Tyrosyl Peptides Research Articles

Conjugation of Glutathione to Oxidized Tyrosine Residues in Peptides and Proteins

Tyrosine residues are sensitive to oxidation and can be converted to hydroperoxides either by superoxide reacting with the Tyr radical or by singlet oxygen. These hydroperoxides rearrange to bicyclic derivatives that are readily reduced to more stable hydroxides. The aromatic character of tyrosine is lost, but the product contains an α-β unsaturated carbonyl group and is, therefore, an electrophile. We have generated hydroxide derivatives of several Tyr-containing peptides and shown using liquid chromatography/mass spectrometry that they undergo Michael addition with GSH. For Tyr-Gly, rate constants of 9.2 and 11.8 m(-1)min(-1) were measured for the two chromatographically distinct isomers. Unusual for GSH addition to an electrophile, the reaction is reversible, with a half-life of many hours for the reverse reaction. These kinetics indicate that with a typical cellular concentration of 5 mm GSH, >95% Tyr-Gly hydroxide would become conjugated with a half-life of ∼15 min. Sperm whale myoglobin forms a hydroperoxide on Tyr-151 in a hydrogen peroxide/superoxide-dependent reaction. We show that its hydroxide derivative reacts with GSH to form a conjugate. Detection of the conjugate required stabilization by reduction; otherwise, the reverse reaction occurred during tryptic digestion and analysis. Our findings represent a novel mechanism for peptide or protein glutathionylation involving a carbon-sulfur cross-link between oxidized Tyr and Cys. As with other electrophiles, the oxidized Tyr should undergo a similar reaction with Cys residues in proteins to give intramolecular or intermolecular protein cross-links. This mechanism could give rise to protein cross-linking in conditions of oxidative stress.

Superoxide-mediated Formation of Tyrosine Hydroperoxides and Methionine Sulfoxide in Peptides through Radical Addition and Intramolecular Oxygen Transfer

The chemistry underlying superoxide toxicity is not fully understood. A potential mechanism for superoxide-mediated injury involves addition to tyrosyl radicals, to give peptide or protein hydroperoxides. The rate constant for the reaction of tyrosyl radicals with superoxide is higher than for dimerization, but the efficiency of superoxide addition to peptides depends on the position of the Tyr residue. We have examined the requirements for superoxide addition and structurally characterized the products for a range of tyrosyl peptides exposed to a peroxidase/O(2)(.) system. These included enkephalins as examples of the numerous proteins and physiological peptides with N-terminal tyrosines. The importance of amino groups in promoting hydroperoxide formation and effect of methionine residues on the reaction were investigated. When tyrosine was N-terminal, the major products were hydroperoxides that had undergone cyclization through conjugate addition of the terminal amine. With non-N-terminal tyrosine, electron transfer from O(2)(.) to the peptide radical prevailed. Peptides containing methionine revealed a novel and efficient intramolecular oxygen transfer mechanism from an initial tyrosine hydroperoxide to give a dioxygenated derivative with one oxygen on the tyrosine and the other forming methionine sulfoxide. Exogenous amines promoted hydroperoxide formation on tyrosyl peptides lacking a terminal amine, without forming an adduct. These findings, plus the high hydroperoxide yields with N-terminal tyrosine, can be explained by a mechanism in which hydrogen bonding of O(2)(.) to the amine increases is oxidizing potential and alters its reactivity. If this amine effect occurred more generally, it could increase the biological reactivity of O(2)(.) and have major implications.

The effect of neighboring amino acid residues and solution environment on the oxidative stability of tyrosine in small peptides.

The effects of neighboring residues and formulation variables on tyrosine oxidation were investigated in model dipeptides (glysyl tyrosine, N-acetyl tyrosine, glutamyl tyrosine, and tyrosyl arginine) and tripeptide (lysyl tyrosyl lysine). The tyrosyl peptides were oxidized by light under alkaline conditions by a zero-order reaction. The rate of the photoreaction was dependent on tyrosyl pK(a), which was perturbed by the presence of neighboring charged amino acid residues. The strength of light exposure, oxygen headspace, and the presence of cationic surfactant, cetyltrimethylammonia chloride had a significant effect on the kinetics of tyrosyl photo-oxidation. Tyrosine and model tyrosyl peptides were also oxidized by hydrogen peroxide/metal ions at neutral pH. Metal-catalyzed oxidation followed first-order kinetics. Adjacent negatively charged amino acids accelerated tyrosine oxidation owing to affinity of the negative charges to metal-ions, whereas positively charged amino acid residues disfavored the reaction. The oxidation of tyrosine in peptides was greatly affected by the presence of adjacent charged residues, and the extent of the effect depended on the solution environment.

Mutant rat trypsin selectively cleaves tyrosyl peptide bonds

A double mutant of rat trypsinogen (Asp189Ser, ΔAsp223) was constructed by site-directed mutagenesis. The recombinant protein was produced in Escherichia coli under the control of a periplasmic expression vector. The purified and enterokinase-activated enzyme was characterized by synthetic fluorogenic tetrapeptide and natural polypeptide substrates and by a recently developed method. In case of this latter method the specificity profile of the enzyme was examined by simultaneous digestion of a mixture of oligopeptide substrates each differing only at the P 1 site residue, and the results were analyzed by high-performance liquid chromatography. All these assays unanimously demonstrated that the recombinant proteinase lacks trypsin-like activity but acquired a rather unique selectivity: it preferentially hydrolyses peptide bonds C-terminal to tyrosyl residues. This narrow specificity should be useful in peptide-analytical applications such as sequence-specific fragmentation of large proteins prior to sequencing.

Transmembrane Nitration of Hydrophobic Tyrosyl Peptides: LOCALIZATION, CHARACTERIZATION, MECHANISM OF NITRATION, AND BIOLOGICAL IMPLICATIONS

We have shown previously that peroxynitrite-induced nitration of a hydrophobic tyrosyl probe is greater than that of tyrosine in the aqueous phase (Zhang, H., Joseph, J., Feix, J., Hogg, N., and Kalyanaraman, B. (2001) Biochemistry 40, 7675-7686). In this study, we have tested the hypothesis that the extent of tyrosine nitration depends on the intramembrane location of tyrosyl probes and on the nitrating species. To this end, we have synthesized membrane spanning 23-mer containing a single tyrosyl residue at positions 4, 8, and 12. The location of the tyrosine residues in the phospholipid membrane was determined by fluorescence and electron spin resonance techniques. Nitration was initiated by slow infusion of peroxynitrite, co-generated superoxide and nitric oxide ((.)NO), or a myeloperoxidase/hydrogen peroxide/nitrite anion (MPO/H(2)O(2)/NO(2)(-)) system. Results indicate that with slow infusion of peroxynitrite, nitration of transmembrane tyrosyl peptides was much higher (10-fold or more) than tyrosine nitration in aqueous phase. Peroxynitrite-dependent nitration of tyrosyl-containing peptides increased with increasing depth of the tyrosyl residue in the bilayer. In contrast, MPO/H(2)O(2)/ NO(2)(-)-induced tyrosyl nitration decreased with increasing depth of tyrosyl residues in the membrane. Transmembrane nitrations of tyrosyl-containing peptides induced by both peroxynitrite and MPO/H(2)O(2)/NO(2)(-) were totally inhibited by (.)NO that was slowly released from spermine NONOate. Nitration of peptides in both systems was concentration-dependently inhibited by unsaturated fatty acid. Concomitantly, an increase in lipid oxidation was detected. A mechanism involving (.)NO(2) radical is proposed for peroxynitrite and MPO/H(2)O(2)/NO(2)(-)-dependent transmembrane nitration reactions.

Oxidative phenol coupling—tyrosine dimers and libraries containing tyrosyl peptide dimers

The principles of phenol oxidation are outlined and summarized. Various procedures were used to dimerize tyrosine derivatives, especially linear tripeptides and cyclohexapeptides, via cross-linking of their phenolic side-chains. The coupling was performed by potassium hexacyanoferrate(III), phenyliodine(III)-bis(trifluoroacetate), vanadium(V)-oxyfluoride or horseradish peroxidase. The individual dityrosine and isodityrosine derivatives obtained in preparative scale, as well as the peptide-dimer libraries generated, were characterized by HPLC and electrospray ionization mass spectrometry. Further analyses were carried out by NMR spectroscopy, fluorescence spectroscopy and gas chromatography.

Tyrosine Versus Serine/Threonine Phosphorylation by Protein Kinase Casein Kinase-2: A STUDY WITH PEPTIDE SUBSTRATES DERIVED FROM IMMUNOPHILIN Fpr3

Protein kinase casein kinase-2 (CK2) is a spontaneously active, ubiquitous, and pleiotropic enzyme that phosphorylates seryl/threonyl residues specified by multiple negatively charged side chains, the one at position n + 3 being of crucial importance (minimum consensus S/T-x-x-E/D/S(P)/T(P). Recently CK2 has been reported to catalyze phosphorylation of the yeast nucleolar immunophilin Fpr3 at a tyrosyl residue (Tyr(184)) fulfilling the consensus sequence of Ser/Thr substrates (Wilson, L.K., Dhillon, N., Thorner, J., and Martin, G.S. (1997) J. Biol. Chem. 272, 12961-12967). Here we show that, by contrast to other tyrosyl peptides fulfilling the consensus sequence for CK2, a peptide reproducing the sequence around Fpr3 Tyr(184) (DEDADIY(184)DEEDYDL) is phosphorylated by CK2, albeit with much higher K(m) (384 versus 4. 3 microM) and lower V(max) (8.4 versus 1,132 nmol.min(-1).mg(-1)) than its derivative with Tyr(184) replaced by serine. The replacement of Asp at position n + 1 with alanine and, to a lesser extent, of Ile at n - 1 with Asp are especially detrimental to tyrosine phosphorylation as compared with serine phosphorylation, which is actually stimulated by the Ile to Asp modification. In contrast the replacement of Glu at n + 3 with alanine almost suppresses serine phosphorylation but not tyrosine phosphorylation. It can be concluded that CK2 is capable to phosphorylate, under special circumstances, tyrosyl residues, which are specified by structural features partially different from those that optimize Ser/Thr phosphorylation.

Alcoholysis and Strand Joining by the Flp Site-specific Recombinase: MECHANISTICALLY EQUIVALENT REACTIONS MEDIATED BY DISTINCT CATALYTIC CONFIGURATIONS

The strand joining step of recombination mediated by the Flp site-specific recombinase involves the attack of a 3'-phosphotyrosyl bond by a 5'-hydroxyl group from DNA. The nucleophile in this reaction, the 5'-OH, can be substituted by glycerol or other polyhydric alcohols. The strand joining and glycerolysis reactions are mechanistically equivalent and are competitive to each other. The target diester in strand joining can be a 3'-phosphate covalently linked either to a short tyrosyl peptide or to the whole Flp protein via Tyr-343. By contrast, only the latter type of 3'-phosphotyrosyl linkage is a substrate for glycerolysis. As a result, in activated DNA substrates (containing the scissile phosphate linked to a short Flp peptide), Flp(Y343F) can mediate the joining reaction utilizing the 5'-hydroxyl attack but fails to promote glycerolysis. Wild type Flp promotes both reactions in these substrates. The strand joining and glycerolysis reactions are absolutely dependent on the catalytic histidine at position 305 of Flp. Our results fit into a model in which a Flp dimer, with one monomer covalently attached to the 3'-phosphate, is essential for orienting the target diester or the nucleophile (or both) during glycerolysis. The requirement for this dimeric complex is relaxed in the strand joining reaction because of the ability of DNA to orient the nucleophile (5'-OH) by complementary base pairing. The experimental outcomes described here have parallels to the "cleavage-dependent ligation" carried out by a catalytic variant of Flp, Flp(R308K) (Zhu, X.-D., and Sadowski, P. D. (1995) J. Biol. Chem. 270, 23044-23054).

Peroxidase-release associated with phagocytosis in Mytilus galloprovincialis haemocytes

Fluorescence spectra of the supernatant of adherent haemocyte monolayers from Mytilus galloprovincialis, supplemented with homovanillic acid or with a tyrosyl peptide glycylglycyltyrosine (GGY), were recorded before and after stimulation by zymosan. The formation of fluorescent derivatives was observed to have spectral characteristics similar to those of fluorescent compounds generated by the exposure of homovanillic acid or GGY to a horseradish peroxidase/hydrogen peroxide system in vitro. Lucigenine-enhanced chemiluminescence (CL luc) of M. galloprovincialis haemocytes stimulated by zymosan or by phorbol ester (PMA) was measured in the presence and absence of sodium azide, a peroxidase inhibitor. Sodium azide inhibited the CL luc of haemocytes stimulated by zymosan, an effective stimulus for myeloperoxidase secretion in human polymorphonuclear leukocytes, but not the CL luc of haemocytes stimulated by PMA, indicating the presence of peroxidases with some properties of myeloperoxidase, in adherent haemocytes from M. galloprovincialis.

The yeast site-specific recombinase Flp mediates alcoholysis and hydrolysis of the strand cleavage product: mimicking the strand-joining reaction with non-DNA nucleophiles

The yeast site-specific recombinase Flp is covalently linked to DNA via a 3′-phosphotyrosyl bond during the strand-breakage step of recombination. We show that this phosphotyrosyl diester bond formed between Flp and DNA can serve as the target for alcoholysis or hydrolysis in an Flp-assisted reaction. Flp does not mediate alcoholysis of the labile phosphodiester bond within the DNA chain under our assay conditions. The body of available evidence supports the notion that the alcoholysis/hydrolysis reaction is mechanistically analogous to the strand-joining step of the recombination pathway. The only difference is that the DNA 5′-hydroxyl group that acts as the nucleophile during recombination is substituted by a non-DNA nucleophile. We find that the alcoholysis reaction occurs only within the normal cleavage complex produced by the “shared active site” assembled at the interface of two Flp monomers. Unlike the strand-joining reaction, alcoholysis does not occur on an activated DNA substrate linked at its 3′-phosphate end to a short tyrosyl peptide (not to the full-length Flp), and bound non-covalently by a Flp monomer. However, even in this substrate that mimics the strand-cleaved state, the joining reaction is competitively inhibited by a polyhydric alcohol such as glycerol.

An antiestrogen: a phosphotyrosyl peptide that blocks dimerization of the human estrogen receptor.

We have previously identified tyrosine-537 as a constitutively phosphorylated site on the human estrogen receptor (hER). A 12-amino acid phosphotyrosyl peptide containing a selected sequence surrounding tyrosine-537 was used to investigate the function of phosphotyrosine-537. The phosphotyrosyl peptide completely blocked the binding of the hER to an estrogen response element (ERE) in a gel mobility shift assay. Neither the nonphosphorylated tyrosyl peptide nor an unrelated phosphotyrosyl peptide previously shown to inhibit the signal transducers and activators of transcription factor (STAT) blocked binding of the hER to the ERE. The hER phosphotyrosyl peptide was shown by molecular sizing chromatography to dissociate the hER dimer into monomers. The hER specifically bound the 32P-labeled phosphotyrosyl peptide, indicating that the inhibition of ERE binding was caused by the phosphotyrosyl peptide binding directly to the hER and blocking dimerization. These data suggest that the phosphorylation of tyrosine-537 is a necessary step for the formation of the hER dimer. In addition, we propose that the dimerization of the hER occurs by a previously unrecognized Src homology 2 domain (SH2)-like phosphotyrosyl coupling mechanism. Consequently, the phosphotyrosyl peptide represents a class of antagonists that inhibits estrogen action by a mechanism other than interacting with the receptor's hormone binding site.

The utility of the prodrug approach to improve peptide absorption

A major obstacle to the application of peptides as clinically useful drugs is their poor biomembrane penetration, rapid enzymatic degradation and short biological half-lives. A possible approach to solve these delivery problems of peptide drugs is derivatization of the peptides to produce prodrugs or transport forms which are more lipophilic than the parent peptides and capable of protecting these against degradation by enzymes present at the mucosal barrier or in the blood. The potential utility of this prodrug approach to protect peptides against degradation by various proteolytic enzymes is illustrated with a number of examples including prodrugs of the peptide bond itself, the C-terminal amide group in peptide amides, the tyrosine phenol group in tyrosyl peptides and the N-terminal amino group. Lipophilic prodrugs of the tripeptide TRH have been found useful to achieve transdermal delivery of the peptide. It is concluded that the prodrug technology may offer a promising means to overcome the enzymatic and penetration barriers in the delivery of several peptide drugs.

Hormone formation in the isolated fragment 1–171 of human thyroglobulin involves the couple tyrosine 5 and tyrosine 130

The 22 kDa fragment (Asn 1—Met 171) purified from iodine-poor human thyroglobulin (hTg) is capable by itself to synthesize thyroxine at Tyr 5, the preferential hormonogenic acceptor site of the protein, after iodination in vitro. To identify the corresponding donor site in this model we studied the fate of the six Tyr residues present in the 22 kDa peptide after in vitro hormone synthesis. Structural studies of the tyrosyl peptides showed that Tyr 5 was the only thyroxine-forming site, the other tyrosines (29, 89, 97 and 107) were noniodinated and Tyr 130 was recovered in alanine form after CNBH 4 treatment of the Tyr 130-containing peptide. Taking into account that alanine could arise from aminoreduced pyruvate species, these results showed that in the 22 kDa fragment (1) hormone formation involves the couple Tyr 5 (acceptor)-Tyr 130 (donor), and (2) dehydroalanine, the resultant product of donor tyrosine after hormone synthesis, has evolved in pyruvoyl form. To test whether Tyr 130 could also act as donor in hTg hormone synthesis, the 22 kDa peptide was isolated from hTg iodinated under conditions leading to iodotyrosine formation followed or not by hormone formation and the tyrosyl peptides were analyzed. After hTg iodination and before coupling (i.e. hormone synthesis) only Tyr 5 and Tyr 130 were recovered in iodotyrosine form; after coupling thyroxine was found at Tyr 5 whereas Tyr 130 disappeared. Taken together these results, correlated with the previously reported cleavage of hTg chain at Tyr 130 occurring during in vivo hormone synthesis, support the theory that the couple Tyr 5 (acceptor)-Tyr 130 (donor) would be the preferential hormonogenic site in human Tg.

Transport of peptides in renal brush border membrane vesicles. Suitability of 125I-labelled tyrosyl peptides as substrates

Three tyrosine-containing peptides (Tyr-Pro, Tyr-Pro-Phe and Tyr-Pro-Phe-Pro) were [ 125I]iodo-labelled and their uptake characteristics in renal brush border membrane vesicles was investigated to assess the suitability of these peptides as substrates in peptide transport studies. Hydrolysis of these peptides during uptake measurements was avoided by using brush border membrane vesicles prepared from the kidneys of Japan Fisher 344 rats which genetically lack dipeptidylpeptidase IV activity. The uptake of 125I-Tyr-Pro and 125I-Tyr-Pro-Phe in these membrane vesicles was found to be stimulated by an inwardly directed H + gradient. In both cases, the time course of uptake exhibited an overshoot, an indication for active transport. The uptake processes were electrogenic since they were stimulated by an inside-negative membrane potential. Free amino acids had no effect on the uptake of Tyr-Pro and Tyr-Pro-Phe whereas many di- and tripeptides effectively blocked the uptake. These results demonstrated that the [ 125I]iodo-labelled Tyr-Pro and Tyr-Pro-Phe are suitable substrates for peptide transport studies because their uptake characteristics clearly suggest participation of the peptide transporter in their uptake. In contrast, the uptake of 125I-Tyr-Pro-Phe-Pro was not active, and was not stimulated by the H + gradient or by the membrane potential. In addition, competition experiments with unlabelled amino acids and peptides indicated that the uptake did not occur via the peptide transporter. However, when allowed to be hydrolyzed to 125I-Tyr-Pro and Phe-Pro by using dipeptidylpeptidase IV-positive renal brush border membrane vesicles prepared from USA Fisher 344 rats, the radiolabel from the [ 125I]iodo-labelled tetrapeptide was found to be taken up into the vesicles via the peptide transporter. These data provide direct evidence to show that intact tetrapeptides are not substrates for the renal peptide transporter.

Prodrugs of peptides. 14. Bioreversible derivatization of the tyrosine phenol group to effect protection of tyrosyl peptides against α-chymotrypsin

Various derivatives of the tyrosine phenol group in N-protected tyrosine amides and esters, used as models of tyrosyl peptides, were synthesized to assess their suitability as prodrug forms with the aim of protecting the tyrosyl peptide bond against cleavage by α-chymotrypsin. The derivatives studied included aliphatic and aromatic carboxylic acid esters, a glutarate ester, carbonate esters and carbamates. All derivatives greatly improved the stability of the parent tyrosine compounds toward hydrolysis by α-chymotrypsin but some esters were themselves readily attacked by the enzyme. Thus, the usefulness of this prodrug approach is greatly dependent on the type of derivatization performed. Useful derivatives were found to include aliphatic carboxylic acid and carbonate esters with a short or branched side chain, a glutarate ester and carbamate esters. Most of these esters are readily bioreversible, the conversion to the parent peptide being catalyzed by plasma or liver esterases.

Selective alteration of substrate specificity by replacement of aspartic acid-189 with lysine in the binding pocket of trypsin.

To test the role of Asp-189 which is located at the base of the substrate binding pocket in determining the specificity of trypsin toward basic substrates, this residue was replaced with a lysine residue by site-directed mutagenesis. Both rat trypsinogen and Lys-189 trypsinogen were expressed and secreted into the periplasmic space of Escherichia coli. The proteins were purified to homogeneity and activated by porcine enterokinase, and their catalytic activities were determined on natural and synthetic substrates. Lys-189 trypsin displayed no catalytic activity toward arginyl and lysyl substrates. Further, there was no compensatory change in specificity toward acidic substrates; no cleavage of aspartyl or glutamyl bonds was detected. Additional studies of substrate specificity involving gas-phase sequence analyses of digested natural substrates revealed an inherent but low chymotrypsin-like activity of trypsin. This activity was retained but modified by the Asp to Lys change at position 189. In addition to hydrolyzing phenylalanyl and tyrosyl peptide bonds, the mutant enzyme has the unique property of cleaving leucyl bonds. On the basis of computer graphic modeling studies of the Lys-189 side chain, it appears that the positively charged NH2 group is directed outside the substrate binding pocket. The resulting hydrophobic cavity may explain the altered substrate specificity of the mutant enzyme. The relatively low chymotrypsin-like activity of both recombinant enzymes may be due to distorted positioning of the scissile bond with respect to the catalytic triad rather than to the lack of sufficient interaction between the hydrophobic side chains and the substrate binding pocket of the enzyme.

Pentafluorobenzenesulfonyl chloride: a new electrophoric derivatizing reagent with application to tyrosyl peptide determination by gas chromatography with electron capture detection.

La detection de 100 fg du dipeptide derive, N-pivaloyl-o-[(pentafluorophenyl)sulfonyl] glycyltyrosine ethyl ester abaisse la limite de detection pour les peptides par chromatographie en phase gazeuse

Iodination by thyroid peroxidase

The Iodination of tyrosyl moieties in thyroglobulin and other proteins is catalyzed by the thyroid peroxidase (TPO), an integral membrane, heme glycoprotein. Tyrosine and tyrosyl peptides can also be iodinated by TPO. The iodination reaction requires H 2 O 2 , I - , an enzyme-associated iodinating intermediate (TPO-I oxid ), and an iodide acceptor, such as free or protein-bound tyrosine. The TPO-catalyzed reaction produces both free and protein-bound mono and diiodotyrosine. TPO can also catalyze the formation of tetraiodothyronine by a coupling mechanism. In the presence of I - and H 2 O 2 , TPO catalyzes the iodination of tyrosyl moieties on proteins such as thyroglobulin and bovine serum albumin (BSA). For routine assays during purification, BSA is used as an I - acceptor. The protein-bound, iodinated tyrosines and thyronines can be identified by paper chromatography after Pronase digestion in the presence of 4m M 1-methyl-2-mercaptoimidazole (MMI). The tripeptide Glu-Tyr-Glu has also been used in TPO assays and the iodinated product has been identified by thin-layer chromatography and autoradiography.

Crosslink precursors for the dipteran puparium

During sclerotization of puparial proteins, tyrosine, lysine, and histidine were converted to highly basic aromatic metabolites. Peptides generated from the sclerotized cuticle with N-bromosuccinimide included the basic derivatives among the hydrolysis products. The absorbance maxima of the aromatic metabolites were 25 nm lower than those of the conventional tyrosyl peptides, with phenolic character poorly expressed or absent. Post-translational modification of the structural proteins preceded visual expression of tanning because aromatic conjugates also were present prior to pupariation. These results are consistent with a crosslinking mechanism favoring covalent bonding between protein chains.

Chemical cleavage of tryptophanyl and tyrosyl peptide bonds via oxidative halogenation mediated by o-iodosobenzoic acid.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemical cleavage of tryptophanyl and tyrosyl peptide bonds via oxidative halogenation mediated by o-iodosobenzoic acidAngelo Fontana, Daniele Dalzoppo, Claudio Grandi, and Marcello ZamboninCite this: Biochemistry 1981, 20, 24, 6997–7004Publication Date (Print):November 1, 1981Publication History Published online1 May 2002Published inissue 1 November 1981https://doi.org/10.1021/bi00527a034RIGHTS & PERMISSIONSArticle Views260Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts