Role of carbohydrates in oxidative modification of fibrinogen and other plasma proteins

Oxidative stress is related to the mechanism of oncogenesis, cell death, and the pathogenesis of many human diseases. Proteins are important targets for oxidative modification, and a Western blot assay that can identify individual oxidized proteins in whole tissue extracts has been described. Using that assay, it was found that plasma proteins show different susceptibilities to oxidative modification. Here, we examine the possibility that the carbohydrate groups of glycoproteins may contribute to the assessment of protein oxidation by carbonyl assays. We used fibrinogen as a model because it is highly susceptible to oxidative modification and contains subunits that are differentially glycosylated. When oxidation-induced carbonyls were measured in fibrinogen subunits by Western blot immunoassay, it was found that the A alpha-chains, which contain no associated carbohydrate groups, were most highly oxidized while the B beta- and gamma-chains, which are glycosylated, were oxidized far less. However, no major difference in the oxidation pattern was obtained when fibrinogen was deglycosylated prior to or after exposure to oxidants. This argues against a possible protective role of the carbohydrate moieties in oxidation of the different fibrinogen subunits. Similar results were obtained with purified human immunoglobulin G and transferrin as well as whole plasma. The results show that carbohydrate moieties are not good targets for oxidative attack by metal-catalyzed oxidation systems. Oxidant-induced carbonyl formation in glycoproteins derives largely, if not entirely, from amino acid oxidation and not from oxidation of carbohydrate groups.