Contribution of functional groups of 19-nor-progestogens to binding to progesterone and estradiol-17beta receptors in rabbit uterus.

The structural elements of 19-norprogestogens which may be essential for binding to progesterone and estradiol-17beta(E2) receptors were investigated in the rabbit uterine cytosol. The kinetic study showed that 19-nor-progestogens are competitive inhibitors of progesterone-receptor (8S) binding and E2-receptor binding. The affinities of steroids for the progesterone receptor were as follows: norethindrone (Ki of 2.3 X 10(-9)M) greater than 5alpha-dihydronorethindrone greater than norethindrone acetate greater than lynestrenol greater than 17alpha-ethynyl-estra-4-ene-3beta, 17beta-diol greater than ethynodiol diacetate (Ki of 1.3 X 10(-7) M). The affinities of steroids for the E2 receptor were as follows: ethynodiol diacetate (Ki of 1.3 X 10(-7)M) greater than 17alpha-ethynyl-estra-4-ene-3beta, 17beta-diol greater than norethindrone acetate greater than norethindrone greater than 5alpha-dihydronorethindrone greater than lynestrenol (Ki of 8.4 X 10(-7)M). The results indicate that 3-ketone and 17beta-hydroxyl groups, and the plane of ring A/B of 19-norprogestogen are important for binding to the progesterone receptor. The affinities of 19-nor-progestogens for the E2 receptor were very weak. Their affinities for the E2 receptor increased with addition of acetate or hydroxyl groups at the 3beta and 17beta positions, and were decreased by the elimination of a 3 oxygen function or the reduction of ring A.