Corticosteroid-binding globulin. A review of some recent aspects

Among the numerous corticosteroid-binding proteins that have been demonstrated to exist throughout the vertebrates, the corticosteroid-binding globulins (CBG or transcortin) of man and guinea pig have been most thoroughly investigated. Both are glycoproteins that contain one binding site for corticosteroid hormones and other steroids. The binding forces are predominantly of a hydrophilic nature, in addition to hydrophobic interaction. Human CBG forms a reversible dimer that maintains full binding affinity and capacity. The monomeric form is composed of two electrophoretic variants that appear indistinguishable in their chemical, immunological, and steroid binding properties. Thermodynamic analysis indicates different binding mechanisms for human and cavian CBG at the lower and higher temperature range. Entrance of hydroxy groups into the steroid molecule increases the affinity to CBG; the contributions to the free energy of complex formation, resulting from individual substitutions or other alterations in the steroid molecule, are additive. Determination of binding specificity leads to a conceptual image of the steroid binding site with respect to the nature of the various binding domains.