Structural Requirements for Steroid Binding to Androgen Binding Proteins

The physicochemical properties of androgen binding protein (ABP) and sex steroid binding globulin (SBG) are virtually indistinguishable, whereas both of these proteins are remarkably different from the androgen cytoplasmic receptor (CR). Certain compounds, such as cyproterone acetate and BOMT, inhibited binding of labelled androgen to CR but did not inhibit binding to ABP. These data suggested that a fundamental difference exists between the binding sites of ABP and CR. We have therefore examined the steroid specificity of binding to a large number of steroidal compounds in order to characterize the binding sites of these binding proteins and to determine their binding specificities. Binding to partially purified ABP and SBG was assayed by a modification of the steady‐state PAGE technique described by Ritzen et al. (1974) by dissolving competitive inhibitors along with 0.2 nM ³H‐DHT in the gel solution. Binding of CR was assayed by the ammonium sulfate precipitation assay described by Verhoeven et al. (1975). A group of steroids was found which had high affinity for CR but low affinity for ABP. The structures of those steroids revealed that alterations at position 11 resulted in the greatest changes in binding affinity for ABP without changing significantly the binding affinity for CR. Whereas removal of the 19‐methyl group reduced binding for ABP, CR was not affected. A second group of steroids had high affinity for ABP but low affinity for CR. The structures of these compounds demonstrate that modifications at positions C‐2 and C‐17 resulted in the greatest changes in affinity for CR without significantly changing the binding affinity for ABP. The 3‐keto and 17α‐hydroxy moieties appear to be essential for CR binding, whereas both of these positions can be modified and still maintain ABP ***