Steroid-protein interactions. Human corticosteroid-binding globulin: characterization of dimer and electrophoretic variants

Human corticosteroid-binding globulin (CBG) forms a dimer that was isolated by gel filtration, has full binding affinity and capacity, and can be dissociated to the monomer. Monomeric CBG consists of two distinct molecular variants, which were detected by polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate. The two monomeric CBG species were separated by preparative gel electrophoresis and were found to bind cortisol, as well as progesterone, with equal affinity. They have one steroid binding site per CBG molecule. Amino acid and carbohydrate analyses are essentially the same for both of the CBG variants. Removal of sialic acid or 90% of the carbohydrate did not affect the existence of the two molecular forms. The two CBG species were isolated from each of the sera from five individual donors, indicating that the observed heterogeneity does not result from pooling genetic variants. The two species are immunologically identical. A possible explanation for the existence of the two electrophoretic variants is a difference in amidation.