Comparative structural analyses of corticosteroid binding globulin.

Some physicochemical characteristics of corticosteroid binding globulin (CBG) in several species have been determined. Molecular radii were determined from Ferguson plots and were used in conjunction with sedimentation coefficients determined by sucrose density gradient centrifugation to calculate the molecular weights of the CBG. These were found to range from 44,200 (dog) to 60,000 (turtle) for most species. The squirrel monkey was found to have a molecular weight twice that of other species (119,800). Purified CBG was prepared from human, rat, and guinea pig sera. The molecular weights of the purified material, as determined by gel electrophoresis in the presence of sodium dodecyl sulfate, were in excellent agreement with those determined by Ferguson analysis. Careful examination of the purified proteins by electrophoresis at pH 8.3 revealed that each consisted of two closely related electrophoretic variants. Tryptic peptides were prepared from the purified proteins and separated by reversed phase HPLC chromatography. The peptide patterns were identical for the three proteins with the exception of three hydrophilic peptides. Amino terminal sequence analysis of the rat and human proteins revealed no apparent homology, however. The immunologic relatedness of the three purified proteins was also examined, but no crossreactivity was observed. The results obtained suggest that while the molecular size and hydrophobicity of peptides have been conserved across species considerable surface differences must exist.