Neutrophils play a major role in acute inflammation in part by generating superoxide and an array of other reactive species. These white blood cells also contribute to protection against inflammatory pain by releasing opioid peptides. The biochemical interactions of enkephalins with neutrophil-derived oxidants are not well understood. In this investigation we reveal that neutrophils use myeloperoxidase to oxidize enkephalins to their corresponding tyrosyl free radicals, which react preferentially with the superoxide to form a hydroperoxide. In methionine enkephalin, rapid intramolecular oxygen transfer from the hydroperoxide to the Met sulfur results in the formation of a sulfoxide derivative. This reaction may occur at sites of inflammation where enkephalins are released and neutrophils generate large amounts of superoxide. Hydroperoxide formation destroys the aromatic character of the Tyr residue by forming a bicyclic structure via conjugate addition of the terminal amine to the phenol ring. As the N-terminal Tyr and its amino group are essential for their opiate activity, we hypothesize that oxidative modification of this residue should affect the analgesic activity of enkephalins.
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