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Norethindrone (17beta-hydroxy-19-nor-17alpha-pregn-4-en-20-yn-3-one) and norethindrone acetate (17beta-acetoxy-19-nor-17alpha-pregn-4-en-20-yn-3-one) interfered to a varying degree, by competitive inhibition, with the binding of progesterone and oestradiol to respective cytoplasmic receptors in the human uterus. Progesterone binding to 4S macromolecule was saturable and co-specific for progestins. Competitors like norgestrel (17beta-hydroxy-18-methyl-19-nor-17alpha-pregn-4-en-20-yn-3-one), 19-norprogesterone, medroxyprogesterone acetate (17alpha-acetoxy-6alpha-methylpregn-4-ene-3,20-dione) and compound R(5020) (17,21-dimethyl-19-norpregna-4,9-diene-3,20-dione) possessed higher binding affinities for the progestin receptor. The dissociation constant (K(d)) for the progesterone-receptor interaction was 0.6-1.6nm and the receptor concentration ranged between 6600 and 8200 sites/cell. Norethindrone and norethindrone acetate competed for the progesterone receptor with inhibition constants (K(i)) of 6.8 and 72nm respectively. Gradient displacement and competitive-receptor assays indicated that norethindrone acetate-binding affinity for progestin receptor was approximately one-tenth that of norethindrone and progesterone. The progestins also inhibited oestradiol binding to 4.6S oestrogenic receptor by 8-12%, involving interaction at the oestradiol-binding site with a calculated K(i) value of 0.5-0.8mum. The competitive interaction of progestins with steroid receptors may be of putative importance in explaining the progestin action at the target site.

Interaction of progestins with steroid receptors in human uterus