The reactive centre loop of corticosteroid-binding globulin (CBG) is a protease target for cortisol release

Abstract

Corticosteroid-binding globulin (CBG) binds more than 90% of circulating cortisol and is a non-inhibitory member of the family of serine protease inhibitors (SERPINS) with an exposed elastase sensitive reactive centre loop (RCL). At sites of inflammation neutrophil activation can release elastase which may cleave the RCL and result in cortisol release from CBG. The RCL sequence also has two theoretical chymotrypsin cleavage sites and we used a monoclonal antibody with specificity for the RCL to investigate chymotrypsin cleavage of CBG. Here we show, for the first time, rapid chymotrypsin cleavage of the RCL of CBG, resulting in undetectable levels of intact CBG, whereas total CBG levels were unchanged. Coincident with both chymotrypsin and elastase cleavage there was an increase in the free cortisol fraction of serum to levels similar to when CBG had been inactivated by heat indicating total cortisol release from CBG. These findings demonstrate a new mechanism for cortisol release from its binding globulin.