A covalent thymine-tyrosine adduct involved in DNA-protein crosslinks: synthesis, characterization, and quantification.

A thymine-tyrosine adduct, (3-[(1,3-dihydro-2,4-dioxopyrimidin-5-yl)methyl]-L-tyrosine), was synthesized using a simple, single-step condensation between 5-(hydroxymethyl)uracil and L-tyrosine. This approach provides access to useful quantities (mg-g) of analytically pure reference material, and with minor modification, to stable isotope-labeled analogues (isotopomers). With reference material and a suitable internal standard available, isotope-dilution liquid chromatography-electrospray ionization-tandem mass spectrometry (LC/MS/MS) was used to assay the adduct in a model system purged of oxygen, i.e., a gamma-irradiated N2O-saturated aqueous solution of thymine and tyrosine. The convenient synthetic route to standards and the method for quantification reported here will prove useful in assessing the significance of the adduct in biological systems. These studies also highlight the potential for artefactual adduct formation if the appropriate substrates are present under acidic conditions.