Research in Molecular Mechanism of Prolactin Binding to its Receptor

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We have cleared two points in the mammary prolactin (PRL) receptor.(1) PRL accelerated the dissociation of PRL from its receptor in a dose- dependent manner. Scatchard analysis revealed that this reaction is not a simple bimolecular reaction. The Hill plot was non-linear. At PRL higher than 30 ng/ml, the Hill coeficient changed from 1.0 to 0.7. Using the PRL-preloaded receptor, the receptor remained active for association after PRL-induced dissociation. As PRL accelerated the PRL-binding reaction, it was speculated that PRL at high levels can actively generate a hormonal signal. Negative cooperativity is of much biological importance under conditions where levels of hormone are elevated exceedingly or abnormally.(2) PRL bound to the cell receptor was dissociated by the treatment with acid in a molarity-dependent manner rather than pH-dependent. The ratio of acid-sensitive to acid-insensitive receptor changed independently after parturiton. By acid treatment of subcellular membranes, about 80% of PRL bound with plasma and intracellular membrane receptors were acid-sensitive and acid-insensitive, respectively. We concluded that plasma membrane PRL receptor is intrinsically acid-sensitive and regulated independently.