Purification and Characterization of a Histidine-Rich Glycoprotein That Binds Cadmium from the Blood Plasma of the BivalveMytilus edulis

An unusual cadmium-binding protein was purified for the first time from the blood plasma of the blue mussel,Mytilus edulis.The protein was isolated and purified to homogeneity using ammonium sulfate precipitation and immobilized metal-ion affinity chromatography. It was identified as a glycoprotein with an apparentMrof 63 kDa and a pIof 4.8. Electrophoresis of the protein under denaturing conditions on polyacrylamide gels produced four bands of 35, 37, 39 and 29 kDa. Isoelectric focusing under denaturing conditions produced 12 closely spaced bands with pIs of 4.2 to 5.8, revealing charge microheterogeneity. Molecular proterties (Mrand pI), carbohydrate content (11.6%) and composition, high histidine content (13.7%), as well cadmium-binding property of the protein (approximate logK≥ 5.4) indicated that it is similar to the mammalian histidine-rich glycoprotein, hitherto unreported in aquatic invertebrates. The cadmium-binding ability of the protein was retained even after heat denaturation and polyacrylamide gel electrophoresis.