A new simple method for determination of C1-esterase inhibitor activity in plasma.

A convenient method for the determination of C1-esterase inhibitor activity in plasma samples is described. The method is based on addition of purified C1s to plasma and measuring excess C1s with a new chromogenic tripeptide-p-nitroanilide substrate with a recording spectrophotometer. Addition of C1s to C1-esterase inhibitor-depleted plasma did not result in any appreciable inactivation of the enzyme for three hours. The concentration of C1-esterase inhibitor in 19 healthy individuals was estimated as 1.63 +/- 0.27 (SD) mumol/l. The correlation with C1-esterase inhibitor antigen in these individuals and 19 patients with varying concentrations of C1-esterase inhibitor was excellent. The correlation with an antikallikrein assay was found to be poor.