A neutral thiol protease secreted from newly excysted metacercariae of trematode parasite Paragonimus westermani: purification and characterization.

1. A neutral thiol protease was purified from the culture filtrate of newly excysted metacercariae of Paragonimus westermani to homogeneity as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, having a monomeric form with mol. wt 22,000. 2. It expressed activity on t-butyloxycarbonyl-valyl-leucyl-lysyl-4-methyl-coumaryl-7-amide in the presence of cysteine at an optimal pH of 7.5, and also the activity was significantly affected by thiol protease inhibitors, indicating that the enzyme belongs to a neutral thiol protease family. 3. The enzyme hydrolyzed protein substrates, azocoll, casein and fluorescein isothiocyanate-labeled collagen, and showed low specificity toward hemoglobin, but no activity with elastin Congo Red and bovine serum albumin. 4. Catalytic property on fluorogenic substrates demonstrated that the enzyme cleaved preferentially the carboxylic side of the basic residue in N-substituted peptides.