Update on the source of phenoloxidase activity in the hemolymph of kuruma prawn Marsupenaeus japonicus

Crustacean phenoloxidase (PO) and hemocyanin (Hc) are classified as type 3 copper proteins. PO catalyzes the oxidation of mono- and di-phenol compounds, which is the rate-limiting step of melanization, while Hc generally functions as a dioxygen-transporting protein in the hemolymph of arthropods. To date, many studies have shown PO activity in Hc, which is inspired by their structural similarity. Here, the source of PO activity in crustaceans was re-examined by purifying Hc and PO exclusively from the hemolymph of kuruma prawn. The conventional procedure for the preparation of arthropod Hc, which includes precipitation of Hc by ultracentrifugation and subsequent purification by size exclusion chromatography, was not able to completely remove hemolymph-type PO from Hc. In contrast, fractionation with 50% saturation of ammonium sulfate and subsequent hydrophobic chromatography yielded sufficiently pure Hc, which contained no detectable PO protein and virtually no PO enzymatic activity. These results indicate that the main source of PO activity in the hemolymph of kuruma prawn is hemolymph-type PO and that the improved purification method of Hc is preferable for evaluating the PO activity of Hc.