Effects of β-l, 3-glucan on the Activation of Prophenoloxidase Cascade in Penaeus japonicus Hemocyte

Phenoloxidase (PO) in crustaceans is believed to be derived from its precursor prophenoloxidase (proPO), however, the conversion mechanism remains unclear. Possible roles of β-l, 3-glucan, laminarin, of hemocyte lysate supernatant (HLS) from Penaeus japonicus in proPO activation were studied. PO activity was induced in HLS from Penaeus japonicus by treatment of laminarin, although laminarin had no influence on purified PO and proPO. p-APMSF and leupeptin, typical serine protease inhibitors, completely hampered this activation. Laminarin also enhanced activity of protease(s) in HLS, which specifically hydrolyzes synthetic peptides containing Arg in their P1 sites, while the activity was significantly suppressed by the serine protease inhibitors. These results indicate that the activation ofproPO in hemocyte of Penaeus japonicus may be achieved after at least three steps: that is the recognition of β-l, 3-glucan, activation of serine protease, and processing of proPO.