Affinity Labeling Steroids for Characterization of Steroid Binding Sites

Abstract

Because enzyme active sites have been identified by affinity labeling with substrate derivatives, we presumed that a similar approach might be used to characterize macromolecular steroid-binding sites. Affinity labeling (site directed irreversible binding) depends upon attaching to the steroid a reagent group capable of reaction with amino acid residues present at a macromolecular steroid binding site. Concentration of an appropriate reagent group at the steroid binding site by the steroid moiety favors covalent bond formation at the binding site as compared to the protein molecule in general. This basic scheme is illustrated in Fig. 1, which contrasts the reversible finding of receptor and steroid with the case where the same steroid bears a reagent group (X) capable of reacting with a residue at the binding site (Y). Note that the covalent bonding step (if a true covalent bond is formed) is irreversible and the steroid is not capable of leaving the site. While some affinity labeling compounds form only pseudo-covalent bonds and will dissociate from the site, it is possible to make derivatives that will form covalent bonds which endure even acid hydrolysis of the macromolecule.