| Abstract: | The interactions of 3H]estradiol, 3H]testosterone and 3H]progesterone with soluble proteins from porcine and calf liver were studied. The specific binding of 3H]progesterone and 3H]testosterone in calf liver cytosol seems to be due to serum transcortin or its intracellular precursor (analog). Contrariwise, the specific binding of 3H]progesterone observed in porcine liver cytosol was absent in the serum. This binding was characterized by slow association and dissociation dynamics, moderate affinity for the 3H]-ligand and a high binding capacity. The structural determinants of the ligands were studied by competitive inhibition of the 3H]-ligand binding. The delta 4-3-keto group in the steroid A-ring was found to be the most important determinant. An intensive metabolism of 3H]progesterone was observed during its incubation with cytosol (data from thin-layer chromatography). A 3H-metabolite (presumably, 20 beta-dihydroprogesterone) was predominant in the bound ligand fraction. The data obtained suggest that proteins of a steromodulin type are widely distributed in the mammalian liver. |
