Serum Phenoloxidase Activity in the Hemolymph of Giant Freshwater Prawn Macrobrachium rosenbergii (De Man, 1879)

Abstract

Melanization is one of the major immune responses in arthropods, particularly crustaceans. The prophenoloxidase (proPO) catalyzes the oxidation of mono or di or polyphenols, a reaction that is the key initial step of melanin formation. We attempted to study the immune response in M. rosenbergii by melanization reaction as produced by serum phenoloxidase (PO) activity. The substrate affinity of the PO enzyme was determined using different phenolic substrates and it was found that the mono, di and poly phenols were oxidized hence the enzyme was characterized as catecholoxidase and l-3,4-dihydroxyphenylalanine (L–DOPA) showed the highest substrate affinity to the enzyme. The biochemical parameters that determined optimum enzyme activity were found such as 1 mM L–DOPA concentration showing at the absorbance of 470 nm, 10 mM Tris–HCl buffer pH 7.5 (brown colour formation), temperature 40 ° C and 10 min incubation. Kinetic characteristics of serum from the prawn were also determined. Determination of optimal conditions of PO activity in the serum has also been attempted. We also identified the serum having a potent PO activity was inhibited by phenylthiourea (PTU) and activation of such as trypsin, sodium dodecyl sulphate (SDS) and laminarin in the freshwater prawn M. rosenbergii.