Effects of urea and guanidine hydrochloride on the activity and the dynamical structure of equine liver alcohol dehydrogenase

The inactivation of equine liver alcohol dehydrogenase by guanidine hydrochloride and urea has been studied by monitoring the intrinsic tryptophan fluorescence and phosphorescence emission. The use of triplet-state lifetimes to probe the flexibility of protein structure at the site of tryptophan-314 reveals a distinct behavior between the two denaturants. At predenaturational concentrations, the loss of enzyme activity in guanidine hydrochloride is associated with a loosening of intramolecular interactions resulting in a greater fluidity of the interior region of the macromolecule. In contrast, the interaction with urea, even at high concentrations, does not alter the dynamics of the native conformation. Enzyme activity is irreversibly lost as a result of a drastic unfolding of the macromolecule which occurs in a highly cooperative two-stage process.