INVOLVEMENT OF ALPHA-2-MACROGLOBULIN AND C-REACTIVE PROTEIN IN A COMPLEMENT-LIKE HEMOLYTIC SYSTEM IN THE ARTHROPOD, LIMULUS-POLYPHEMUS

Homologues of two plasma proteins of vertebrates, alpha2-macroglobulin and C-reactive protein, participate in a hemolytic system of the anci ent arthropod, Limulus polyphemus. C-reactive protein, which can under the appropriate circumstances activate the classical pathway of the m ammalian complement system, is an essential element of the hemolytic s ystem of Limulus. The selective removal of C-reactive protein from the plasma with phosphorylethanolamine-agarose inactivated hemolysis. Add ition of affinity-purified C-reactive protein to inactive plasma resto red activity. Exposure of plasma to phosphorylethanolamine in solution potentiated hemolysis. Alpha2-macroglobulin is a member of the same p rotein family as the complement protein C3 and both require an intact thiol ester for activity. Treatment of Limulus plasma with methylamine under conditions that inactivate thiol-ester-containing proteins redu ced the hemolytic activity of some plasma preparations. Addition of pu rified Limulus alpha2-macroglobulin to the methylamine-treated plasma restored hemolytic activity. However, alpha2-macroglobulin is not nece ssary for hemolysis since the hemolytic activity of some pooled plasma preparations was insensitive to methylamine treatment under condition s that inactivated alpha2-macroglobulin. Purified C-reactive protein w as hemolytic in the absence of alpha2-macroglobulin. These observation s suggest that the proteins in Limulus plasma that participate in hemo lysis represent the components of an ancient invertebrate defense syst em with distant evolutionarily affinities to the vertebrate complement system.