Cysteine protease is a major component in the excretory/secretory products of Euclinostomum heterostomum (Digenea: Clinostomidae).

Cysteine proteases of parasite organisms play numerous indispensable roles in tissue penetration, feeding, immunoevasion, virulence, egg hatching and metacercarial excystment. They are critical key enzymes in the biology of parasites and have been exploited as serodiagnostic markers, therapeutic and vaccine targets. In the present study, the cysteine proteases in the in vitro released excretory/secretory (E/S) products of the digenetic trematode parasite, Euclinostomum heterostomum have been analysed. The encysted progenetic metacercariae of E. heterostomum collected from the infected liver and kidney of Channa punctatus were excysted in vitro and incubated in phosphate buffer at 37 ± 1 °C, and the E/S products released were analysed. The spectrophotometric analysis of the proteases revealed active hydrolysis of chromogenic substrate, azocoll, in a time-, temperature- and pH-dependent manner. Optimum activity was observed at pH 7.0 at 37 ± 1 °C, and with 1 mM each of various protease inhibitors (Mini Protease Inhibitor Cocktail, ethylene diaminetetraacetic acid, phenyl methyl sulphonyl fluoride, iodoacetamide and 1,10-phenanthroline) used, significant inhibition was observed by iodoacetamide and 85% of inhibition at a concentration of 2 mM, suggesting that cysteine protease is a major component in the E/S of this parasite. Four discrete protease bands of Mr 36, 39, 43 and 47 kDa were identified by gelatin-substrate zymography. Maximum gelatinolytic activity was observed at pH 7.0, and among various inhibitors used, almost complete disappearance of protease bands was observed by 2 mM iodoacetamide. The proteolytic cleavage of bovine serum albumin, bovine haemoglobin and human haemoglobin in vitro were also studied.