Site-specific oxidation of angiotensin I by copper(II) and L-ascorbate: conversion of histidine residues to 2-imidazolones.
The reaction of a histidine-containing peptide (angiotensin I) with copper (II)/ascorbate under physiological conditions has been studied chemically. In the presence of a catalytic amount of copper(II) ion, ascorbate mediated the oxidative damage to the peptide via selective loss of the histidine residue. Furthermore, the reaction of copper(II)/ascorbate with the peptide gave two products (AGT-1 and AGT-2) selectively. From amino acid analysis of the modified peptides, it was found that either of the two histidine residues within the native peptide was modified. Amino-terminal sequence analysis indicated that AGT-1 and AGT-2 were modified at the His9 and the His6, respectively. In addition, the data of FAB-MS and 1H NMR suggested that the unknown residues (modified histidine) within AGT-1 and AGT-2 should have the 2-imidazolone structure. In order to confirm the 2-imidazolone residue in both modified peptides, they were hydrolyzed and analyzed by reverse-phase HPLC. The result demonstrated that the acid hydrolysis of modified peptides gave a product which was identical to authentic 2-imidazolone residue. Consequently, it was confirmed that the reaction of Cu(II)/ascorbate occurs specifically at the C-2 position of the imidazole ring of the histidine residue within a peptide.
Uchida
K
K
Department of Food Science & Technology, Nagoya University, Japan.
Kawakishi
S
S
D000595:Amino Acid Sequence; D000803:Angiotensin I / Q000737:chemistry*; D001205:Ascorbic Acid / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D019327:Copper Sulfate; D006639:Histidine; D007093:Imidazoles; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction
D007093:Imidazoles; D006639:Histidine; D003300:Copper; D000803:Angiotensin I; D019327:Copper Sulfate; D001205:Ascorbic Acid