Profiles Of Oxidation Products Research Articles

Mass Spectrometric Quantification of Markers for Protein Oxidation by Tyrosyl Radical, Copper, and Hydroxyl Radical in Low Density Lipoprotein Isolated from Human Atherosclerotic Plaques

Lipoprotein oxidation has been implicated in the pathogenesis of atherosclerosis. However, the physiologically relevant pathways mediating oxidative damage have not yet been identified. Three potential mechanisms are tyrosyl radical, hydroxyl radical, and redox active metal ions. Tyrosyl radical forms o,o'-dityrosine cross-links in proteins. The highly reactive hydroxyl radical oxidizes phenylalanine residues to o-tyrosine and m-tyrosine. Metal ions oxidize low density lipoprotein (LDL) by poorly understood pathways. To explore the involvement of tyrosyl radical, hydroxyl radical, and metal ions in atherosclerosis, we developed a highly sensitive and quantitative method for measuring levels of o, o'-dityrosine, o-tyrosine, and m-tyrosine in proteins, lipoproteins, and tissue, using stable isotope dilution gas chromatography-mass spectrometry. We showed that o,o'-dityrosine was selectively produced in LDL oxidized with tyrosyl radical. Both o-tyrosine and o, o'-dityrosine were major products when LDL was oxidized with hydroxyl radical. Only o-tyrosine was formed in LDL oxidized with copper. Similar profiles of oxidation products were observed in bovine serum albumin oxidized with the three different systems. Applying these findings to LDL isolated from human atherosclerotic lesions, we detected a 100-fold increase in o,o'-dityrosine levels compared to those in circulating LDL. In striking contrast, levels of o-tyrosine and m-tyrosine were not elevated in LDL isolated from atherosclerotic tissue. Analysis of fatty streaks revealed a similar pattern of oxidation products; compared with normal aortic tissue, there was a selective increase in o,o'-dityrosine with no change in o-tyrosine. The detection of a selective increase of o,o'-dityrosine in LDL isolated from vascular lesions is consistent with the hypothesis that oxidative damage in human atherosclerosis is mediated in part by tyrosyl radical. In contrast, these observations do not support a role for free metal ions as catalysts of LDL oxidation in the artery wall.

Thermal oxidation of anhydride cured epoxies II—Depth distribution of oxidation products

Samples of a flexibilized anhydride cured epoxy were exposed to thermal ageing in air at various temperatures and oxygen pressures. For samples of 2–4-mm thickness, oxygen diffusion controls the oxidation kinetics. The depth distribution of oxidation products (anhydrides), was studied by IR titration on microtome sections. The kinetics were modelled by Fick's law modified by a term representing the rate of oxygen consumption. When this term is first order with respect of O 2 concentration, as established from a study of the influence of P(O 2), the integration of Fick's law leads to a concentration depth profile of oxidation products which has an exponential shape as observed experimentally. The model predicts that the depth of the oxidized layer must be proportional to the reciprocal of the square root of the oxidation rate constant. Thus, autoretarded kinetics must lead to a continuous increase in the thickness of the oxidized layer during ageing as effectively observed. The consequences of the spatial heterogeneity of the oxidation rate on weight loss kinetics are examined. A simple model having a relatively good predictive value, is proposed for the early period of exposure.