Lysine derivatives experimentally formed

The calculated mass for these tautomers is 393.1801 (monoisotopic mass) or 393.45 (average mass), which shows excellent agreement with the experimental mass of 393.1830 (monoisotopic mass from accurate mass measurement, sample 2) or 393.45 (average mass from LC/ESIMS experiment, sample 1) for the derivatized active site cofactor. The underivatized cofactor itself has a structure of the type shown as 3. This is consistent with the strong evidence for a quinone-like structure in LO (6, 8, 9, 23), and with the conclusion that the cofactor is comprised of a crosslink between a tyrosine derivative and a lysine residue. This structure could arise from an initial hydroxylation of Tyr to form dopa, followed by the oxidation of dopa to dopa quinone, and subsequent nucleophilic attack by the e-amino group of a lysine side chain to generate an aminoquinol (13). 

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