Ascaris suum: are trypsin inhibitors involved in species specificity of Ascarid nematodes?

Inhibitors of porcine trypsin were prepared from aqueous extracts of the parasitic nematodes Ascaris suum (hogs) and Ascaris lumbricoides (human). In this study three experiments were performed. (1) Polyclonal antibodies were prepared against one isoform of trypsin inhibitor from each parasitic nematode. Each antibody reacted with all isoforms from itself as well as all isoforms from the other parasite. (2) Association equilibrium constants were measured by titrating host trypsins (porcine or human) with the isoforms of trypsin inhibitors from A. suum and A. lumbricoides. While three of the combinations formed tight complexes that can be precipitated, the fourth complex, A. suum trypsin inhibitor-human trypsin has a Ka that is a 300 to 1000 times weaker interaction than the three other titration pairs. (3) Live A. suum worms were incubated in isosmotic media that contained either porcine trypsin or human trypsin. A suum worms survived in porcine trypsin and in the controls but were killed and digested after exposure for 5 days in human trypsin. The first experiment suggests that the trypsin inhibitors from A. suum and A. lumbricoides have similar epitopes, while the second experiment suggests that there are differences near the reactive site of the inhibitors. The consequences of these differences are dramatically demonstrated by the third experiment in which live A. suum worms in the presence of human trypsin die and are digested but those in porcine trypsin survive. These experiments suggest that in order to parasitize a host, a nematode requires a complement of protease inhibitors that interact strongly with those host proteases that are in their environment.