Progesterone in Uterus and Plasma. I. Binding in Rat Uterus 105,000 g Supernatant

In the rat uterus 105,000 g supernatant, progesterone is bound by a specific (high affinity, low capacity) component and also by nonspecific (low affinity, high capacity) component (s). The specific component has many features identical with those of plasma corticosteroid binding globulin (sedimentation coefficient, electrophoretic mobility on paper and polyacrylamide gel, chromatographic behavior through Sephadex G-200, thermolability, steroid specificity, association constant at 4 C and antigenicity). The specific binding site concentration per mg of protein in uterus supernatant is 70 % that of plasma in estradiol-primed castrated rat s, whereas it is very low in kidney or diaphragm supernatant. The specific binding site concentration per uterus shows very marked variations with the endocrine status of the rat. It is very low in prepuberal rats, 12 times higher in recently castrated adult rats, 30 times higher in estradiolprimed castrated rats and 100 times higher in normal adult rats (random as far as the period of the cycle). The localization (extra- or intracellular), the origin (extra-uterine or local) and the role of this proteinare not understood, but quantitative data rule out simple contamination by blood or plasma.(Endocrinology87: 276, 1970)