285. Metabolism of progesterone in small intestine of normal and pregnant rat

1.1. Equilibrium paper-strip electrophoresis was used to characterize the corticosteroid-binding proteins of several mammalian sera in the presence of 14C-labeled corticosteroid hormones. The highest specific activity of [4-14C]corticosterone and [4-14C]cortisol in the serum of rat and rabbit, respectively, was associated with the α1-globulin fraction. The α-globulin fraction of bovine and horse serum also showed the highest binding activity for these corticosteroid hormones. It was concluded that the high-affinity corticosteroid-binding proteins in the sera are α-globulins.2.2. Continuous-flow paper electrophoresis was applied in order to determine whether one or more than one electrophoretic serum protein component is responsible for the high-affinity binding of different corticosteroids. It was found that the same electrophoretic fraction of human serum had the highest binding affinity for corticosterone, cortisol and aldosterone. Cortisol displaced aldosterone from its binding sites. In rat serum, one electrophoretic component was responsible for high-affinity binding of corticosterone and cortisol. Similar results were obtained for rabbit serum.3.3. Corticosteroid-binding globulin fractions of rat serum which were free of albumin, bound corticosterone much more firmly than cortisol. The same relative binding was observed in human serum although the difference in binding of the two corticosteroids was small.4.4. At low pH values, corticosteroid-binding globulin of rat serum migrated in the electrophoretic field as a pre-albumin.