Ascorbate-mediated specific oxidation of the imidazole ring in a histidine derivative

Abstract

In the presence of a catalytic amount of metal ion, ascorbate gave rise to site-specific damage to the protein and peptides accompanied with the selective loss of histidine residues. Hence, we undertook a model reaction of N-benzoylhistidine with the metal/ascorbate system to chemically characterize the site-specific mechanism. Under the physiological conditions, the substrate (N-benzoylhistidine) was almost completely degraded in this system (5 mm ascorbate and 0.05 mm Cu2+) within 24 h of incubation. The reaction of Cu2+/ascorbate toward the substrate was considerably O2-dependent. We have confirmed eight products (II-IX) including a 2-oxo compound, N-benzoyl-β-(2-oxo-imidazolonyl)alanine (III), as the main product and various ring-ruptured products such as N-benzoylasparagine (II), N-benzoylaspartate (VI), N-benzoylaspartylurea (V), and N-benzoyl-N′-formylas-paragine (VIII). Oxidation of the imidazole group was assumed to be initiated at the C-2 position of the imidazole ring to yield III, which was then followed by the formation of the ring-ruptured products. In addition, other products, 5-(2-benzamidovinyl)imidazole (VIII), benzamide (IV), and benzoate (IX), were also detected, which were tentatively formed by the hydrogen abstraction of the α- or β-position of the substrate. © 1989.