Identification and characterization of proteins with phenoloxidase-like activities in the sea urchin Strongylocentrotus nudus

• Three phenoloxidase (PO)-like proteins were identified in Strongylocentrotus nudus.
• The proteins identified in S. nudus had laccase-type PO activities.
• The proteins identified in S. nudus had extremely small molecular masses.
• The proteins identified in S. nudus were thermophilic.
• The proteins identified in S. nudus had different biochemical characteristics.

Three proteins with PO-like activities in the coelomocytes of sea urchin Strongylocentrotus nudus were identified using electrophoretic method and named as SnPO1, SnPO2 and SnPO3 according to their molecular mass from high to low. The SnPOs were characterized for substrate specificity and the effects of temperature, pH, divalent metal ions and inhibitors on PO activities. They showed oxidative activities to L-3,4-dihydroxyphenylalanine. (l-DOPA), dopamine and hydroquinone, but failed to oxidize tyrosine, which illustrated the three proteins had laccase-like PO activities. The optimum temperature for the activities of SnPO1, SnPO2 and SnPO3 was 75 °C, 70 °C, 40 °C, and the optimum pH was 7.0, 9.0, 8.0, respectively. The SnPOs were notably activated after being incubated in boiled water for 60 min, suggesting that the three proteins are thermophilic. The activity of SnPO1 was greatly enhanced by Cu2+, Mn2+ and Fe2+ and inhibited by Pb2+, Cd2+, EDTA, DETC, sodium sulfite and ascorbic acid, but SnPO2 and SnPO3 were not obviously affected by Pb2+ and Cd2+, suggesting the three proteins are copper-containing, and the catalytic properties of SnPO1 might be different from those of SnPO2 and SnPO3. Taken together, SnPO1, SnPO2 and SnPO3 might play different roles in the immune and physiological processes of S. nudus.