An enzymic method for the trace iodination of immunoglobulins and other proteins

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Abstract
1. A method is described for the trace iodination of immunoglobulins and other serum proteins by a system consisting of lactoperoxidase, hydrogen peroxide and iodide. 2. γG immunoglobulin that had been labelled to a specific radioactivity of 5μc/μg. by use of carrier-free [125I]iodide gave no evidence of denaturation when analysed by electrophoresis and density-gradient ultracentrifugation. 3. Tryptic hydrolysis and peptide ‘mapping’ of a completely characterized peptide radioiodinated by this method showed that the [125I]iodide was bound to tyrosyl residues. 4. Proteins differ in their susceptibility to iodination by this method. Human γG immunoglobulin, for example, is iodinated more than ten times as readily as is human α2-macroglobulin under the same conditions. 5. Lactoperoxidase catalyses the iodination of proteins much more readily than does horseradish peroxidase.