Upon heat treatment of the pyruvate dehydrogenase complex from Bacillus stearothermophilus, the most thermostable component is a dihydrolipoamide dehydrogenase (E3c). To understand this stability, the thermal disintegration of E3 dissociated from the complex (E3d) was examined, comparing with that of E3c. Judging from residual activity and inactivation rate, E3d was less thermostable than E3c; E3d and E3c lost half of their original activities upon incubations for 30 min at 79°C and 90°C, respectively. Heat treatment of E3d raised the fluorescence intensities of Trp residue, intrinsic FAD, and extrinsic 8-anilinonaphthalene-1-sulfonate. E3d lost FAD, and inactive E3d polypeptides were aggregated. The sulfonate bound to the aggregate became notably fluorescent. The thermal disintegration of E3d was speculated to be a consecutive reaction that was different from the concurrent disintegration reaction of the complex. Some interactions with other component polypeptides was suggested to improve the thermostability of E3c.
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