A sensitive fluorometric assay for protein-bound DOPA and related products of radical-mediated protein oxidation

Oxidative attack on proteins results in the hydroxylation of tyrosyl residues to protein-bound DOPA (3,4-dihydroxyphenylalanine). Existing methods for assaying protein-bound DOPA have poor sensitivity and numerous possible interferences, such that accurate determination (especially of very low DOPA concentrations) has required time-consuming acid hydrolysis and HPLC analysis with fluorometric detection. This work presents a sensitive and selective assay for peptide or protein-bound o-benzoquinones derived from DOPA based on fluorometric detection of ethylenediamine derivatives. Detection limits for protein-bound DOPA are in tbe range 0.53-4.70 ng/mL for the assay mixture, corresponding to sample DOPA concentrations of 0.59-5.30 ng/mL (representing a minimum of 6-54 pmole detected), depending on the particular protein/peptide under study. The assay response increases linearly with DOPA concentration, and also with the extent of radical exposure of the protein. The assay is a simple and fast way to assess DOPA formation and thus oxidative damage in a protein.