Affinity of Heavy Metal Cations to a Protein in Hepatopancreas of Scallop

In this work, we obtained a protein that passed through an ultrafiltration membrane of 100 kDa and was retained on that of 30 kDa from the hepatopancreas of scallop. We measured the molecular weight of the protein by MALDI-TOF-MS, and evaluated the affinity of metal cations to it. The molecular weight of the protein was found to be around 20 kDa, despite the fact that the proteins were retained on a 30-kDa ultrafiltration membrane. Consequently, we poured a portion of EDTA solution onto the ultrafiltration membrane holding the protein. As a result, it passed through the 30-kDa ultrafiltration membrane. This implies that the protein may be dissociated by removing metal cations. Next, we studied the affinity of Cd2+, Cu2+, Ni2+, and Pb2+ to the protein immobilized on Sepharose (p-Sepharose). Adsorption isotherms were prepared to obtain the adsorption capacity and the adsorption constant by employing the Langmuir model. The order of the adsorption capacity was Cu2+ > Ni2+ > Cd2+ (sic) Pb2+. The mole ratios of metal cations to protein were 0.24 for Cd2+, 2.00 for Cu2+, 0.37 for Ni2+, and 0.24 for Pb2+. Mole ratios less than 1 support a hypothesis derived from MS measurements, that the proteins may associate with each other via metal cations. The order of the adsorption constant was Cu2+ > Cd2+ > Ni2+ > Pb2+. The Cd2+, which has a large ionic radius, has a low adsorption capacity, but does have the second largest adsorption constant. We confirmed a strong affinity of heavy metal cations to the protein in hepatopancreas of scallop.