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

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Abstract
An assay is described for measurement of a serum inhibitor of an esterase derived from preparations of the first component of complement (C[image] 1-esterase). Esterolysis of N-acetyl-L-tyrosine ethyl ester by C[image] 1-esterase is inhibited instantaneously and stoichiometrically by fresh human serum. Units of C[image] 1-esterase and of serum inhibitor are defined. The inhibitor in human serum is stable at -45[degree] for at least 4 months, at 37[degree] for at least 24 hr., and at 48[degree] for at least 30 min. It is completely inactivated during 30 min. incubation at 60[degree] and is labile at 0[degree] at pH values below 6. The inhibitor is inactivated at -5[degree] in the presence of methanol concentrations of 15% or greater but is recovered quantitatively in a 40% (NH4)2 SO4 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. Monkey and guinea pig sera inhibit human C[image] 1-esterase to about the same extent as human serum, while rabbit serum is about 1/3 as active. No marked species specificity exists in the interaction of inhibitor in human or rabbit serum with human or rabbit C[image] 1-esterase. These observations are being employed in investigation of the possible role of inhibitor in experimental hypersensitivity.