Hemocyanin of the chiton, Stenoplax conspicua (Dall)

1. The hemocyanin of the chiton, Stenoplax conspicua, has a molecular weight determined by light-scattering of 4.2 X 10(6) daltons, (dt) and a sedimentation coefficient of 60 S. 2. The fully dissociated subunits in 6.0 and 8.0 M urea, and at pH 8.9-10 in the absence of divalent ions, have molecular weights of 4.15-4.30 x 10(5) and 4.17-4.75 x 10(5) dt, which is close to one-tenth of the molecular weight of the parent hemocyanin assembly. 3. The pH dependence of the molecular weights from pH 4.5 to 11 exhibit bell-shaped transition profiles, best accounted for by a three-species, decamer to dimer to monomer scheme of subunit dissociation, with one acidic and one basic ionizing group per dimer and 5-8 acidic and basic groups per monomer. 4. In the absence of stabilizing divalent ions S. conspicua hemocyanin is relatively unstable. At pH 7.4 in the presence of 0.01 M EDTA, it is predominantly in the dimeric state, characterized by a sedimentation constant of 18 S. It is also more readily dissociated to monomers at high pHs (8-9 and above) than are the C. stelleri and A. granulata hemocyanins. 5. Urea and GdmCl are effective dissociating agents of S. conspicua hemocyanin. The urea dissociation profile obtained at pH 8.5, 0.01 M Mg2+, 0.01 M Ca2+, and analyzed by means of the decamer-dimer-monomer scheme of subunit dissociation gave estimates of about 30 amino acid groups (Napp) at the dimer contacts within the hemocyanin decamers and about 120 groups per monomer within each dimer, suggesting hydrophobic stabilization of hemocyanin assembly.