Interaction of oxidized and reduced uteroglobin with progesterone.

Binding of added progesterone to native uteroglobin requires the reduction of the disulfide bonds that hold together the two polypeptide chains of the protein. The hypothesis that in the native oxidized state of uteroglobin the steroid binding cavity is preformed and occupied by a progesterone molecule has been tested by several experimental means. The results demonstrate that progesterone does not interact with oxidized uteroglobin, and show that the majority of the oxidized uteroglobin molecules purified from pseudopregnant rabbits do not contain a progesterone molecule. Oxidation of reduced uteroglobin in the presence of saturating amounts of progesterone does not result in significant retention of the steroid inside the oxidized protein.