Reactivity of peptidyl-tyrosine to hydroxylation and cross-linking

Tyrosine residues of neuroendocrine peptides are frequently the targets of oxidation reactions, one of which involves hydroxylation to peptidyl-3, 4-d ihydroxy-phenyl-L-alanine (DOPA). The reactivity in vitro of peptidyl-DOPA in two neuroendocrine peptides, a neurotensin fragment (pELYENK) and procto lin (RYLPT), was investigated using ultraviolet-visible scanning spectropho tometry and matrix-assisted laser desorption ionization mass spectrometry f ollowing oxidation by tyrosinase and periodate. The peptides form covalentl y coupled dimers and trimers, and their masses are consistent with the pres ence of diDOPA cross-links. Lysine does not appear to participate in multim er formation because it is efficiently recovered in fragmentation ladders u sing subtilisin. While multimer formation in the neurotensin-derived peptid e can be blocked effectively by adding N-acetyl-DOPA-ethylester to the reac tion medium, the DOPA ethylester couples itself four to five times to each peptide.