In the presence of O2, Fe(III) or Cu(II), and an appropriate electron donor, a number of enzymic and nonenzymic oxygen free radical-generating systems are able to catalyze the oxidative modification of proteins. Whereas random, global modification of many different amino acid residues and extensive fragmentation occurs when proteins are exposed to oxygen radicals produced by high energy radiation, only one or a few amino acid residues are modified and relatively little peptide bond cleavage occurs when proteins are exposed to metal-catalyzed oxidation (MCO) systems. The available evidence indicates that the MCO systems catalyze the reduction of Fe(III) to Fe(II) and of O2 to H2O2 and that these products react at metal-binding sites on the protein to produce active oxygen (free radical?) species (viz; OH, ferryl ion) which attack the side chains of amino acid residues at the metal-binding site. Among other modifications, carbonyl derivatives of some amino acid residues are formed; prolyl and arginyl residues are converted to glutamylsemialdehyde residues, lysyl residues are likely converted to 2-amino-adipylsemialdehyde residues; histidyl residues are converted to asparagine and/or aspartyl residues; prolyl residues are converted to glutamyl or pyroglutamyl residues; methionyl residues are converted to methionylsulfoxide residues; and cysteinyl residues to mixed-disulfide derivatives. The biological significance of these metal ion-catalyzed reactions is highlighted by the demonstration: (i) that oxidative modification of proteins "marks" them for degradation by most common proteases and especially by the cytosolic multicatalytic proteinase from mammalian cells; (ii) protein oxidation contributes substantially to the intracellular pool of catalytically inactive and less active, thermolabile forms of enzymes which accumulate in cells during aging, oxidative stress, and in various pathological states, including premature aging diseases (progeria, Werner's syndrome), muscular dystrophy, rheumatoid arthritis, cataractogenesis, chronic alcohol toxicity, pulmonary emphysema, and during tissue injury provoked by ischemia-reperfusion. Furthermore, the metal ion-catalyzed protein oxidation is the basis of biological mechanisms for regulating changes in enzyme levels in response to shifts from anaerobic to aerobic metabolism, and probably from one nutritional state to another. It is also involved in the killing of bacteria by neutrophils and in the loss of neutrophil function following repeated cycles of respiratory burst activity.