Structural requirements for the action of steroids as quenchers of albumin fluorescence.
1. Androgens, corticoids, gestagens, estrogens and related steroids are effective quenchers of the intrinsic fluorescence of bovine serum albumin. The quenching effect involves the formation of a steroid albumin complex which formation constant (Kf) and free energy of formation (delta G 0) can be determined by fluorescence titration. The fluorimetrically determined delta G 0 values range from -6.5 to -7.5 kcal/mol. 2. 5 alpha-Androstane and 5 alpha-pregnane are effective quenchers of albumin fluorescence, in accord with the essentially hydrophobic nature of the steroid-albumin interaction. Introduction of hydroxy or oxo groups in 5 alpha-androstane decreases the fluorescence quenching action, but the effect of each group declines when other polar groups are present in the steroid molecule. Similar effects occur with 5 alpha-pregnane except that 20-hydroxy (or oxo) duo-polar derivatives are more effective than the parent hydrocarbon. 3. Comparison of delta G 0 values for steroids differing in a single grouping shows that the steroid-albumin interaction is increased by (a) the benzenoid A-ring; (b) sulfate or carboxylate ions in the vicinity of C-3; (c) the 3-oxo group in place of the 3 alpha-hydroxyl (with 5 beta-pregnane derivatives; not with 5 alpha-androstane derivatives); (d) 17 beta-acetyl or 17 beta-hydroxyethyl residues; (e) acetylated or propionated 17 beta-hydroxy groups; (f) acetylated or methylated hydroxy groups at the C-3 of estrogens; (g) delta 5 and delta 6 double bonds; and (h) the 19 beta-methyl group. The maximal variation of delta G 0 determined by affinity-enhancing groups is -0.8 kcal/mol. Conversely, the steroid-albumin interaction is decreased by introduction of (i) oxygen atoms at C-3, C-6, C-11, C-16, and C-17; (j) 17 alpha-ethynyl and 17 alpha-acetoxyl residues; (k) benzoylated or hexahydro-benzoylated beta-hydroxy groups at C-17; (l) acetylated and benzoylated hydroxy groups at C-3; and delta 1 (conjugated) double bond. Oxo groups at C-3, C-6, C-16 and the 16 alpha, 17 alpha-epoxy group are more effective than the corresponding alpha-hydroxyl in decreasing affinity, while at C-11 and C-17, the alpha-hydroxyl is more effective than the beta-hydroxyl and the oxo group. The effect of substituents is influenced by the whole molecular structure, particularly, by the stereostructure at the A/B juncture, and the presence of an oxo group at C-17. 4. The stereospecific effect of substituents at different positions in the steroid molecule suggests that with non-aromatic, A/B trans (planar) steroids, binding to albumin primarily involves the (alpha) rear surface of the B-, C- and D-ring, and possibly, the 17 beta-side chain. With estrogens and A/B cis (dihedral) steroids, the benzenoid A-ring and electron attracting groups at C-3, respectively, may participate in binding.
D000731:Androstanes; D000736:Androstenes; D000818:Animals; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D004963:Estrenes; D005453:Fluorescence; D011278:Pregnanes; D011283:Pregnenes; D012710:Serum Albumin, Bovine; D013050:Spectrometry, Fluorescence; D013256:Steroids
D000731:Androstanes; D000736:Androstenes; D004963:Estrenes; D011278:Pregnanes; D011283:Pregnenes; D013256:Steroids; D012710:Serum Albumin, Bovine