Functional analysis of plasma prophenoloxidase system in the marine mussel Perna viridis

The role of prophenoloxidase (proPO) system in recognition and phagocytosis of yeast cells by hemocytes was examined in vitro using whole plasma and proPO system isolated from the plasma of the marine mussel, Perna viridis. The proPO was isolated from the plasma by ammonium sulphate precipitation and gel filtration. The final proPO preparation was homogeneous in native PAGE, and could be activated by trypsin, α-chymotrypsin and pronase-E. Laminarin (a polymer of β-1, 3-glucan) and lipopolysaccharides (LPS) from diverse bacterial species effectively activated the isolated proPO, demonstrating the ability of this proenzyme to interact directly with microbial surface components. The susceptibility of proPO activation to inhibition by serine protease inhibitors such as soybean trypsin inhibitor (STI) or p-nitrophenyl-p′-guanidinobenzoate (p-NPGB), indicates that the isolated fraction may contain an integral serine protease domain in an inactive state. The presence of laminarin- or LPS-activated whole plasma of P. viridis facilitated adherence of yeast cells to hemocyte surface as well as eventually stimulated phagocytic uptake of the target cells by hemocytes, and no such hemocytic response was recorded with STI controls. This and other results strongly suggest that the intermediary factors generated during activation of plasma proPO system by non-self molecules play a key role in recognition and opsono-phagocytosis by hemocytes. However, the proPO system isolated from P. viridis plasma, after activation with microbial surface components, failed to show an opsonic effect.