[54] Radiolabeling of proteins

Proteins may be labeled for use in immunoassays, as substrates for degradative enzymes, in binding experiments with receptors or other binding proteins in tissues, or to determine their distribution and metabolic fate in vivo. Regardless of the purpose, it is important to minimize denaturation and biologic inactivation. The high specific activities obtainable for iodine isotopes largely explain the great popularity of radioiodine as an indicator molecule for proteins. As an alternative, enzyme immunoassays that utilize fluorogenic or chemilucinogenic substrates or those that are coupled to a second enzyme system for amplification, can have comparable sensitivities and are increasingly being utilized. Regardless of the iodination procedure, precautions should be taken for the possible release of radioactive vapor, which is a potential inhalation hazard during the period of time agents, such as chloramine-T are active. Iodination reactions should be capped and performed in an efficient fume hood behind lead shielding. Direct iodination methods result in most of the radioactivity being incorporated into tyrosyl and histidyl residues. Losses of immune reactivity may be because of the direct steric effects of introduced iodine atoms on amino acids essential for binding, to oxidative damage occurring in the presence of various oxidized iodine species or the iodinating reagent itself, or to changes in protein charge associated with iodine substitution.