DETECTION AND CHARACTERIZATION OF AN OVINE PLACENTAL-LACTOGEN STABLE INTERMEDIATE IN THE UREA-INDUCED UNFOLDING PROCESS

The urea-induced equilibrium unfolding of ovine placental lactogen, pu rified from ovine placenta, was followed by size-exclusion chromatogra phy, far-UV CD, and intrinsic tryptophan fluorescence. The data obtain ed by each of these methods showed a poor fit to a two-state model inv olving only a native and an unfolded form. A satisfactory fit required , instead, a model that involved a stable, partially folded form in ad dition to the native and unfolded ones. The results obtained from the best-fitting theoretical curves for the three-state model indicated th at this intermediate state, which is thr predominant species in soluti on at 3.6 M of urea activity, is compact, largely alpha-helical, and c hanges considerably the native-like tertiary packing around its trypto phan residues. These findings suggest that this stable intermediate ex hibits properties similar to those that characterize the molten globul e state.