Non-Enzymatic Glycosylation (or Glycation) and Inhibition of the Pig Heart Cytosolic Aspartate Aminotransferase by Glyceraldehyde 3-Phosphate

Glycated proteome: from reaction to intervention

Glycation, a nonenzymatic reaction between reducing sugars and proteins, is a proteome wide phenomenon, predominantly observed in diabetes due to hyperglycemia. Glycated proteome of plasma, kidney, lens, and brain are implicated in the pathogenesis of various diseases, including diabetic complications, neurodegenerative diseases, cancer, and aging. This review discusses the strategies to characterize protein glycation, its functional implications in different diseases, and intervention strategies to protect the deleterious effects of protein glycation.

Multiple binding partners

GAPDH interacts with a plethora of diverse cellular proteins. The network of interacting partners, or interactome, is presented for GAPDH with the interacting molecules grouped into specific functional and structural categories. By organizing the binding partners in this way, certain common structural features are beginning to surface, such as acidic dipeptide sequences that are found in several of these binding proteins. Additionally, the consensus sequences for target polynucleotides are being brought to light. The categories, which are presented according to function, offer an opportunity for research into the corresponding structural correlates to these interactions. Recent discoveries of interacting proteins have revealed novel relationships that are generating emerging mechanisms. Proteins that are associated with age-related neurodegenerative diseases appear to be particularly prone to binding GAPDH, suggesting that GAPDH may be playing a role in these diseases. Neurodegenerative diseases that are discussed are the conformational diseases of aging, suggesting that GAPDH may be a global sensor for cellular conformational stress. In addition to GAPDH's oxidoreductase activity, several other enzymatic functions have been discovered, including peroxidase, nitrosylase, mono-ADP-ribosylase and kinase activities.

Fluorescence, birefringence and confocal microscopy of the abdominal aorta from nonobese diabetic (NOD) mice

In this study, nonenzymatic glycosylation was assessed in aorta extracellular matrix (ECM) from nonobese diabetic (NOD) mice, using nitroblue tetrazolium (NBT). Molecular and structural changes were investigated in elastic lamellae and collagen fibers of diabetic mice aortas after staining with dansyl chloride and anilinonaphthalene sulfonate (ANS). Alterations in arterial autofluorescence and birefringence of collagen fibers were investigated in unstained aortas. Proliferation of smooth muscle cells (SMC) was also investigated by Feulgen reaction staining assessed by confocal microscopy and image analysis. Assessment of nonenzymatic glycosylation demonstrated glycosylation products in the aorta ECM of NOD mice. Elastic lamellae and collagen fibers from NOD mouse aortas presented less intense fluorescence after staining with dansyl chloride and ANS when compared to aortas of control nondiabetic mice. However, unstained NOD aortas showed more intense autofluorescence when compared to controls. Birefringence analysis suggests alterations in the higher molecular packing of the arterial collagen fibers in NOD aortas. In aortas stained by Feulgen reaction, no evidence of SMC proliferation was observed in diabetic aortas.

Glycation-induced inactivation of aspartate aminotransferase, effect of uric acid

Glycation is common posttranslational modification of proteins impairing their function, which occurs during diabetes mellitus and aging. Beside extracellular glycation of long-lived proteins, intracellular modifications of short-lived proteins by more reactive sugars like fructose are possible. The process includes free oxygen radicals (glycoxidation). In an attempt to reduce glycoxidation and formation of advanced glycation products (AGE), influence of 0.2-1.2 mM uric acid as endogenous antioxidant on glycoxidation of purified pig heart aspartate aminotransferase (AST) by 50 mM and 500 mM D-fructose in vitro was studied. Uric acid at 1.2 mM concentration reduced AST activity decrease and formation of total AGE products caused by incubation in vitro of the enzyme with sugar up to 25 days at 37 degrees C. The results thus support the hypothesis that uric acid has beneficial effects in controlling protein glycoxidation. The in vitro system AST-fructose proved to be a useful tool for investigation of glycation process.

Anti-crosslinking properties of carnosine: significance of histidine

Carnosine, a histidine-containing dipeptide, is a potential treatment for Alzheimer's disease. There is evidence that carnosine prevents oxidation and glycation, both of which contribute to the crosslinking of proteins; and protein crosslinking promotes beta-amyloid plaque formation. It was previously shown that carnosine has anti-crosslinking activity, but it is not known which of the chemical constituents are responsible. We tested the individual amino acids in carnosine (beta-alanine, histidine) as well as modified forms of histidine (alpha-acetyl-histidine, 1-methyl-histidine) and methylated carnosine (anserine) using glycation-induced crosslinking of cytosolic aspartate aminotransferase as our model. beta-Alanine showed anti-crosslinking activity but less than that of carnosine, suggesting that the beta-amino group is required in preventing protein crosslinking. Interestingly, histidine, which has both alpha-amino and imidazolium groups, was more effective than carnosine. Acetylation of histidine's alpha-amino group or methylation of its imidazolium group abolished anti-crosslinking activity. Furthermore, methylation of carnosine's imidazolium group decreased its anti-crosslinking activity. The results suggest that histidine is the representative structure for an anti-crosslinking agent, containing the necessary functional groups for optimal protection against crosslinking agents. We propose that the imidazolium group of histidine or carnosine may stabilize adducts formed at the primary amino group.

Carnosine disaggregates glycated alpha-crystallin: an in vitro study

Protein glycation, which promotes aggregation, involves the unwanted reaction of carbohydrate oxidation products with proteins. Glycation of lens alpha-crystallin occurs in vivo and may contribute to cataractogenesis. Anti-glycation compounds such as carnosine may be preventive, but interestingly carnosine reverses lens opacity in human trials. The mechanism for this observation may involve carnosine's ability to disaggregate glycated protein. We investigated this hypothesis using glycated alpha-crystallin as our in vitro model. Methylglyoxal-induced glycation of alpha-crystallin caused aggregation as evidenced by increased 90 degrees light scattering. After addition of carnosine, light scattering returned to baseline levels suggesting that the size of the glycation-induced aggregates decreased. Additionally, carnosine decreased tryptophan fluorescence polarization of glycated alpha-crystallin, suggesting that carnosine increased peptide chain mobility, which may contribute to the controlled unfolding of glycated protein. Comparatively, guanidine-HCl and urea had no effect. Our data support the hypothesis that carnosine disaggregates glycated alpha-crystallin.

Methylglyoxal-induced glycation affects protein topography

Methylglyoxal is a metabolic byproduct that is elevated in diabetic tissue. We examined the effects of methylglyoxal on cytosolic aspartate aminotransferase (cAAT), which is an enzyme previously shown to be modified by glyceraldehyde, acrolein, and ribose 5-phosphate. In the present study we observed that methylglyoxal caused real-time changes in tryptophan (intrinsic) fluorescence. Millimolar concentrations of methylglyoxal predominately decreased the fluorescence emission at 388 nm. While micromolar concentrations also decreased emission at 388 nm, low levels of methylglyoxal caused a prominent redshift in the wavelength of maximal emission. The changes in intrinsic fluorescence reflect definable changes in protein topography. These observations are consistent with a change in conformation that is more compact than that of native cAAT, suggesting that intramolecular cross-links (i.e., lysine-lysine) or hydrophobic pockets (i.e., carboxyethyl-lysines) were formed. Methylglyoxal also inhibited activity, and the inhibition correlated with the methylglyoxal-induced change in protein conformation.

Different influences of hyperglycemic duration on phosphorylated extracellular signal-regulated kinase 1/2 in rat heart

Extracellular signal-regulated kinase (ERK) 1/2 is an important intracellular proteinase associated with myocardial protection against heart injury. Hyperglycemia was also reported to be highly involved in heart injury by the formation of advanced glycation end products (AGEs) in myocardial protein, resulting in its altered structure and function. However, the effect of this glycation on mitogen-activated protein kinases, particularly ERK1/2, in the myocardium is largely unclarified. In this study, we investigated whether the glycation of an intracellular protein, ERK1/2, would result in ERK1/2-AGEs formation that adversely affects ERK1/2 activation in the rat heart under hyperglycemia. Hyperglycemia was induced by injection of streptozotocin (STZ) and hearts were examined 4 and 20 weeks after STZ treatment. By immunohistochemical staining and Western blotting, it was determined that the level of phosphorylated ERK1/2 in the rat heart under hyperglycemia 20 weeks after STZ treatment decreased markedly by about 50% of that of the time-matched control group, whereas in the case of 4 weeks after STZ treatment, it increased by about 2.7-fold that of the time-matched group. The level of deposition of AGEs in proteins of the myocardium increased significantly depending on the duration of hyperglycemia. Twenty weeks after STZ treatment, two clear bands corresponding to 44- and 42-kDa AGEs were detected by Western blotting: these corresponded to protein sizes of ERK1/2. The immunoprecipitation method further confirmed the formation and the increased intensity of ERK1/2-AGEs in the rat heart under hyperglycemia for 20 weeks. These results demonstrate that long-term hyperglycemia may inhibit ERK1/2 phosphorylation in the myocardium, whereas a short-term (4 weeks) hyperglycemia enhances its phosphorylation. The ERK1/2 phosphorylation under long-term hyperglycemia is very different from that under short-term hyperglycemia. In addition, this inhibition of ERK1/2 activation appears to be dependent on the formation of ERK1/2-AGEs under long-term hyperglycemia, which may be related in part to the etiology of diabetic cardiomyopathy. It also suggests that the formation of AGEs in intracellular enzymes and proteins under hyperglycemia could play important roles in the development of diabetes complications.

Carnosine promotes the heat denaturation of glycated protein

Glycation alters protein structure and decreases biological activity. Glycated proteins, which accumulate in affected tissue, are reliable markers of disease. Carnosine, which prevents glycation, may also play a role in the disposal of glycated protein. Carnosinylation tags glycated proteins for cell removal. Since thermostability determines cell turnover of proteins, the present study examined carnosine's effect on thermal denaturation of glycated protein using cytosolic aspartate aminotransferase (cAAT). Glycated cAAT (500 microM glyceraldehyde for 72h at 37 degrees C) increased the T(0.5) (temperature at which 50% denaturation occurs) and the Gibbs free energy barrier (DeltaG) for denaturation. The enthalpy of denaturation (DeltaH) for glycated cAAT was also higher than that for unmodified cAAT, suggesting that glycation changes the water accessible surface. Carnosine enhanced the thermal unfolding of glycated cAAT as evidenced by a decreased T(0.5) and a lowered Gibbs free energy barrier. Additionally, carnosine decreased the enthalpy of denaturation, suggesting that carnosine may promote hydration during heat denaturation of glycated protein.

Effects of thermal denaturation on protein glycation

Protein denaturation occurs at sites of inflammation. We hypothesized that denatured protein may provide a more susceptible target for glycation, which is a known mediator of inflammation. We examined the effects of thermal denaturation on the susceptibility of protein glycation using glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and aspartate aminotransferase (AAT) as our target proteins. GAPDH and AAT are ubiquitous proteins that exhibited very different thermal stabilities. Glycating agents, methylglyoxal (MG) and glyceraldehyde (Glyc), caused an increase in the formation of advanced glycation endproducts (AGEs) in native and denatured GAPDH and AAT. The effects of the glycating agents were more pronounced with the denatured proteins. In addition to nitroblue tetrazolium (NBT)- reactivity, our measured endpoints were absorbance (lambda = 365 nm) and fluorescence (lambda(ex) = 370 nm; lambda(em) = 470 nm) properties that are typically associated with protein glycation. We also looked at carnosine's ability to prevent glycation of native and denatured protein. Carnosine, an endogenous histidine dipeptide, exhibits anti-inflammatory activity presumably due to its anti-oxidant and anti-glycation properties. Carnosine prevented Glyc-induced AGE formation in both native and denatured AAT suggesting that carnosine's anti-inflammatory activity may be due in part to carnosine's ability to prevent glycation of denatured protein.

Glycation of aspartate aminotransferase and conformational flexibility

Glycation of proteins alters biological function and changes cellular processes. Our study investigated the conformational changes that accompany glycation using the cardiac aspartate aminotransferase (cAAT). We examined the effects of brief and prolonged exposure of cAAT to glyceraldehyde (Glyc) and ribose 5-phosphate (R5P). When cAAT was briefly incubated (3.5 h) with Glyc (500 microM) or R5P (5 mM) at 37 degrees C, cAAT activity and 1-anilinonaphthalene 8-sulfonate (ANS) binding increased relative to control. After prolonged incubation (64 h) with Glyc (500 microM) or R5P (5 mM) at 37 degrees C, activity and ANS binding decreased relative to control. Furthermore, upon prolonged incubation of cAAT with 500 microM Glyc (14.5 h) or 2 mM R5P (64.25 h) at 37 degrees C, the denaturation curves shifted to the right relative to control. We conclude that upon brief incubation with Glyc and R5P, cAAT exhibited a more open and flexible structure and upon prolonged incubation, a more rigid structure.

Carnosine prevents the glycation-induced changes in electrophoretic mobility of aspartate aminotransferase

Carbohydrate-derived aldehydes cause irreversible loss of protein function via glycation. We previously observed that glyceraldehyde 3-phosphate (Glyc3P) abolishes the enzyme activity of cardiac aspartate aminotransferase (cAAT). We also examined the protective effects of carnosine against Glyc3P-induced loss of enzyme activity. The present study looked at carnosine's prevention of Glyc3P-induced change in protein structure. Purified cAAT (2 mg protein/mL) was incubated with various concentrations of carnosine (1-20 mM) in the presence of Glyc3P (500 microM) for 4 days at 37 degrees C. Following incubation, samples were analyzed by SDS-polyacrylamide gel electrophoresis. Carnosine showed prevention of protein modification at carnosine-to-Glyc3P ratios of 10:1 or greater. There was a progressive loss of the unmodified cAAT protein band as Glyc3P concentration was increased. Additionally, the gel position of the Glyc3P-modified cAAT protein varied over time. The apparent molecular weight (MWapp) of the Glyc3P-modified cAAT protein that formed after 1 day at 37 degrees C (500 microM) was greater than its MWapp after 2 days, suggesting that a chemical rearrangement of the initial adduct occurs. These observations support the hypothesis that carnosine is an antiglycation agent and that its mechanism of action involves prevention of protein modification.