Inactivation of C-1 inhibitor by proteases: demonstration by a monoclonal antibody of a neodeterminant on inactivated, non-complexed C-1 inhibitor.

Monoclonal antibodies were raised against kallikrein-C-1 inhibitor and factor XIIa-C-1 inhibitor complexes. One of the monoclonal antibodies (KII) appeared to react predominantly with C-1 inhibitor complexes in an ELISA. However, the apparent binding of KII to C-1 inhibitor complexes was probably due to the presence of proteolytically inactivated C-1 inhibitor in the complex mixture used for the coating:KII did not bind either kallikrein-C-1 inhibitor or factor XIIa-C-1 inhibitor complexes generated in plasma by dextran sulphate. SDS-PAGE analysis of C-1 inhibitor incubated with proteases revealed that KII-reactive C-1 inhibitor has a lower molecular weight than native C-1 inhibitor. We propose that the determinant that reacts with KII is exposed after cleavage of C-1 inhibitor in its reactive site. The monoclonal antibody KII will enable us to study the inactivation of C-1 inhibitor in human inflammatory disease.