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, purified from ovine placenta, was followed by size‐exclusion chromatography, far‐UV CD, and intrinsic tryptophan fluorescence. The data obtained by each of these methods showed a poor fit to a two‐state model involving only a native and an unfolded form. A satisfactory fit required, instead, a model that involved a stable, partially folded form in addition to the native and unfolded ones. The results obtained from the best‐fitting theoretical curves for the three‐state model indicated that this intermediate state, which is the predominant species in solution at 3.6 M of urea activity, is compact, largely α‐helical, and changes considerably the native‐like tertiary packing around its tryptophan residues. These findings suggest that this stable intermediate exhibits properties similar to those that characterize the molten globule state.