Binding activity of 5alpha-reduced gestagens to the progestin receptor from African elephant (Loxodonta africana).

Recent findings in the African elephant (Loxodonta africana) indicate that the major progestins contained within and biosynthesized by corpora lutea are 5alpha-reduced metabolites and that progesterone is quantitatively of minor importance. The specific gestagenic action within the reproductive tract of elephants was determined by measurement of relative binding affinity of the respective progestins to the gestagen receptor extracted from elephant endometrium. The cytosol was incubated with 40 nmol/liter [3H]ORG-2058 and increasing concentrations of the tested progestin. Progesterone (P4), 5alpha-pregnane-3,20-dione (DHP), and 5alpha-pregnane-3alpha-ol-20-one (5alpha-P-3OH) were used. The competition for binding sites on the progestin receptor was shown by decreasing counts measured after extraction with scintillation fluid. The progestin concentration which induced a 50% reduction of measured counts was estimated (C50) and relative binding affinity of progesterone to other progestins was calculated (RBA = C50progestin/C50p4). The relative binding affinity of DHP to P4 at the gestagen receptor of elephant endometrium was equivalent. The other 5alpha-reduced progestin (5alpha-P-3OH) showed no competition to the [3H]ORG-2058 receptor binding. We conclude that the biological significance of P4 and DHP at the receptor level is very similar. The higher quantitative levels of DHP in corpus luteum and serum support the hypothesis that this progestin is the major gestagen in the elephants, whereas 5alpha-P-3OH is an inactive metabolite.