Abstract
A spectrophotometric method is proposed for studying the interaction of progesterone with purified rabbit uteroglobin. The method exploits the depression in the absorbance of the α,β-unsaturated keto-group of the steroid which takes place upon binding to uteroglobin. The magnitude of this depression at around 260 nm is correlated to the binding affinity and can be used to calculate the apparent equilibrium association constant and the number of steroid binding sites per protein molecule. The results obtained demonstrate that reduction of the disulphide bonds of native uteroglobin is a prerequisite for progesterone binding, and that the sulphydryl groups are not directly implicated in the interaction with the steroid. The only tyrosine residue of the uteroglobin subunit changes its absorption spectrum as a consequence of steroid binding, and the binding reaction is accompanied by a positive entropy change. These findings suggest an association of progesterone binding with conformational changes of the polypeptide structure of uteroglobin.
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