Retinoid affinity label for the binding site of retinol-binding protein

Three radioactive retinoid bronoacetates were synthesized as potential retinoid affinity labels for the retinol binding site of human plasma retinol-binding protein (RBP). The compounds synthesized were beta-[9-3H]ionyl bromoacetate (IBA), beta-[11-3H]ionylideneethyl bromoacetate (IEBA), and [15-3H]retinyl bromoacetate (RBA). When excess ligand was incubated with RBP for 5 h at 37 degrees C, IBA and IEBA formed nearly 1:1 molar complexes with RBP, whereas RBA bound only approximately one-third as well. Subsequent addition of retinol to the retinoid-RBP complex resulted in complete displacement of IBA and RBA from the protein, whereas a large proportion (37%) of the [3H]IEBA remained bound to the retinol binding site of RBP. For maximization of covalent bonding of IEBA to RBP, IEBA was incubated with RBP for varying lengths of time, followed, in each instance, by addition of retinol to displace noncovalently bound IEBA. The amount of IEBA remaining bound to RBP increased with increasing incubation time, reaching a maximum of about 0.66 mol/mol of RBP at 18 h. Moreover, at each time point, the binding of retinol to RBP was inhibited to an extent that was equivalent to the amount of [3H]IEBA that was not displaced from RBP by retinol. Only a very small proportion of the bound [3H]IEBA that was not displaced with retinol could be extracted from the protein with chloroform-methanol. Taken together, these several lines of evidence strongly suggest that the IEBA was bound in the retinol binding site of RBP and was attached to the protein in a covalent manner. Thus, IEBA appears to be an effective affinity label for the retinol binding site of RBP.