Nonenzymatic peptide alpha-amidation. Implications for a novel enzyme mechanism.

An abiotic system is described which chemically catalyzes the formation of less than Glu-His-Pro-NH2 (thyrotropin-releasing hormone) from less than Glu-His-Pro-amino acid in the presence of copper, ascorbate, and molecular oxygen. Evidence is presented to support the participation of hydroxyl and carbon radicals as reaction intermediates in the production of a peptide amide and an aldehyde or ketone. The characteristics of this model system closely mimic the characteristics of enzymatic peptide amidation, and an oxidative, free-radical mechanism for enzymatic peptide amidation is proposed as an alternative to the mechanism for enzymatic amidation offered by Bradbury et al. (Bradbury, A. F., Finnie, M. D. A., and Smyth, D. G. (1982) Nature 298, 686-688).