OXIDATION OF FREE AMINO-ACIDS AND AMINO-ACID-RESIDUES IN PROTEINS BY RADIOLYSIS AND BY METAL-CATALYZED REACTIONS

Basic mechanisms that underlie the oxygen free radical-promoted oxidation of free amino acids and amino acid residues of proteins are derived from radiolysis studies. Results of these studies indicate that the most common pathway for the oxidation of simple aliphatic amino acids involves the hydroxyl radical-mediated abstraction of a hydrogen atom to form a carbon-centered radical at the α-position of the amino acid or amino acid residue in the polypeptide chain. Addition of O2 to the carbon-centered radicals leads to formation of peroxy radical derivatives, which upon decomposition lead to production of NH3 and α-ketoacids, or to production of NH3, CO2, and aldehydes or carboxylic acids containing one less carbon atom. As the number of carbon atoms in the amino acid is increased, hydrogen abstraction at other positions in the carbon chain becomes more important and leads either to the formation of hydroxy derivatives, or to amino acid cross-linked products as a consequence of carbon-centered radical recombination processes. α-Hydrogen abstraction plays a minor role in the oxidation of aromatic amino acids by radiolysis. Instead, the aromatic ring is the primary site of attack leading to hydroxy derivatives, to ring scission, and in the case of tyrosine to the formation of Tyr-Tyr cross-linked dimers. The basic pattern for the oxidation of amino acids by metal ion-catalyzed reactions (Fenton chemistry) is similar to the α-hydrogen abstraction pathway. But unlike the case of oxidation by radiolysis, this Fenton pathway is the major mechanism for the oxidation of all aliphatic amino acids, regardless of chain length, as well as for the oxidation of free amino acids. Curiously, the Fe(III)-catalyzed oxidation of free amino acids is almost completely dependent upon the presence of bicarbonate ion, and is greatly stimulated by iron chelators at chelator/Fe(III) ratios less than 1.0, and is inhibited at chelator/Fe(III) ratios greater than 1.0. It is deduced that the most active catalytic complex is composed of two equivalents of HCO3-, an amino acid, and at least one equivalent of iron; however, two forms of iron, an iron-chelate and another form, must somehow be involved. In contrast to the situation with radiolysis, the aromatic rings of aromatic amino acids are only minor targets for metal-catalyzed reactions. All amino acid residues in proteins are subject to attack by hydroxyl radicals generated by ionizing radiation; however, the aromatic amino acids and sulfur-containing amino acids are most sensitive to oxidation. The oxidation of amino acid residues in proteins by radiolysis, as in the case of free amino acids, involves α-hydrogen abstraction by hydroxyl radicals, and in the presence of oxygen leads to the generation of peroxy intermediates, which upon decomposition lead to peptide bond cleavage by the α-amidation mechanism. Under anaerobic conditions little or no peptide bond cleavage occurs; instead, higher-molecular-weight aggregates are formed due to the formation of protein-protein (Tyr-Tyr; -S-S-) cross-linkage reaction. In addition, the aromatic amino acid residues, especially tyrosine and tryptophan residues, are degraded to yield as yet ill-defined products. In contrast, metal ion-catalyzed oxidation of proteins is mainly a site-specific process in which only one or a few amino acids at metal-binding sites on the protein are preferentially oxidized. Histidine, proline, arginine, and lysine residues have been identified as major targets for oxidation. Histidine residues are converted to aspartate or asparagine residues; proline residues are converted to glutamic acid, pyroglutamic acid, and γ-glutamicsemialdehyde residues; arginine residues are converted to γ-glutamicsemialdehyde residues; and lysine residues are converted to 2-amino-adipicsemialdehyde residues. Little or no peptide bond cleavage or aggregation occurs except under conditions where proteins are exposed to high concentrations of metal chelate complexes for extended periods of time. In contrast to the case of radiolysis, aromatic amino acid residues and to a lesser extent, the sulfur-containing amino acids are not major targets for oxidation, presumably because they are not commonly present at metal-binding sites.