Abstract
Purified preparations of the esterase derived from the first component of complement (C'1 esterase) increased vascular permeability in guinea pig skin, an effect inhibited by triprolidine, an antihistaminic agent, but not by soy bean trypsin inhibitor. The permeability-increasing and esterolytic properties of C'1 esterase were inhibited in parallel by the serum inhibitor of C'1 esterase, diisopropylphosphofluoridate and extremes of temperature and pH. Moreover, the permeability-increasing and esterolytic properties evolved in parallel when C'1 esterase was generated from its subcomponents. How C'1 esterase induces changes in vascular permeability remains unexplained, although the possibility that its action is mediated through a histamine-like agent is attractive.