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Recombinant human monoclonal antibodies (mAbs)2 have been the main products for the biotechnology industry for more than a decade (1-3). A key strength of antibodies as therapeutics is that their clinical potential can readily be increased by improving their existing properties through a range of antibody engineering technologies (4, 5). As therapeutic agents, mAbs are produced in large scale cell culture, purified, and stored under various conditions and administered to patients (6, 7). Exposure to these production/storage conditions may reduce the stability and efficacy of the mAb by increasing the chance for introducing undesirable modifications such as oxidation, proteolytic cleavage, deamidation, and isomerization. A better understanding of the whole spectrum of possible degradation pathways, particularly new pathways, could facilitate the engineering of mAbs with improvement in the production of stable, efficacious, and safe biotherapeutics.A recent study indicated that antibodies have the intrinsic capacity to convert molecular oxygen into hydrogen peroxide (H 2 O 2 ) (8) and in this process to produce some short lived hydroxyl radical species (HO ⅐ ) at the interface of the light and heavy chains (9 -12). These observations were further supported by a more recent observation that the light chains (three and three types) from the urine of six patients who had multiple myeloma and light chain proteinuria were found capable of generating H 2 O 2 (13). Substantial evidence suggests that the production of H 2 O 2 is an important signaling event triggered by the activation of various cell surface receptors, such as antibody-receptor interaction (14 -19). It has been demonstrated that H 2 O 2 -mediated redox chemistry can regulate the biological function of proteins through interactions with specific residues such as cysteine (Cys) (20 -23); thus, H 2 O 2 may represent a key signaling molecule in mammalian systems. Stamler and Hausladen (20) have proposed a continuum of H 2 O 2 -mediated Cys-SH modifications that constitute important biological signaling events on the one hand and irreversible hallmarks of oxidative stress on the other. Quite commonly, Cys-SH reacts with H 2 O 2 and yields oxidized forms of reversible or irreversible modified residues; reversible modified groups can be stabilized within the protein environment and recycled (21). Irreversible oxidation can lead to the degradation of proteins via a hydroxyl radical mediated mechanism to cleave a peptide bond at the ␣-carbon position through either the diamide or ␣-amidation pathways (24 -27). Although extensive studies have been conducted, because of the transient nature of a radical reaction and unstable intermediate products, the mechanisms underlying the formation of some specific reaction products is still not fully understood. It is well known that Cys residues of an IgG molecule form the intrachain or interchain disulfide bonds (1, 5); thus, the effect of H 2 O 2 -mediated Cys redox chemistry on the structure and stability of a human antibo...

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