Assay and Properties of Serum Inhibitor of C'1-Esterase

Abstract

An assay is described for measurement of a serum inhibitor of an esterase derived from preparations of the first component of complement (C'1-esterase). Esterolysis of N-acetyl-L-tyrosine ethyl ester by C'1-esterase is inhibited instantaneously and stoichiometrically by fresh human serum. Units of C'1-esterase and of serum inhibitor are defined. The inhibitor in human serum is stable at 45 for at least 4 months, at 37° for at least 24 hr. and at 48° for at least 30 min. It is completely inactivated during 30 min. incubation at 60° and is labile at 0° at pH values below 6. The inhibitor is inactivated at —5 in the presence of methanol concentrations of 15° or greater but is recovered quantitatively in a 40° ammonium sulfate supernatant fraction. These data are being applied to purification of the inhibitor from human serum. Purification procedures and properties of the purified inhibitor will be reported elsewhere.