Purification and characterization of phenoloxidase from clam Ruditapes philippinarum

Using L-dihydroxyphenylalanine (L-DOPA) as a specific substrate, phenoloxidase (PO) from clam (Ruditapes philippinarum) was purified by Q Sepharose Fast Flow ion-exchange chromatography and Sephacryl S-100 gel-filtration, and characterized biochemically and enzymatically in this study. The molecular mass of PO in SDS-PAGE is about 76.9 kDa, and the prophenoloxidase (proPO) molecule, isolated as a monomeric protein, is 84.1 kDa. The PO molecule had a high oxidative activity, and the proPO molecule had almost no oxidative activity. The PO activity was optimal at pH 7.0 and temperature of 40 degrees C. The Km value of the PO for L-DOPA was 2.2 mmol l(-1). The PO was extremely sensitive to benzoic acid and sodium sulfite, very sensitive to citric acid, thio urea, 1-phenyl-2-thiourea and cysteine, but not sensitive to ascorbic acid. Combined with its specific enzyme activity on tyrosine and L-DOPA, it can be concluded that the Ruditapes PO is probably a kind of tyrosinase-type phenoloxidase. The PO activity was strongly inhibited by ethylenediaminetetraacetic acid (EDTA), diethyldithiocarbamate (DETC), Zn2+, Ca2+ and Cu2+, as well as by Mg2+. The results with EDTA, DETC, and some metal ions, combined with the perfect recovery effect of Cu2+ on DETC-inhibited PO activity, indicate that Ruditapes PO is most probably a copper-containing metalloenzyme.