Partial Purification and Characterization of Chicken Corticosteroid-Binding Globulin

Affinity chromatography of pooled chicken serum yielded a solution in which 45% of the protein was corticosteroid-binding globulin (CBG). Further purification by single passes through hydroxylapatite and Biogel A-.5m resulted in a final product with about 91% purity. The molecular weight was 56,000 daltons as determined by gel filtration and 63,000 as determined by sodium dodecysulfate-polyacrylamide gel electrophoresis. The amino acid composition was similar to that of mammalian CBG's when compared on the basis of total amino acid residues of each type. However, the avian CBG was much lower than mammalian CBG in half-cystine residues. The corticosteroid-binding ability of the purified CBG was increased up to tenfold by treatment with dithiothreitol (DTT). Of several cations tested, Mg2+, Ca2+, Zn2+, and Mn2+ exhibited some synergism with DTT, but were ineffective by themselves; Cu2+ was inhibitory. The association constant toward corticosterone at 4 degrees was 3.6 X 10(-7) M-1. Unlike mammalian CBG, which does not bind dexamethasone, the chicken CBG binds dexamethasone with almost the same affinity as corticosterone.