Factors in the Iodination of Histidine in Proteins

Abstract

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 #?-lactoglobulin. Relatively fewer histidyl residues than tyrosyl residues are iodianated a t 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 So/, histidine iodine. However, mono-and diiodohist,idine formation in equilibrium-labelled thyroids accounts for < lo/, 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.Indirect evidence for the iodination of histidyl residues in proteins was adduced long ago by Bauer and Strauss [l] for globin and by Fraenkel-Conrat [2] for lysozyme. More recently, the decrease of histidyl residues in hydrolysates of iodoproteins has been taken as evidence that iodohistidyl residues had been formed 13-61, and it is generally held that iodohistidines are destroyed during acid hydrolysis [2,3,5,6]. It is well established, although not generally appreciated, that mono-and diiodohistidyl residues are readily formed during the iodination of a variety of proteins in vitro. Roche et al. [7] demonstrated the presence of iodohistidines in globin and thyroglobulin. With quantitative chromatographic techniques it has been possible to show that monoiodohistidine and/or diiodohistidine are formed in good yield during the iodination of lysozyme [S], pancreatic ribonuclease A [9], fructose 1,6-diphosphate [lo], and insulin [ll]. I n lysozyme and insulin up to one third of the total organic iodine was present in histidyl residues.There was considerable variation in the reactivity of different histidyl residues of the same protein. This was influenced, in part, by the presence of certain hgands believed to bind to histidine [9,11], and presumably in part by conformational factors. We therefore attempted t o assess the importance of histidine iodination in a series of other proteins, and in particular, thyroglobulin; moreover, it was important to decide, if possible, whether incomplete iodination was due primarily to increased reactivity of some residues or low reactivity of the others. We also studied certain aspects of histidine iodination with the hope of differentiating intrinsic react...

A Novel Post-translational Modification Involving Bromination of Tryptophan

“…Co-elution of Natural and Synthetic Bromosleeper Peptide N-terminal (1)(2)(3)(4) Fragment-Approximately equal amounts of the endoproteinase Asp-N-digested N-terminal fragment of the bromosleeper peptide and the synthetic peptide Trp*-Ala-Thr-Ile-OH were mixed (where Trp* ϭ bromotryptophan). The mixture was applied onto a C 18 analytical column and eluted at 1 ml/min using a gradient of acetonitrile in 0.085% trifluoroacetic acid.…”

“…One notable set of modified amino acids are halogenated derivatives of tyrosine and histidine. Naturally occurring halogenated tyrosine and histidine residues have previously been identified from proteins (1)(2)(3)(4). A particularly well documented example of in vivo posttranslational halogenation of an amino acid residue in a protein is afforded by thyroglobulin (5).…”

Studies on Mono- and Diiodohistidine. I. THE IDENTIFICATION OF IODOHISTIDINES FROM THYROIDAL IODOPROTEINS AND THEIR PERIPHERAL METABOLISM IN THE NORMAL MAN AND RAT

“…Because of their low specific activity iodohistidines from normal human iodoproteins could not have been detected by these The high iodohistidine content of Ta as compared with Tg is not explained by the calculated tyrosyl/histidyl residue ratios which, in man, have been found respectively to be 1.0 for Tg and 0.9 for Ta (40). This may be due to known variations in reactivity of the histidyl residue from different proteins (2). Both for Tg and Ta the amount of iodohistidine present is so small that it is usually disregarded as a side product of the normal iodination and hormonogenesis.…”

“…The normal in vivo iodination of histidine is usually overlooked, although monoiodohistidine (MIH ) 1 has been identified in the rat thyroid (1,2). No biosynthetically formed diiodohistidine (DIH) has ever been detected.…”

CLC Cl − /H + transporters constrained by covalent cross-linking

“…For cross-linking in the presence of I 2, it was necessary to remove all traces of imidazole (26). After elution from the Co-affinity column with 400 mM imidazole, the protein was chromatographed by FPLC on a Superdex 200 in cross-link buffer [CB; 100 mM NaCl, 20 mM Tris⅐HCl (pH 7.5), 10 mM decylmaltoside].…”