Photoaffinity Labels for Estrogen Binding Proteins of Rat Uterus

A number of estrogen derivatives bearing a photosensitive functional group have been synthesized for use as photoaffinity labels for the high-affinity estrogen binding protein of rat uterus: diazoacetate derivatives of estradiol and estrone, 16-diazoestrone, and 3-diazo-2-ketopropyl ether and ortho-azide derivatives of estradiol, estrone, and hexestrol. All of these compounds are reasonably stable and have been carefully purified and characterized. The binding affinity of these compounds and their precursors for the rat uterine estrogen binding protein has been measured by a competitive binding assay. Introduction of substituents lowers the binding affinit in all cases, especially when one or both of the hydroxyl groups are blocked. Substituents at positions 2, 4, and 16 are reasonably well tolerated, and some preference for polar substituents is noted at these sites. Derivatives in which the A-ring hydroxyl is internally hydrogen bonded to a nitro group (2-nitro) have very low affinity, but the nitro group is well tolerated at position 4, where it does not hydrogen bond to the hydroxyl. The binding affinity of all the derivatives reflects the relative binding affinity of the parent ligands: hexestrol > estradiol > estrone. The requirements for selective affinity labeling of the estrogen binding protein from rat uterus are outlined. Based on the binding data, a number of the derivatives prepared appear to be promising candidates for covalent labeling of this protein in unpurified or only semi-purified preparations.