Hemocyanin of the horseshoe crab, Limulus polyphemus. Structural differentiation of the isolated components.

The high molecular weight hemocyanin found in the hemolymph of the horseshoe crab, Limulus polyphemus, is composed of at least eight different kinds of subunits. Ion exchange chromatography at high pH in the presence of EDTA yields five major zones, hemocyanins I to V, three of which are electrophoretically heterogeneous. The subunits have similar molecular weights, 65,000 to 70,000, and their amino acid compositions are remarkably similar to each other and to other arthropod and molluscan hemocyanins. Digestion of the native subunits of Limulus hemocyanin by formic acid or trypsin shows considerable structural diversity which is supported by cyanogen bromide cleavage patterns and by peptide mapping of the tryptic peptides prepared from denatured hemocyanin subunits. The structural differentiation of the subunits is accompanied by functional differentiation, as shown in previous investigations of their O2 and CO affinities (Sullivan, B., Bonaventura, J., and Bonaventura, C. (1974) Proc. Natl. Acad. Sci. U.S.A. 71, 2558-2562; Bonaventura, C., Bonaventura, J., Sullivan, B., and Bourne, S. (1975) Biochemistry 13, 4784-4789). The subunit diversity of Limulus hemocyanin suggests that other electrophoretically heterogeneous hemocyanins may be composed of structurally distinct subunits.