Iodination of proteins

The following protocol is representative of those found in the literature for iodination of protein molecules using chloramine-T. 

We thank the Royal Society (UK) for a postdoctoral fellowship (GRM) and the University of Otago for a research grant. We are grateful to Michael Crawford for production of biological artwork, Jesse Gale for technical advice on radio-iodination of proteins, Drs Sarah L. Heath and David S. Larsen for helpful discussions, Drs Sally P.A. McCormick, Stephen Faulkner, and Wolfgang Mohr for critically reading this manuscript, Tanya K. Ronson for her help with the references and Dr Katie Heslop for additional help with manuscript production. 

The objectives of investigators using iodination of proteins have been (a) to study the structure function relationships in proteins, (b) to provide a label for proteins in order to investigate their metabolic fate, (c) to provide a method of increasing the sensitivity for assay procedures of proteins such as in radioimmunoassays, and (d) more recently, as a tool for the investigation of the arrangement of proteins in macromolecular structures such as membranes. A number of reviews dealing with various aspects of peroxidase-catalyzed iodination have appeared. 

The iodination of proteins has long been a very popular modification reaction. This reaction, performed under mild conditions, has been applied to such varied ends as the detection of side-chains at the catalytic sites of proteins, investigation of the relative reactivity of tyrosyl side-chains, and the introduction of heavy atoms in connection with X-ray diffraction studies. The availability of two radioactive iodine isotopes, 1 with a half-life of 56 days, and 1 with a half-life of 8 days, has been very extensively exploited in the preparation of hormone derivatives of high specific activity for radioimmunoassay and receptor studies. 

For procedures for non-enzymatic iodination of proteins, most frequently carried out with iodine monochloride, reference should be made to the lucid treatment by Roholt and Pressman (1972). 

In vitro studies on the ability of horseradish peroxidase or lacto-peroxidase to catalyze iodination of proteins at tyrosine residues are becoming quite frequent (244-248). The studies are designed either to provide better understanding of the incorporation of iodine into thyroid proteins or to develop better ways of producing radioactively labeled proteins for tracer studies. An immobilized enzyme system has been developed to achieve this second purpose (248). A large number of proteins incorporate iodine by this method which is superior to iodination with chloroamine-T or IC1. These iodinated proteins should prove invaluable in nutritional studies. 

Several reports have pointed out that iodination of proteins may affect their adsorption to solid surfaces and their chromatographic behavior (40-43). Consequently, we have chosen to determine the quantum yield of unlabeled, adsorbed lysozyme via fluorescence lifetimes. We are currently in the process of modifying our fluorescence equipment to obtain such fluorescence lifetimes. 

Salacinski, R, et al. (1981). Iodination of Proteins, Glycoproteins, and Peptides Using a Solid Phase Oxidizing... 

Iodized molecules with low MW serve as ligands in binding assays, for the indirect iodination of proteins, or as photoaffinity ligands. Typically these are phenol compounds. 

An iodination reaction often creates a mixture of different molecules. For example, the iodination of proteins that contain several tyrosine residues yields monoiodized, di-iodized. 

We thank the Royal Society (UK) for a postdoctoral fellowship (GEM) and the University of Otago for a research grant. We are grateful to Michael Crawford for production of biological artwork, Jesse Gale for technical advice on radio-iodination of proteins,... 

CARRAWAY, 1975). The method has been most widely applied to the covalent labeling of free amino groups of plasma membrane constituents and therefore seems somewhat restrictive and nonspecific, Similarly, the iodination of proteins with 131i is not target specific but certainly it has yielded much insight about transverse asymmetry of membrane macromolecules. 

Iodination of proteins has produced measurable changes in the titration curves. In zein (280), insulin (125), and pepsin (6) the region of the titration curves usually assigned to the phenolic group of tyrosine (pK=10) is displaced in iodinated proteins in the direction of increased acidity by nearly 2 pH units. This is apparently not true for iodinated globin (299). 

---