Conformational studies on rat alpha-lactalbumin.

The reversible unfolding of rat alpha-lactalbumin, which, in contrast to other alpha-lactalbumins, has a 17-amino-acid extension at the carboxyl terminus and a carbohydrate unit at Asn-45, was studied by circular dichroism between 193 and 310 nm as a function of pH, heat and guanidine hydrochloride ( GdnHCl ). The native structure of rat alpha-lactalbumin was similar to that of bovine alpha-lactalbumin. Acidification changes rat alpha-lactalbumin to a state similar to the 'A state' of bovine alpha- lactalbumin . The heat-reduced unfolding of the tertiary structure of rat alpha-lactalbumin is highly cooperative. The GdnHCl -induced unfolding of rat alpha-lactalbumin could not be expressed by a two-state mechanism as in the case of bovine alpha-lactalbumin. However, the midpoints of transitions suggested lower stabilities of secondary and tertiary structures in rat alpha-lactalbumin than in bovine alpha-lactalbumin consistent with the differences in the local structure between rat and bovine alpha-lactalbumins.