The ovarian progestogen receptor in the spotted seatrout, Cynoscion nebulosus, demonstrates steroid specificity different from progesterone receptors in other vertebrates.

A nuclear progestogen receptor has previously been characterized in the ovary of the spotted seatrout. The steroid specificity of this receptor was further defined in the present study by determining the binding affinity of a wide variety of progestin and corticosteroid agonists and antagonists. The addition of a hydroxyl or keto group to the 11 position resulted in a 10-100-fold decrease in relative binding affinity (RBA). The significance of the 17, 20, and 21 positions in determining the RBA of closely related steroids was investigated in detail. Modification of the 17alpha-hydroxyl to an acetyl or carbyne group resulted in a 10-fold decrease in RBA. The substitution of a ketone group with a hydroxyl group at the 20 position increased binding, whereas the addition of a 21-hydroxyl group consistently decreased RBA by 40-60%. The effect of the 17alpha-hydroxyl group on RBA was dependent on what functional group was present at the 20 position. The addition of a 17alpha-hydroxyl decreased affinity by one- to 10-fold if a ketone group was present at position 20. However, the RBA increased five- to 10-fold upon addition of the 17alpha-hydroxyl group if a hydroxyl was present at the 20 position. The effects of the different substitutions at the 17, 20 and 21 positions explain why the two teleost maturation-inducing steroids 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-P) and 17alpha,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S) have higher affinities than progesterone for this receptor. It is concluded that the seatrout progestogen receptor demonstrates steroid specificity different from progesterone receptors in other vertebrates.