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

Abstract

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 2 distinct molecular variants, which were detected by polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate. The 2 monomeric CBG species were separated by preparative gel electrophoresis, and bound cortisol and progesterone with equal affinity. They have 1 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 2 molecular forms. The 2 CBG species were isolated from each of the sera from 5 individual donors, indicating that the observed heterogeneity does not result from pooling genetic variants. The 2 spp. are immunologically identical. A possible explanation for the existence of the 2 electrophoretic variants is a difference in amidation.