A CYSTEINE PROTEINASE IN THE CERCARIAE OF DIPLOSTOMUM-PSEUDOSPATHACEUM (TREMATODA, DIPLOSTOMATIDAE)

A cysteine proteinase was detected in extracts from cercariae of the t rematode Diplostomum pseudospathaceum. The enzyme preferred protein su bstrates over synthetic, chromogenic peptides. The optimal pH for hydr olysis of substrates was 7.2 for azocoll, 6.4 and 7.6 for azocasein, 7 .6 for azoalbumin, and 6.8 for N-benzoyl-L-arginine-4-nitroanilide. El astin-Congo red and certain N-blocked L-aminoacyl- and L-peptidyl nitr oanilides bearing L-phenylalanine, L-alanine, L-tyrosine, and L-leucin e at the P-1 subsite were not hydrolyzed. Thiol-reducing and divalent cation-complexing agents stimulated the proteinase activity, whereas t hiol-blocking agents inhibited it. The relative molecular weight of th e enzyme was approximately 40 000 as determined by SDS-PAGE. Detection of an identical proteinase in water after treatment of living cercari ae with praziquantel suggests that the enzyme occupied the penetration glands in the larvae. Thus, when secreted by the parasite during inva sion of an appropriate host, the enzyme might act as a penetration-pro moting factor.