Selective reduction of the disulfide bonds of ovine placental lactogen

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 alpha-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.

Identification of a tyrosine residue in ovine placental lactogen as essential for its binding to receptors

Nitration of ovine placental lactogen (oPL) with a 10-fold molar excess of tetranitromethane over protein content resulted in the modification of 0.8 tyrosine residue. No conformational changes were observed by either fourth-derivative spectral analysis or circular dichroism. Nitration significantly decreased the binding capacity of the hormone to lactogenic and somatogenic rat liver receptors. This binding capacity was not restored by reduction of the nitro groups, thus indicating that the decrease was not due to the difference in pK between tyrosine and nitrotyrosine. The nitrotyrosine-containing peptide was isolated from a tryptic digest by HPLC and its modification extent was of 67%, which is consistent with the decrease in binding capacities (65% and 70%). Its amino acid sequence was determined and the modified tyrosine residue was identified as Tyr-46. These results provide the first evidence of the involvement of a tyrosine residue in the binding of oPL to both lactogenic and somatogenic receptors. This tyrosine appears to be a shared binding epitope between oPL and the prolactins.

Selective modification of tryptophan-150 in ovine placental lactogen

1. Ovine placental lactogen was modified by reaction with o-nitrophenylsulfenyl chloride. Fluorescence measurements indicated that one of the two tryptophan residues of the molecule had reacted. Besides, there was some reagent not covalently bound. 2. The reagent was covalently bound to Trp-150. No evidence of modification of Trp-90 was found. 3. Binding capacity to lactogenic as well as somatogenic receptors was diminished but not abolished upon modification, indicating that absolute molecular integrity of Trp-150 is not required for binding. 4. This behavior is similar to that of the tryptophan residues of ovine prolactin.