Pregnancy-associated corticosteroid-binding globulin: high resolution separation of glycan isoforms

Corticosteroid-binding globulin (CBG) is a glycoprotein that functions as a specific carrier of cortisol in the circulation. CBG contains six sites for N-glycosylation with, on average, five sites occupied by a mixture of biantennary and triantennary oligosaccharides with variable additional terminal sialic acid residues leading to glycoforms with significant heterogeneity in mass and isoelectric points. During pregnancy, a form of CBG possessing only triantennary oligosaccharides comprising approximately 10 % of total CBG appears specifically. We describe the first application of two-dimensional gel electrophoresis to the separation of human CBG glycoforms. This technique resolved a greater degree of charge heterogeneity than previous studies, and allowed simultaneous visualization of changes to the size and isoelectric points of CBG during pregnancy. Profiles of CBG glycoforms during pregnancy showed a general increase in size followed by a shift to lower pI in a large proportion of the glycoprotein. This may result from the enhancement of triantennary glycosylation, with the extent of incorporation of sialic acid increasing with the number of available sites for its addition. The pregnancy-specific CBG previously defined probably represents a subset of the acidic and high molecular weight glycoforms we have resolved by two-dimensional electrophoresis and now describe as pregnancy-associated CBG.