Detection and characterization of phenoloxidase in the freshwater planarian Dugesia japonica

Freshwater planarians have traditionally been favored animal models in regenerative and developmental biology. Little is known about the immunological research in these animals. Here, we provide spectrophotometric, electrophoretic and histochemical evidence demonstrating the presence and characterization of phenoloxidase (EC 1.14.18.1; PO) activity in the planaria Dugenia japonica. PO activity was optimal at pH 7.5 and at a temperature of 45 °C. ProPO in the humoral fluid was readily activated by trypsin and zymosan, and was susceptible to phenylthiourea. Two bands of PO activity in the humoral fluid with 150 and 300 kDa by native PAGE and a single band with 150 kDa after SDS-PAGE were resolved. Enzyme histochemical staining demonstrated the presence of PO in the epidermal cell and pharyngeal epithelium, which indicate that PO could serve as a protecting agent against environmental pathogens in D.japonica.