The source of the oxygen atom in the α-hydroxyglycine intermediate of the peptidylglycine α-amidating reaction

Peptidylglycine α-amidating activity catalyses the oxidation of a C-terminally glycine-extended peptide to a desglycine α-amidated peptide at the expense of ascorbate and O₂ in the presence of Cu²⁺. The reaction involves oxidative N-dealkylation within the terminal glycine residue with retention of the glycine N atom and release of the remainder as glyoxylate. Recent studies by us and others have revealed that the reaction consists of two steps via a carbinolamide as an intermediate (peptidyl α-hydroxyglycine), and also that two separate enzymes derived from a common precursor protein catalyse these steps, formation of the carbinolamide and its conversion into α-amide and glyoxylate. As for the mechanism of carbinolamide formation, two distinct pathways can be considered: direct mono-oxygenation at the glycine α-C atom and dehydrogenation leading to an imine followed by hydration. To draw a distinction between them, we carried out the reaction with D-Tyr-Val-Gly as the substrate either in the H₂¹⁸O-enriched medium or under an atmosphere of ¹⁸O₂, and isolated the α-hydroxyglycine intermediate. The fast-atom-bombardment mass-spectral analyis demonstrated that the hydroxy O atom comes from O₂, but not from H₂O, indicating that the α-hydroxylation should be a mono-oxygenase reaction.