Schistosoma mansoni: inhibition of cercarial "penetration" proteases by components of mammalian blood.

1. Normal human sera and plasma were fractionated in order to identify inhibitors of the "penetration" proteases of Schistosoma mansoni cercariae. 2. The main inhibitor, accounting for 90% of the total activity of serum, appears to be alpha 1-antitrypsin (alpha 1-AT) as identified by separation on DEAE-cellulose and Sephadex, by immunoelectrophoresis and by anticercarial protease activity of purified alpha 1-AT preparations. 3. The inhibition profiles of purified preparations of the 6 known serum antiproteases suggest that the parasite protease is similar to vertebrate chymotrypsin. 4. On a molar basis, the order of inhibitory activity against the cercarial protease is: alpha 1-AT = alpha 2-macroglobulin; C'-1-inactivator; alpha 1-antichymotrypsin. No inhibition was obtained with inter-alpha-inhibitor or antithrombin III.