Factors in the Iodination of Histidine in Proteins

Factors that determine the reactivity toward iodine of the histidyl residues of various proteins are compared. The proteins studied include lysozyme, ribonuclease, insulin, lactic, malic and glutamic dehydrogenases, phosphoglucomutase, thyroglobulin, trypsin, chymotrypsin and β‐lactoglobulin. Relatively fewer histidyl residues than tyrosyl residues are iodianated at pH 8.5 both in water and in guanidine. The fraction of iodine present in histidyl residues increases with increasing pH. Diiodohistidine is formed primarily in proteins that do not contain tryptophane. Thyroglobulin can be iodinated to contain 8% histidine iodine. However, mono‐ and diiodohistidine formation in equilibrium‐labelled thyroids accounts for < 1% of the total iodine. Histidine iodination requires the dissociable proton of the imidazole ring as shown by lack of iodination of N ‐methylated model compounds. The assumption that iodination proceeds by an attack on the imidazole anion (rather than via an N‐I intermediate) is supported by the finding that the sterically hindered 2‐ t ‐butyl imidazole is iodinated. It is concluded that, although conformational factors such as electrostatic facilitation and ligand binding influence the reactivity of certain histidyl residues, the major factors in histidyl iodination are the instrinsic properties of the residue.