Cold denaturation of alpha-lactalbumin.

We have investigated the thermal unfolding of bovine alpha-lactalbumin by means of circular dichroism spectroscopy in the far- and near-ultraviolet regions, and shown that the native alpha-lactalbumin undergoes heat and cold denaturation. The guanidine hydrochloride-induced unfolding of alpha-lactalbumin was also investigated by circular dichroism spectroscopy at various temperatures from 261 to 318 K. It is shown that the population of the molten globule state is strongly dependent on temperature and that the molten globule state does not accumulate during the guanidine hydrochloride-induced unfolding transition at 261 K. Our results indicate that the molten globule state of alpha-lactalbumin undergoes cold denaturation as the native alpha-lactalbumin does, and that the heat capacity change of unfolding from the molten globule to the unfolded state is positive and significant. The present results further support the idea that the molten globule and the unfolded states do not belong to the same thermodynamic state, and that the native, molten globule and unfolded states are sufficient for interpreting the guanidine hydrochloride-induced unfolding behavior of alpha-lactalbumin.