Oxidative stress, insulin signaling, and diabetes.

What Is Responsible for the Initiating Chemistry of Iron-Mediated Lipid Peroxidation: An Update

Neurodegenerative diseases are associated with the misfolding and deposition of specific proteins, either intra- or extracellularly in the nervous system. Although familial mutations play an important role in protein misfolding and aggregation, the majority of cases of neurodegenerative diseases are sporadic, suggesting that other factors must contribute to the onset and progression of these disorders. Post-translational modifications are known to influence protein structure and function. Some of these modifications might affect proteins in detrimental ways and lead to their misfolding and accumulation. Reducing sugars play important roles in modifying proteins, forming advanced glycation end-products (AGEs) in a non-enzymatic process named glycation. Several proteins linked to neurodegenerative diseases, such as amyloid beta, tau, prions and transthyretin, were found to be glycated in patients, and this is thought to be associated with increased protein stability through the formation of crosslinks that stabilize protein aggregates. Moreover, glycation may be responsible, via the receptor for AGE (RAGE), for an increase in oxidative stress and inflammation through the formation of reactive oxygen species and the induction of NF-kappaB. Therefore, it is essential to unravel the molecular mechanisms underlying protein glycation to understand their role in neurodegeneration. Here, we reviewed the role of protein glycation in the major neurodegenerative disorders and highlight the potential value of protein glycation as a biomarker or target for therapeutic intervention.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/path.2682

The sour side of neurodegenerative disorders: the effects of protein glycation†

In recent years, glycated hemoglobin (HbA1c) has been increasingly accepted as a functional metric of mean blood glucose in the treatment of diabetic patients. Importantly, HbA1c provides an alternate measure of total glycemic exposure due to the representation of blood glucose throughout the day, including post-prandially. In this article, we propose and demonstrate the potential of Raman spectroscopy as a novel analytical method for quantitative detection of HbA1c, without using external dyes or reagents. Using the drop coating deposition Raman (DCDR) technique, we observe that the nonenzymatic glycosylation (glycation) of the hemoglobin molecule results in subtle but discernible and highly reproducible changes in the acquired spectra, which enable the accurate determination of glycated and nonglycated hemoglobin using standard chemometric methods. The acquired Raman spectra display excellent reproducibility of spectral characteristics at different locations in the drop and show a linear dependence of t...

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Raman Spectroscopy-Based Sensitive and Specific Detection of Glycated Hemoglobin

Effect of nonenzymatic glycation on functional and structural properties of hemoglobin.

Heme proteins––hemoglobin and myoglobin possess esterase activities. Studies with purified hemoglobin from normal individuals and diabetic patients revealed that the esterase activity as measured from hydrolysis of p-nitrophenyl acetate (p-NPA) was higher in diabetic condition and increased progressively with extent of the disease. HbA1c, the major glycated hemoglobin, which increases proportionately with blood glucose level in diabetes mellitus, exhibited more esterase activity than the non-glycated hemoglobin fraction, HbA0, as demonstrated spectrophotometrically as well as by activity staining. Glycation influenced esterase activity of hemoglobin by increasing the affinity for the substrate and the rate of the reaction. Both HbA0 and HbA1c-mediated catalysis of p-NPA hydrolysis was pH-dependent. Esterase activity of in vitro-glycated myoglobin (GMb) was also higher than that of its non-glycated analog (Mb). The amplified esterase activities of hemoglobin and myoglobin might be associated with glycation-induced structural modifications of the proteins.

Effect of non-enzymatic glycation on esterase activities of hemoglobin and myoglobin.

Trifluoperazine (TFZ), a phenothiazine drug, penetrates into human erythrocytes and releases oxygen by interaction with hemoglobin. TFZ-induced oxygen release from hyperglycemic erythrocytes isolated from diabetic patients is considerably less compared to that from the cells of normoglycemic individuals. In diabetes mellitus, hemoglobin is significantly glycated by glucose. Non-glycated hemoglobin, HbA0 and its major glycated analog, HbA1c have been separated from the blood samples of diabetic patients. TFZ releases considerable amount of oxygen from HbA0, but very little from HbA1c. Spectrofluorimetric studies reveal that TFZ forms excited state complexes with both HbA0 and HbA1c. Titration of HbA0 with TFZ in a spectrophotometric study exhibits two isosbestic points. Similar experiment with HbA1c causes gradual loss of the Soret peak without appearance of any isosbestic point indicating a possibility of heme loss during interaction, which is also supported by gel filtration experiment and SDS-PAGE experiment followed by heme staining. The results suggest that drug action on hemoglobin is influenced by glycation-induced structural modification of the protein.

Effect of Glycation of Hemoglobin on its Interaction with Trifluoperazine

Metmyoglobin (Mb) was glycated by glucose in a nonenzymatic in vitro reaction. Amount of iron release from the heme pocket of myoglobin was found to be directly related with the extent of glycation. After in vitro glycation, the unchanged Mb and glycated myoglobin (GMb) were separated by ion exchange (BioRex 70) chromatography, which eliminated free iron from the protein fractions. Separated fractions of Mb and GMb were converted to their oxy forms -MbO2 and GMbO2, respectively. H2O2-induced iron release was significantly higher from GMbO2 than that from MbO2. This free iron, acting as a Fenton reagent, might produce free radicals and degrade different cell constituents. To verify this possibility, degradation of different cell constituents catalyzed by these fractions in the presence of H2O2 was studied. GMbO2 degraded arachidonic acid, deoxyribose and plasmid DNA more efficiently than MbO2. Arachidonic acid peroxidation and deoxyribose degradation were significantly inhibited by desferrioxamine (DFO), m...

In vitro nonenzymatic glycation enhances the role of myoglobin as a source of oxidative stress.

Increased glucose concentration in diabetes mellitus causes glycation of several proteins, leading to changes in their properties. Although glycation-induced functional modification of myoglobin is known, structural modification of the protein has not yet been reported. Here, we have studied glucose-modified structural changes of the heme protein. After in vitro glycation of metmyoglobin (Mb) by glucose at 25°C for 6 days, glycated myoglobin (GMb) and unchanged Mb have been separated by ion exchange (BioRex 70) chromatography, and their properties have been compared. Compared to Mb, GMb exhibits increased absorbance around 280 nm and enhanced fluorescence emission with excitation at 285 nm. Fluorescence quenching experiments of the proteins by acrylamide and KI indicate that more surface accessible tryptophan residues are exposed in GMb. CD spectroscopic study reveals a change in the secondary structure of GMb with decreased α-helix content. 1-anilino-naphthaline-8-sulfonate (ANS) binding with Mb and GMb indicates that glycation increases hydrophobicity of the heme protein. GMb appears to be less stable with respect to thermal denaturation and differential calorimetry experiments. Heme-globin linkage becomes weaker in GMb, as shown by spectroscopic and gel electrophoresis experiments. A correlation between glycation-induced structural and functional modifications of the heme protein has been suggested.

Anjana Roy

Papers

Effect of nonenzymatic glycation on functional and structural properties of hemoglobin.

Effect of non-enzymatic glycation on esterase activities of hemoglobin and myoglobin.

Effect of Glycation of Hemoglobin on its Interaction with Trifluoperazine

In vitro nonenzymatic glycation enhances the role of myoglobin as a source of oxidative stress.

Non-enzymatic glycation induces structural modifications of myoglobin