Steroid-protein interactions. Human corticosteroid binding globulin: some physicochemical properties and binding...

Area of Science:

• Biochemistry
• Molecular Endocrinology

Background:

• Human corticosteroid-binding globulin (CBG) plays a crucial role in regulating the bioavailability of steroid hormones.
• Understanding the factors influencing CBG-steroid interactions is essential for endocrinology and drug development.

Purpose of the Study:

• To investigate the impact of reducing agents on CBG-steroid binding affinity.
• To determine the influence of pH and temperature on the binding constants of CBG complexes.
• To elucidate the structural requirements for optimal steroid binding to human CBG.

Main Methods:

• Equilibrium association constant (Ka) measurements using varying concentrations of reducing agents (dithiothreitol, beta-mercaptoethanol).
• pH-dependent binding studies and van't Hoff analysis of temperature effects on Ka.
• Systematic evaluation of binding affinities for 47 different steroids.

Main Results:

• Reducing agents reversibly decrease CBG-cortisol binding affinity, yielding more consistent Ka values.
• CBG exhibits a broad pH optimum (pH 8-11) for cortisol and progesterone binding, with relative affinities varying with pH.
• Steroid structure, particularly the 20-oxo group, 10 beta-methyl group, and 4-position double bond, significantly impacts binding energy.

Conclusions:

• Reducing agents can modulate CBG-steroid interactions and improve binding assay consistency.
• The binding site of human CBG accommodates specific steroid structural features, providing insights into ligand recognition.
• Enthalpy drives the association of cortisol and progesterone with CBG, with entropy changes compensating.