CHARACTERIZATION OF C1 INHIBITOR-TA - A DYSFUNCTIONAL C1INH WITH DELETION OF LYSINE-251

Dysfunctional C1 inhibitor (C1INH)-Ta is a naturally occurring mutant from a patient with type II hereditary angioedema. This mutant has a d eletion of the codon for Lys-251, which is located in the connecting s trand between helix F and strand 3A, overlying beta sheet A. Deletion of this Lys modifies the amino acid sequence at this position from Asn -Lys-Ile-Ser to Asn-Ile-Ser and creates a new glycosylation site. To f urther characterize the mechanism of dysfunction, we have analyzed the recombinant normal and Ta proteins expressed by COS cells in addition to the proteins in serum and isolated from serum. Recombinant C1INH-T a revealed an intermediate thermal stability in comparison with the in tact and reactive center cleaved normal proteins. Analysis of the reac tivity of this recombinant protein with target proteases demonstrated no complex formation with C1s, C1r, or kallikrein. Inefficient complex formation was, however, clearly detectable with beta-factor XIIa. Eac h protease produced partial cleavage of the recombinant mutant inhibit or. Recombinant C1INH-Ta, on 7.5% SDS-polyacrylamide gel electrophores is and by size fractionation on Superose 12, showed a higher molecular weight fraction that was compatible in size with dimer formation. How ever, no multimerization of C1INH-Ta isolated from serum or of C1INH-T a in serum, was observed. The C1INH-Ta dimer expressed the epitopes th at normally are expressed only on the protease complexed or the cleave d inhibitor. These epitopes were not expressed on the monomeric inhibi tor. The data suggest that the mutation in C1INH-Ta results in a foldi ng abnormality that behaves as if it consists of two populations of mo lecules, one of which is susceptible to multimerization and one of whi ch is converted to a substrate, but which retains residual inhibitory activity.