The procedures for worm collection and preparation of C. sinensis crude extracts were carried out as described elsewhere (Hong et al., 1999). Purification of the 17 kDa protein was performed by a two-step column chromatography employing the methods of Chung et al. (1997) including Sephacryl S-200 HR gel filtration and Q-Sepharose anion exchange chromatography. Proteolytic assay was measured fluorometrically using synthetic dipeptide substrate carbobenzoyl-phenylalanyl-arginyl-7-amino-4-methylcoumarin (Cbz-Phe-Arg-AMC; Sigma) with 2 mM dithiothreitol (DTT). One unit of the enzyme activity was determined as the amount of the enzyme that caused releasing of 1 µM of AMC in 1 hr (Chung et al., 1997). The enzyme inhibition test was measured with specific inhibitors such as iodoacetic acid (IAA, 20 µM), trans epoxy-succinly-L-leucyl-amido(4-guanidino) butane (E-64, 10 µM), diisopropylfluorophosphate (DFP, 2 mM), 3,4-dichloroisocoumarin (3,4-DCI, 10 µM), 4-(amidinophenyl) methanesulphonyl fluoride (APMSF, 10 µM), ethylenediamine tetraacetic acid (EDTA, 2 mM). All chemicals were purchased from the Sigma. The immune sera of BALB/c mice raised against the 17 kDa protein were prepared by the methods described by Kong et al. (1997). The purity of the enzyme was analyzed by 7.5-15% SDS-PAGE under reducing condition. Immunoblotting was performed using an anti-17 kDa antibody and antigens of other parasites. After electropho-resis, the protein bands were transferred onto polyvinylidene difluoride membrane (PVDF, Amersham Pharmacia, Uppsala, Sweden). The membrane was incubated overnight with 1:1,000 diluted anti-17 kDa antibody at a room temperature and followed by incubation with 1:1,000 diluted peroxidase-conjugated anti-mouse IgG for 3 hr. The blots were developed with 4-chloro-1-naphtol.

SDS-PAGE revealed that the purified protein from C. sinensis migrated as a 17 kDa (Fig. 1). The protein cleaved Cbz-Phe-Arg-AMC in the presence of DTT and was completely inhibited by cysteine protease specific inhibitors, IAA and E-64, but not inhibited by serine or metalloprotease inhibitors such as DFP, APMSF and EDTA (Table 1). These results indicated that the purified enzyme belongs to the cysteine protease family.

Immunoblot analysis of the crude extracts from other parasites revealed that the anti-17 kDa antibody reacted with the 17 kDa bands of many other trematodes such as C. sinensis, Opisthorchis viverrini, P. westermani, Fasciola hepatica, G. seoi and Metagonimus yokogawai (Fig. 2). In addition, the antibody was specifically recognized purified 17 kDa cysteine proteases from C. sinensis and P. westermani, the 16 kDa cysteine protease of G. seoi, but not the 16 kDa cysteine protease of N. brasiliensis (Fig. 2). The antibody, however, failed to detect any proteins from C. sinensis metacercarial extracts and Schistosoma japonicum extracts (data not shown). These results suggest that the 17 kDa protein is commonly shared by many trematodes but possibly not with cestodes or nematodes. The 17 kDa protein may be a cysteine protease that commonly present in different trematode parasites.