Tyrosine−Lipid Peroxide Adducts from Radical Termination: Para Coupling and Intramolecular Diels−Alder Cyclization

Abstract

Free radical co-oxidation of polyunsaturated lipids with tyrosine or phenolic analogs of tyrosine gave rise to lipid peroxide-tyrosine (phenol) adducts in both aqueous micellar and organic solutions. The novel adducts were isolated and characterized by 1D and 2D NMR as well as by mass spectrometry. The spectral data suggest that the polyunsaturated lipid peroxyl radicals give stable peroxide coupling products exclusively at the para position of the tyrosyl (phenoxy) radicals. These adducts have characteristic 13C chemical shifts at 185 ppm due to the cross-conjugated carbonyl of the phenol-derived cyclohexadienone. The primary peroxide adducts subsequently undergo intramolecular Diels-Alder (IMDA) cyclization, affording a number of diastereomeric tricyclic adducts that have characteristic carbonyl 13C chemical shifts at ~198 ppm. All NMR HMBC and HSQC correlations support the structure assignment of the primary and Diels-Alder adducts, as does MS collision induced dissociation. Kinetic rate constants and activation parameters for the IMDA reaction were determined and the primary adducts were reduced with cuprous ion giving a phenol-derived 4-hydroxycyclohexa-2,5-dienone. No products from adduction of peroxyls at the phenolic ortho position were found either in the primary or the cuprous reduction product mixtures. These studies provide a framework for understanding the nature of lipid-protein adducts formed by peroxyl-tyrosyl radical-radical termination processes. Coupling of lipid peroxyl radicals with tyrosyl radicals leads to cyclohexenone and cyclohexadienone adducts which are of interest in and of themselves since, as electrophiles, they are likely targets for protein nucleophiles. One consequence of lipid peroxyl reactions with tyrosyls may therefore be protein-protein crosslinks via interprotein Michael adducts.

Total ApoE and ApoE4 Isoform Assays in an Alzheimer's Disease Case-control Study by Targeted Mass Spectrometry (n = 669): A Pilot Assay for Methionine-containing Proteotypic Peptides

“…We then looked at whether a performic acid/phenol mixture could increase the oxidation yield. Phenol has been shown not to oxidize tyrosine residues, which may be of interest for future work on proteotypic peptides containing both tyrosine and methionine residues (43). As shown in Fig.…”

Oxygen radicals, nitric oxide, and peroxynitrite: Redox pathways in molecular medicine

“…In this context, molecular oxygen becomes a key modulator of tyrosine oxidation yields when LOO • is the proximal oxidant of tyrosine (59). Moreover, this chemistry facilitates the formation of intertwined products arising from the combination reaction of LOO • with Tyr • , the Diels Alder adducts (97). 2 "flux" will be channeled through the SOD reaction; still a percentage of O •− 2 will react with other targets, most notably • NO, and therefore a continuous flux of peroxynitrite will be generated under basal physiological conditions.…”

Exploring oxidative modifications of tyrosine: An update on mechanisms of formation, advances in analysis and biological consequences

“…with added ethanolamine), providing a pathway to protein hydroperoxides. Such addition reactions may be of significance when TyrO • is generated on membrane-associated protein domains exposed to lipid peroxidation, as has been demonstrated in a model system involving the hydrophobic Tyr analogue 7-methoxy-2-oxo-2H-chromen-4-yl)methyl 3-(4hydroxyphenyl) propanoate and (6E,9E)-pentadecadiene exposed to a thermolabile azoinitiator [115]. Analogous addition products have been documented for Tyr derivatives and methyl linoleate, in addition to the formation of 3,3'-diTyr [115,116].…”

“…Such addition reactions may be of significance when TyrO • is generated on membrane-associated protein domains exposed to lipid peroxidation, as has been demonstrated in a model system involving the hydrophobic Tyr analogue 7-methoxy-2-oxo-2H-chromen-4-yl)methyl 3-(4hydroxyphenyl) propanoate and (6E,9E)-pentadecadiene exposed to a thermolabile azoinitiator [115]. Analogous addition products have been documented for Tyr derivatives and methyl linoleate, in addition to the formation of 3,3'-diTyr [115,116]. These reactions may not only induce protein conformational changes, but result in the formation of covalent crosslinks via subsequent reactions of the protein peroxides [16,[117][118][119], and efficient oxidation of nearby Met residues to Met sulfoxide [111,120].…”