Acid cysteine protease was purified from metacercariae of the mammalian trematode parasite Purugorzimus wesrevrncrrzi. The purified enzyme had a molecular mass of 27 kDa and was a monomeric polypeptide. The protease had an absolute requirement for a reducing agent for full activity towards fluorescein-isothiocyanate-labeled hemoglobin, and it was active in the acidic pH range, with an optimum pH of 4.0. While acidic proteolysis was insensitive to the aspartic protease inhibitor pepstatin A, activity was significantly inhibited by the cysteine protease inhibitors, leupeptin, chymostatin and L-truns-epoxysuccinyl-~-leucylamido(4-guanidino)-butane. The sensitivity of the enzyme to the inhibitors was similar to that of cathepsins B and L, but the specificity of the protease towards chromogenic substrates was slightly different from that of the cathepsins. The purified enzyme was highly specific for N-substituted peptidyl substrates containing arginine in the P, position and phenylalanine in the P2 position, and the protease extensively degraded human native proteins, such as human serum albumin, immunoglobulins, complement components and also endogenous protease inhibitors. Since the protease hydrolyzes both soluble proteins and components of human defense systems, it may facilitate parasite nutrition and evasion of host defense mechanisms.Keywords: Pumgonimus westevmuni; metacercaria: protease; cysteine protease ; acid proteaseThe mammalian trematode Purugonimus westermuni is a medically important paraaite, causing paragonimiasis in humans. The infective forms of larvae, metacercariae, invade the human abdominal cavity via the intestine and migrate into the thorax. The functions of proteolytic enzymes are of importance during the infection and migration processes of Parugonimus in humans, since histolysis of host tissue proteins and digestion of physiologically significant proteins are related to their biochemical properties [ I , 21. The larvae of P westevrnuni produce at least two distinct cysteine-dependent proteolytic enzymes in terms of their pH profiles and substrate specificities [I]. One is neutral cysteine protease (NCP), which has been characterized in detail and shown to have the ability to degrade physically important proteins such as collagen [2-41. This enzyme could suppress the functions of host immune mechanisms, and its physiological activity appears to be important in considering the infection of humans by larvae IS, 61. The other, an acidic proteolytic activity, which preferentially hydrolyzes hemoglobin in the presence of a reducing agent, was also demonstrated to be present in larval Paragoriimus [ I , 21. It is suspected that the substrate specificity of acidic proteolysis is responsible for the nutrition potential for tional Defense Medical College. Tokorozawa, Saitama 359, Japan the infected parasite in mammalian hosts. Little information. however, is available concerning the precise mechanisms by which this enzyme functions as a key element in parasitism. Study of interactions between larva...