Binding Specificity of Rat a-Fetoprotein for a Series of Estrogen Derivatives: Studies Using Equilibrium and Nonequilibrium Binding Techniques*

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Abstract
The binding specificity of rat a-fetoprotein (AFP) for a series of estrogen derivatives has been investigated and compared to that of the uterine cytosol estrogen receptor. In addition, several equilibrium and nonequilibrium binding techniques have been examined for their appropriateness in studying AFP-estrogen interactions under different experimental conditions. The ratio of association constants (RAC) of various estrogen derivatives (compared to 17β-estradiol; RAC = 100%) were determined using a rapid and convenient miniaturized equilibrium assay that utilizes the partitioning properties of Sephadex G-25. AFP has the highest affinity for estrone derivatives and is especially tolerant of small, nonpolar substituents in the D ring; it binds nonsteroidal estrogens and antiestrogens weakly or not at all. Thus, it has a binding specificity quite unlike that of the estrogen receptor. Several potential affinity labels were studied. Of these, 16-diazoestrone, a photoreactive compound, has the highest affinity for AFP (RAC = 121%). Another equilibrium technique, steady state polyacrylamide electrophoresis, effects a clear separation between AFP and albumin. It is superior to conventional polyacrylamide electrophoresis under conditions where dissociation is rapid. Charcoaldextran adsorption also rapidly removes estradiol that is bound to AFP (ti½ = -10 min at 0 C). The AFP-estradiol complex appears to be stabilized by adsorption to hydroxylapatite, however, so that hydroxylapatite can be employed in a batchwise manner to assay binding to AFP. This is the most appropriate technique when high concentrations of ligand are used. These studies highlight the marked differences in binding specificity between rat AFP and the uterine cytosol estrogen receptor and indicate the techniques most appropriate for binding measurements under different conditions. This work should facilitate the design of affinity labeling reagents for rat AFP as well.