Conjugation of Chelating Agents to Proteins and Radiolabeling with Trivalent Metallic Isotopes

Peptides and proteins may be tagged with metallic elements in order to use them as imaging reporters or for other applications. The polypeptide of interest is first conjugated to a suitable chelating agent that forms stable complexes with the element of interest. This conjugation step is undertaken either in aqueous or in non-aqueous conditions depending on the solubility of the substrate. For polypeptides of greater than ∼10 kDa in size, this is normally done in aqueous medium. Most commonly the chelators are reacted with lysine amino groups. The protein is first desalted into a suitable buffer at pH 8–9 and a molar excess of a bifunctional chelating agent is added. After a suitable period of incubation, excess, unreacted or hydrolyzed chelator is removed and the protein conjugate is desalted into an acidic buffer. The conjugate can then be tagged by addition of a suitable metal salt followed, if necessary, by removal of unchelated metal. As described in the protocol that follows, the entire conjugation, purification and labeling procedure takes about 2 d.