No abstract

The demonstration that oxidatively modified forms of proteins accumulate during aging, oxidative stress, and in some pathological conditions has focused attention on physiological and nonphysiological mechanisms for the generation of reactive oxygen species (ROS) 1 and on the modification of biological molecules by various kinds of ROS. Basic principles that govern the oxidation of proteins by ROS were established in the pioneering studies of Swallow (1), Garrison (2, 3), and Scheussler and Schilling (4) who characterized reaction products formed when proteins were exposed to ionizing radiation under conditions where only ⅐ OH, O 2. , or a mixture of both was made available. Results of these studies demonstrated that the modification of proteins is initiated mainly by reactions with ⅐ OH; however, the course of the oxidation process is determined by the availability of O 2 and O 2 . or its protonated form (HO ⅐ 2 ). Collectively, these ROS can lead to oxidation of amino acid residue side chains, formation of protein-protein cross-linkages, and oxidation of the protein backbone resulting in protein fragmentation. In the meantime, it has been shown that other forms of ROS may yield similar products and that transition metal ions can substitute for ⅐ OH and O 2 . in some of the reactions.

We summarize here results of studies designed to elucidate basic mechanisms of reactive oxygen (ROS)-mediated oxidation of proteins and free amino acids. These studies have shown that oxidation of proteins can lead to hydroxylation of aromatic groups and aliphatic amino acid side chains, nitration of aromatic amino acid residues, nitrosylation of sulfhydryl groups, sulfoxidation of methionine residues, chlorination of aromatic groups and primary amino groups, and to conversion of some amino acid residues to carbonyl derivatives. Oxidation can lead also to cleavage of the polypeptide chain and to formation of cross-linked protein aggregates. Furthermore, functional groups of proteins can react with oxidation products of polyunsaturated fatty acids and with carbohydrate derivatives (glycation/glycoxidation) to produce inactive derivatives. Highly specific methods have been developed for the detection and assay of the various kinds of protein modifications. Because the generation of carbonyl derivatives occurs by many different mechanisms, the level of carbonyl groups in proteins is widely used as a marker of oxidative protein damage. The level of oxidized proteins increases with aging and in a number of age-related diseases. However, the accumulation of oxidized protein is a complex function of the rates of ROS formation, antioxidant levels, and the ability to proteolytically eliminate oxidized forms of proteins. Thus, the accumulation of oxidized proteins is also dependent upon genetic factors and individual life styles. It is noteworthy that surface-exposed methionine and cysteine residues of proteins are particularly sensitive to oxidation by almost all forms of ROS; however, unlike other kinds of oxidation the oxidation of these sulfur-containing amino acid residues is reversible. It is thus evident that the cyclic oxidation and reduction of the sulfur-containing amino acids may serve as an important antioxidant mechanism, and also that these reversible oxidations may provide an important mechanism for the regulation of some enzyme functions.