A serine proteinase in the penetration glands of the cercariae of Plagiorchis elegans (Trematoda, Plagiorchiidae).

Cercariae of Plagiorchis elegans secrete a serine proteinase from their penetration glands. The enzyme hydrolyzes both azocoll and gelatin at the optimal pH of 8.4 but is incapable of hydrolyzing elastin at the pH range of 7.2-10.0 Ca2+ and Mg2+ activate it, whereas metal chelators (1 mM EGTA, 1 mM EDTA) and serine proteinase inhibitors (1 mM PMSF and 0.1 mM DFP) act as strong inhibitors. The proteinase activity is insensitive to thiol-blocking compounds and to 5 mM 1,10-phenanthroline, a relatively specific chelator of zinc. It appears, therefore, that the active proteinase represents a metalenzyme complex rather than a metalloenzyme. Being capable of hydrolyzing N-blocked L-alanine-1-naphthylester, N-blocked L-methionine-1-naphthylester, and naphthyl AS-D chloroacetate at pH 6.8, the proteinase is insensitive to 1 mM p-nitrophenyl phosphate, an inhibitor of some mammalian esterproteinases. The enzyme does not split N-blocked DL-phenylalanine-2-naphthylester, nonspecific esterase substrates (1-naphthyl acetate, 1-naphthyl butyrate, 5-bromoindoxyl acetate), or several N-blocked L-aminoacyl- and N-blocked L-peptidyl-naphthylamides bearing L-arginine, L-alanine, L-phenylalanine, L-leucine, or L-proline at the P1 subsite. At operative pH values of 4.8 and 3.8 generated during electrophoresis in a stacking and a resolving gel, respectively, the cercarial proteinase migrates toward the cathode. The separated enzyme produces three bands of proteolysis in a gelatin-containing polyacrylamide gel at the optimal pH of 8.4. The presence of the proteinase in the penetration glands, in the secretory canals, and in the secretion from the glands suggests that the enzyme facilitates penetration, thus enabling the larvae to force their way through tissues of their hosts.