The photoreactivity of a number of photosensitive estrogen derivatives with the estrogen binding protein of rat uterus can be ascertained using a cytosol exchange assay. The competitive effect of estradiol localizes the reaction at the estrogen binding site and permits the assignment of a reaction efficiency for each compound—per centof the binding capacity that is consumed upon photolysis. The kinetics of reaction of several azide and diazo derivatives is biphasic, the initial portion corresponding to the photolysis rate of the photosensitive function and the slower portion to the direct photolylic degradation of the binding protein. Reaction efficiencies of these compounds range from 0 to 21%. In certain cases there is a pronounced dependence on the orientation of the reactive function in the binding site. Investigation of a number of other chromophoric estrogens and antiestrogens shows some to be unreactive (benzil, o-iodophenol, o-nitrophenol, a-oximo ketone, stilbene, and styrene chromophores) and others to be highly reactive (20-51%; triarylethylene chromophores). As opposed to the biphasic kinetics of these derivatives, reaction of 6-oxoestradiol is cleanly first order. A reagent of this type is classified as “photoexcitable” (reversibly photoactivatable), while the others are “photolabile” (irreversibly photoactivatable).
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