Effects of methylamine and heparin on a rapid chromogenic assay of C1-esterase inhibitor in plasma

We describe a rapid assay of C1-esterase inhibitory (C1-inh) activity in plasma. After adding purified C1s serine protease (EC 3.4.21.42) in excess to plasma, we determine the residual C1s activity towards a new chromogenic tripeptide, CH₃CO-Lys(CbO)-Gly-Arg-pNA. Optimal conditions include the addition of methylamine (final concentration 0.12 mol/L) to reduce the potential inhibitory capacity of α₂-macroglobulin towards C1s and the addition of heparin (final concentration 3000 int. units/L) to enhance the reaction of C1s with C1-inh. The correlation with C1-inh antigen concentrations in plasma was excellent. The estimated interassay CV was 4.3%, whereas the intra-assay CV was 2.0% for activity concentrations within the range of normal individuals (x̄ ± 2 SD: 70-124%), 1.3% at lower concentrations. The method is more convenient, rapid, and precise than previous methods, and C1-inh activity in plasma can be assessed within 30 min. We found that concentrations of C1-inh in plasma were low during open-heart bypass surgery.