We have studied the guanidine hydrochloride-induced equilibrium unfolding and the kinetics of refolding of canine milk lysozyme by circular dichroism and fluorescence spectroscopy. The thermodynamic analysis of the equilibrium unfolding measured by circular dichroism and fluorescence has shown that unfolding is represented by a three-state mechanism and that the intermediate state of canine milk lysozyme is remarkably more stable than the intermediates observed in other lysozyme and α-lactalbumin. In the kinetic refolding of this protein, there are at least two kinetic intermediates; a burst-phase intermediate accumulated within the dead time (4 ms) of the measurement and an intermediate that has been observed during the kinetics with a rate constant of 10-20 s-1 after the burst phase. This result is apparently in contrast with those previously observed in the kinetic refolding of α-lactalbumin and equine lysozyme that show only the burst-phase intermediate. The relationship between the extraordinarily stable equilibrium molten globule and the kinetic folding intermediates will be discussed.
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