SDS-induced phenoloxidase activity of hemocyanins from Limulus polyphemus,Eurypelma californicum, and Cancer magister

Phenoloxidase, widely distributed among animals, plants, and fungi, is invo lved in many biologically essential functions including sclerotization and host defense. In chelicerates, the oxygen carrier hemocyanin seems to funct ion as the phenoloxidase. Here, we show that hemocyanins from two ancient c helicerates, the horseshoe crab Limulus polyphemus and the tarantula Eurype lma californicum, exhibit O-diphenoloxidase activity induced by submicellar concentrations of SDS, a reagent frequently used to identify phenoloxidase activity. The enzymatic activity seems to be restricted to only a few of t he heterogeneous subunits. These active subunit types share similar topolog ical positions in the quaternary structures as linkers of the two tightly c onnected 2 x 6-mers. Because no other phenoloxidase activity was found in t he hemolymph of these animals, their hemocyanins may act as a phenoloxidase and thus be involved in the primary immune response and sclerotization of the cuticle. In contrast, hemolymph of a more recent arthropod, the crab Ca ncer magister, contains both hemocyanin with weak phenoloxidase activity an d another hemolymph protein with relatively strong phenoloxidase activity. The chelicerate hemocyanin subunits showing phenoloxidase activity may have evolved into a separate phenoloxidase in crustaceans.