Purification and characterisation of a second cathepsin L proteinase secreted by the parasitic trematode Fasciola hepatica

A 29.5‐kDa cysteine proteinase was purified from medium in which mature Fasciola hepatica parasites were maintained. The N‐terminal sequence (14 residues) of the purified protein is similar to known cathepsin L proteinases, including a 27‐kDa cathepsin L proteinase, also secreted by this parasite, which had been isolated previously in our laboratory [Smith, A. M., Dowd, A. J., Mc Gonigle, S., Keegan, P. S., Brennan, G., Trudgett, A. & Dalton, J. P. (1993) Mol. Biochem. Parasitol. 62, 1–8]. The N‐terminal sequences of the 29.5‐kDa and 27‐kDa cathepsin L proteinases differ only in residue number seven (arginine and proline, respectively). Immunoblot studies, using antiserum that reacts with both cathepsin L proteinases, rule out the possibility of both enzymes arising from a higher molecular sized parent molecule. The reaction kinetics of the two F. hepatica cathepsin L proteinases on a variety of peptide substrates revealed that the two enzymes differ in their substrate specificity. Five peptide substrates that are cleaved with high affinity by the 29.5‐kDa cathepsin L isolated in this study are not cleaved by the previously purified 27‐kDa cathepsin L. The protein‐modifying reagent, tetranitromethane, affected the 29.5‐kDa cathepsin L proteinase only, causing inactivation of the enzyme and changing its migration in polyacrylamide gel electrophoresis. Our studies suggest that the two F. hepatica cysteine proteinases represent two distinct subclasses within the cathepsin L class.