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 exces s of tetranitromethane over protein content resulted in the modificati on of 0.8 tyrosine residue. No conformational changes were observed by either fourth-derivative spectral analysis or circular dichroism. Nit ration 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 tha t the decrease was not due to the difference in pK between tyrosine an d nitrotyrosine. The nitrotyrosine-containing peptide was isolated fro m a tryptic digest by HPLC and its modification extent was of 67%, whi ch is consistent with the decrease in binding capacities (65% and 70%) , Its amino acid sequence was determined and the modified tyrosine res idue was identified as Tyr-46. These results provide the first evidenc e of the involvement of a tyrosine residue in the binding of oPL to bo th lactogenic and somatogenic receptors. This tyrosine appears to be a shared binding epitope between oPL and the prolactins.