Protease inhibitors from the parasitic worm <em>Parascaris equorum</em>.

Two proteic inhibitors (I and II) of serine proteases have been purified from the parasitic worm Parascaris equorum by affinity chromatography on immobilized trypsin followed by preparative electrophoresis. They have an apparent relative molecular mass of 9000 and 7000 as determined by gel filtration, a slightly acid isoelectric point (5.5 and 6.1) and a similar amino acid composition. Both inhibitors lack serine, methionine and tyrosine. They bind bovine trypsin extremely strongly with an association constant, K_a, larger than 10⁹ M^-1 and form a 1:1 complex with this protease. The K_a values for the binding to bovine chymotrypsin are ≈ 3.3 × 10⁸ M^-1 (inhibitor I) and ≈ 2 × 10⁶ M^-1 (inhibitor II). Inhibitor I interacts also with porcine elastase (K_a ≈ 5 × 10⁷ M^-1, while inhibitor II is inactive towards this enzyme. [ABSTRACT FROM AUTHOR]