Rainbow trout, Salmo gairdneri, oocytes were used in an in vitro bioassay to test for the potency of C21 steroids in inducing final maturation. A wide range of steroids was used and most of the assays were replicated at least 3 times and up to 10 times with the most effective steroids. 17 alpha,20 beta-Dihydroxy-4-pregnen-3-one (17,20 beta-P), which has been identified as the natural maturation-inducing steroid in Salmoniformes, was used as the reference steroid. Four steroids, 17 alpha,20 beta,21-trihydroxy-4-pregnen-3-one; 3 alpha,17 alpha,20 beta-trihydroxy-5 alpha-pregnane; 3 beta,17 alpha,20 beta-trihydroxy-5 alpha-pregnane; and 3 alpha,17 alpha,20 beta,21-tetrahydroxy-5 alpha-pregnane were found to be equipotent with 17,20 beta-P. Analysis of the structure-activity relationships indicated that, for maximum biological activity, steroids must have a planar nucleus (as in delta 4 and 5 alpha-reduced steroids), a hydroxyl or keto group at position 3, and hydroxyl groups at positions 17 and 20 beta. Steroids with delta 5 or 5 beta-reduced conformations and/or hydroxyl or keto groups at position 11 had a much reduced biological activity.

http://www.ncbi.nlm.nih.gov/pubmed/3203879?dopt=Abstract