Comparative binding affinity study of progestins to the cytosol progestin receptor of endometrium in different mammals.

The relative binding affinity of 5 alpha-reduced progestins and a newly synthesized antiprogestin J912 (progesterone 100%) was determined in a competitive receptor binding assay using [3H]ORG-2058 as radiolabeled ligand for the progestin receptor. Uteri obtained from 12 different species of four mammalian orders were examined. The relative binding affinity of 75-100% and a blood prevalence of 5 alpha-pregnane-3,20-dione in horses and African elephants suggest a biological role of this particular 5 alpha-reduced progesterone. For pigs the binding affinity of 5 alpha-pregnane-3,20-dione was about 50% of progesterone, but blood levels are unknown. In all other cases the low binding affinity of investigated progestins precludes possible biological role. For 5 alpha-pregnane-3 alpha-ol-20-one, 5 alpha-pregnane-20 alpha-ol-3-one, and 5 alpha-pregnane-3 beta,20 alpha-diol the relative binding affinity was less than 1%. A rather low binding (< 15%) was observed in 5 alpha-pregnane-3,20-dione in all ruminant species investigated. The antiprogestin J912 was found to be highly efficient in displacing progesterone from its endometrial binding sites in carnivores and might therefore be used for pregnancy interruption during diapause in certain species, e.g., in captive bears.