Reactions of Hydrogen Atoms with Met-Enkephalin and Related Peptides

The reactions of hydrogen atoms with enkephalins and related peptides have been investigated by radiolytic methods in aqueous solutions and lipid vesicle suspensions. Pulse radiolysis experiments indicate that methionine residue (Met) is the main target. In Met‐enkephalin (Tyr‐Gly‐Gly‐Phe‐Met) the attack of the hydrogen atom occurs to about 50 % on Met with formation of methanethiyl radical. The remaining percentage is divided roughly evenly between Tyr and Phe. With Leu‐enkephalin (Tyr‐Gly‐Gly‐Phe‐Leu) the site of attack is limited to Tyr and Phe. Using a peptide–liposome (that is, 1‐palmitoyl‐2‐oleoyl phosphatidylcholine vesicles) model, the cis–trans isomerization of phospholipids could be detected due to the catalytic action of thiyl radicals. The radiation chemical yields of the H. and, consequently of CH3S. radical, was modulated by the experimental conditions and the nature of peptide. Large amounts of trans lipids observed in phosphate buffer vesicle suspensions indicated the efficient role of double‐bond isomerization as marker of Met‐containing peptide damage. Hydrogen atoms mainly attack the Met residue (about 50 %) of Met‐enkephalin with subsequent formation of a terminal α‐aminobutyric acid (Abu) and methanethiyl radicals (see scheme). By using a peptide–liposome model, the cis–trans isomerization of phospholipids has been detected. This highlights the role of trans lipid as a marker of this radical damage.