and inactive E3d polypeptides were aggregated. The sulfonate bound to the a ggregate became notably fluorescent. The thermal disintegration of E3d was speculated to be a consecutive reaction that was different from the concurr ent disintegration reaction of the complex. Some interactions with other co mponent polypeptides was suggested to improve the thermostability of E3c.