Purification and characterization of a histidine-rich glycoprotein that binds cadmium from the blood plasma of the bivalve Mytilus 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 immobi lized metal-ion affinity chromatography. It was identified as a glycoprotei n with an apparent M-r of 63 kDa and a pI of 4.8. Electrophoresis of the pr otein under denaturing conditions on polyacrylamide gels produced four band s of 35, 37, 39 and 29 kDa. Isoelectric focusing under denaturing condition s produced 12 closely spaced bands with pls of 4.2 to 5.8, revealing charge microheterogeneity. Molecular proterties (M-r and pI), carbohydrate conten t (11.6%) and composition, high histidine content (13.7%), as well cadmium- binding property of the protein (approximate log K greater than or equal to 5.4) indicated that it is similar to the mammalian histidine-rich glycopro tein, hitherto unreported in aquatic invertebrates. The cadmium-binding abi lity of the protein was retained even after heat denaturation and polyacryl amide gel electrophoresis. (C) 1999 Academic Press.