Twelve human C1 inhibitor P1 variants were constructed by site-directed mutagenesis of the codon for arginine 444 and were expressed in COS-1 cells to analyze the functional properties.
Transiently transfected COS-1 cells were assayed for the presence of C1 inhibitor antigen with the mAb RII RIA (21). Expression levels of all P1 variants were comparable to wild type. . Lysates of transfected COS cells did contain recombinant C1 inhibitor P1 stop antigen, but in substantially lower amounts than the other P1 variants. This indicates that the premature stop codon results in an aberrantly folded protein, which is degraded intracellularly.
P1 His retained the ability to complex with target proteases, but considerably higher amounts of this variant was required compared to recombinant wild-type C1 inhibitor.
P1 Cys was shown to be nonfunctional in kinetic analyses.
Conclusion: P1-Cys variant is nonfunctional toward Cls, beta-factor XIIa, kallikrein, and plasmin. The low activity of P1-His toward Cls is probably not physiologically relevant.
Variants: c.1396C>T; c.1397G>A
CAIDs: CA116521; CA116519