Intact or “active” corticosteroid-binding globulin (CBG) and total CBG in plasma: Determination by parallel ELISAs using monoclonal antibodies

Abstract

The predominant carrier of cortisol in circulation is corticosteroid-binding globulin (CBG) which is a non-functional member of the family of serine protease inhibitors. Corticosteroid-binding globulin possesses an exposed elastase sensitive loop and upon cleavage it adopts a “relaxed” conformation promoting the delivery of cortisol to sites of inflammation. Recently we have developed monoclonal antibodies which recognise only the intact exposed elastase loop, including an N-glycosylation site, which, in concert with another monoclonal antibody to CBG, offered the potential for the determination of intact and total CBG which may both be present in circulation. Here we validate these parallel ELISAs and show that like total CBG there is little diurnal variation of intact plasma CBG. Furthermore in a normal reference population the majority of CBG is in the intact or active form but a significant level of apparently cleaved CBG is evident. In some subjects there is gross discordance between total CBG and intact CBG implying a predominance of apparently cleaved CBG in circulation and this significantly affects calculated free cortisol levels. Gross differences in total and intact CBG levels may not be due to differences in N-glycosylation affecting antibody binding as CBG levels are unaffected by PNGase F treatment.