BINDING-ACTIVITY OF 5-ALPHA-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 corpo ra lutea are 5 alpha-reduced metabolites and that progesterone is quan titatively of minor importance. The specific gestagenic action within the reproductive tract of elephants was determined by measurement of r elative binding affinity of the respective progestins to the gestagen receptor extracted from elephant endometrium. The cytosol was incubate d with 40 nmol/liter [H-3]ORG-2058 and increasing concentrations of th e tested progestin. Progesterone (P4), 5 alpha-pregnane-3,20-dione (DH P), and 5 alpha-pregnane-3 alpha-ol-20-one (5 alpha-P-30H) were used. The competition for binding sites on the progestin receptor was shown by decreasing counts measured after extraction with scintillation flui d. The progestin concentration which induced a 50% reduction of measur ed counts was estimated (C50) and relative binding affinity of progest erone to other progestins was calculated (RBA = C50(progestin)/C50(p4) ). The relative binding affinity of DHP to P-4 at the gestagen recepto r of elephant endometrium was equivalent. The other 5 alpha-reduced pr ogestin (5 alpha-P-30H) showed no competition to the [H-3]ORG-2058 rec eptor binding. We conclude that the biological significance of P-4 and DHP at the receptor level is very similar. The higher quantitative le vels of DHP in corpus luteum and serum support the hypothesis that thi s progestin is the major gestagen in the elephants, whereas 5 alpha-P- 30H is an inactive metabolite. (C) 1997 Academic Press.