Protein oxidation glycation

Ascorbate (AH-) can make two e available, and consequently both AH- and its one e oxidation product, the ascorbyl radical (A -), become antioxidants. The latter dismutates to form A - and dehydroascorbicadd (A), as shown in Scheme 3, or is reduced back by GSH or GSH-dependent enzymes (glutaredoxine, thioredoxin). Immediately, A is irreversibly hydrolized to 2,3-diketogluconic acid and then to oxalate, thre-onate, and many other metabolites. This last point is important because products derived from the hydrolysis of A may potentially damage proteins by glycation (47). 

The impairment of glucose utilization could result from the modification of the glycolytic enzymes under oxidative stress effects. Oxidative stress is an important factor leading to the pathophysiologcal alterations in conformational diseases. Oxidative stress is manifested in protein oxidation, lipid peroxidation, DNA oxidation, and advanced glycation end-products, as well as reactive oxygen species (ROS), and reactive nitrogen species (RNS) formation. Either the oxidants or the products of oxidative stress could modify the proteins or activate other pathways that may lead to additional impairment of cellular functions and to neuronal loss [57, 58]. 

The assays most widely employed are the measurement of thiobarbituric acid-reactive species (TBARS) and the formation of conjugated dienes, markers of lipid peroxidation [31-33] the determination of advanced oxidation protein products (AOPP), a marker of protein oxidation, and of advanced glycation end-products (AGE) [34-37] the measurement of erythrocyte antioxidant potential [38]. Of particular importance is the isoprostanes determination, since these compounds are formed by the free radical catalysed peroxidation of arachidonic acid, which is independent of the cyclooxygenase enzyme, giving rise to stable compounds, measurable in urine [39]. As recently assessed in a Workshop on markers of oxidative damage and antioxidant protection [40], currently available methods for the determination of antioxidant and redox status are not yet generally suitable for routine clinical applications, essentially for the lack of standardized tests. 

Production of ROS, factors involved in the aging process (Finkel and Holbrook, 2000), is elevated in AD brain and may be an important cause of AD (Martins et al., 1986). Elevated levels of oxidized lipids (Upid peroxidation, malondialdehyde, 4-hydroxynonenal) (Markesbery and Carney, 1999), proteins (advanced glycation end product modifications, tyrosine nitration) (Good et al., 1996 Takeda et al., 1998), and nucleic acids (8-hydroxy-deoxyguanosine) have been documented in AD brains (Lyras et al., 1997). Mitochondria and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex... 

Indeed, increased iron levels in the substantia nigra [21], elevation of hpid peroxidation [22], decline in glutathione concentrations [23], enhanced protein oxidation [24], and increase in glycation end products [25] are biomarkers associated with oxidative damage in neurodegeneration and aging. 

PTM analysis with ETD include sulfation, oxidation, glycation, and differentiation between aspartic and isoaspartic acid. A recent study by Coon and co-workers utilized ETD for mapping PTMs on the tail of histone H4 from human embryonic stem cells. These authors were able to decipher 74 discrete combinatorial PTM codes and quantify striking changes in methylation and acetylation patterns occurring as the cells underwent differentiation. So far, ETD has had very limited application to molecules other than peptides and proteins. One example, by McLuckey and co-workers, involves doubly sodiated glycer-ophosphocholine lipids. Here, ETD product ions provided information on carbon number and degree of unsamration. 

Hunt, J.V., Dean, R.T. and Wolff, S. (1988). Hydroxyl radical production and autoxidative glycosylation. Glucose oxidation as the cause of protein damage in the experimental glycation model of diabetes mellitus and ageing. Biochem. J. 256, 205-212. 

Hunt, J., Bottoms, M. and Mitchinson, M.J. (1993). Oxidative alterations in the experimental glycation model of diabetes mellitus are due to protein-glucose adduct oxidation. Biochem. J. 291, 529-535. 

Glucose may auto-oxidize (like other alphahydroxy-aldehydes) and generate hydroxyl radicals in a transition-metal-catalysed reaction, and induce both fragmentation and conformational changes in glycated proteins (Hunt et al., 1990). 

Ahmed, N., Ahmed, U., Thornalley, P. J., Hager, K., Fleischer, G., and Munch, G. (2005). Protein glycation, oxidation and nitration adduct residues and free adducts of cerebrospinal fluid in Alzheimer s disease and link to cognitive impairment. ]. Neurochem. 92, 255-263. 

Ortwerth BJ, Chemoganskiy V Mossine VV Olesen PR. The effect of UVA light on the anaerobic oxidation of ascorbic acid and the glycation of proteins. Invest Ophthalmol Vis Sci 2003 44 3094-3102. 

Oxidation modifications such as carbonylation, thiol oxidation, and aromatic hydroxylation, and Maillard glycation (the reaction of sugars with amino acid side chains) are the protein modifications most frequently reported in foodstuffs that have been subjected to thermal processing. However, condensations and eliminations of side chains or peptide backbone breakdown have also been described (95). 

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