Isolation and identification of 5-methyl-imidazolin-4-one derivative as glyceraldehyde-derived advanced glycation end product

We isolated and identified the glyceraldehyde-derived advanced glycation product (AGE) formed from glyceraldehyde and N(alpha)-acetylarginine. A major product was identified as N(alpha)-acetyl-N(delta)-(5-methyl-imidazolin-4-one-2-yl)-ornithine. The compound has been reported as methylglyoxal-derived AGE, MG-H1. This study suggests that MG-H1 is formed through both glyceraldehyde-related and methylglyoxal-related pathways. There is a possibility that MG-H1 becomes an index of injury to glyceraldehyde and methylglyoxal-related enzymes.     

The Formation of Argpyrimidine in Glyceraldehyde-Related Glycation

Three major glyceraldehyde-related advanced glycation end products (AGEs) were formed from a mixture of N^α-acetyllysine, N^α-acetylarginine, and glyceraldehyde. Two of the compounds were MG-H1 and GLAP, as previously reported, and the other compound was identified as N^α-acetyl-N^δ-(5-hydroxy-4,6-dimethyl-pyrimidin-2-yl)-ornithine, argpyrimidine (APN). APN is a modification product of arginine residue, but it did not form from glyceraldehyde with arginine residue. The coexistence of lysine residue was necessary to APN formation.     

Isolation and Identification of the 3-Hydroxy-5-hydroxymethyl-pyridinium Compound as a Novel Advanced Glycation End Product on Glyceraldehyde-related Maillard Reaction

Glyceraldehyde (200 mM) and N^α-acetyllysine (100 mM) were incubated in 0.2 M sodium phosphate buffer (pH 7.4) at 37°C for a week. A major compound, glyceraldehyde-related Maillard reaction product, was purified from the reaction mixture using reverse phase (ODS)-HPLC. It was identified as 1-(5-acetylamino-5-carboxypentyl)-3-hydroxy-5-hydroxymethyl-pyridinium, named as GLAP (Glyceraldehyde derived Pyridinium compound), using NMR and MS analyses. It was suggested that GLAP as a novel advanced glycation end product (AGE) is one of the key compounds in the glyceraldehyde-related Maillard reaction.     

Glyceraldehyde-derived advanced glycation end-products having pyrrolopyridinium-based crosslinks

Reducing sugars and reactive aldehydes, such as glyceraldehyde, non-enzymatically react with amino or guanidino groups of proteins to form advanced glycation end-products (AGEs) by the Maillard reaction that involves Schiff base formation followed by Amadori rearrangement. AGEs are found relatively in abundance in the human eye and to accumulate at a higher rate in diseases that impair vision such as cataract, diabetic retinopathy or age-related macular degeneration. We identified two novel AGEs of pyrrolopyridinium lysine dimer derived from glyceraldehyde, PPG1 and PPG2, in the Maillard reaction of N^α-acetyl-l-lysine with glyceraldehyde under physiological conditions. Having fluorophores similar to that of vesperlysine A, which was isolated from the human lens, PPGs were found to act as photosensitizers producing singlet oxygen in response to blue light irradiation. Moreover, PPG2 interacts with receptor for AGE (RAGE) in vitro with a higher binding affinity than GLAP, a well-known ligand of the receptor. We also proposed a pathway to form PPGs and discussed how they would be formed in vitro. As glyceraldehyde-derived AGEs have been studied extensively in connection with various hyperglycemia-related diseases, further studies will be required to find PPGs in vivo such as in the lens or other tissues.     

Identification of a novel advanced glycation end product derived from lactaldehyde

Advanced glycation end products (AGEs) are implicated in the development of diabetic complications via the receptor for AGEs (RAGE). We have reported that the 3-hydroxypyridinium (3HP)-containing AGEs derived from α-hydroxyaldehydes physically interact with RAGE and show cytotoxicity. Lactaldehyde (LA) is formed from a reaction between threonine and myeloperoxidase, but no LA-derived AGEs have been characterized. Here, we identify the structure and physiological effects of an AGE derived from LA. We isolated a novel 3HP derivative, 2-acetamido-6-(3-hydroxy-5-methyl-pyridin-1-ium-1-yl)hexanoate, named as N-acetyl-LAPL (lactaldehyde-derived pyridinium-type lysine adduct), from a mixture of LA with N^α-acetyl-L-lysine. LAPL was also detected in the LA-modified protein. LAPL elicited toxicity in PC12 neuronal cells, but the effect was suppressed by the soluble form of RAGE as a decoy receptor. Moreover, surface plasmon resonance-based analysis revealed that LAPL specifically binds to recombinant RAGE. These results indicate that LA generates an AGE containing the 3HP moiety and contributes to RAGE-dependent cytotoxicity.

Abbreviations: AGEs: advanced glycation end products; RAGE: receptor for advanced glycation end products; 3HP: 3-hydroxypyridinium; LA: lactaldehyde; LAPL: lactaldehyde-derived pyridinium-type lysine adduct; BSA: bovine serum albumin; GLAP: glyceraldehyde-derived pyridinium; MPO: myeloperoxidase; HFBA: heptafluorobutyric acid; TFA: trifluoroacetic acid; HPLC: high performance liquid chromatography; LC-ESI-QTOF-MS: liquid chromatography-electrospray ionization-quadrupole time-of-flight-mass spectrometry; NMR: nuclear magnetic resonance; LA-BSA: lactaldehyde-modified bovine serum albumin; PBS: phosphate buffered saline, GST, glutathione S-transferase; SPR: surface plasmon resonance; OP-lysine: 2-ammonio-6-(3-oxidopyridinium-1-yl)hexanoate; GLO1: glyoxalase 1; MG, methylglyoxal     

The formation of argpyrimidine in glyceraldehyde-related glycation

Three major glyceraldehyde-related advanced glycation end products (AGEs) were formed from a mixture of N(alpha)-acetyllysine, N(alpha)-acetylarginine, and glyceraldehyde. Two of the compounds were MG-H1 and GLAP, as previously reported, and the other compound was identified as N(alpha)-acetyl-N(delta)-(5-hydroxy-4,6-dimethyl-pyrimidin-2-yl)-ornithine, argpyrimidine (APN). APN is a modification product of arginine residue, but it did not form from glyceraldehyde with arginine residue. The coexistence of lysine residue was necessary to APN formation.     

Detection and determination of glyceraldehyde-derived advanced glycation end product

Protein is modified by carbonyl compound in the Maillard reaction, and the irreversible structure is formed as the advanced glycation end product (AGE). We identified GLAP (glyceraldehyde-derived pyridinium compound) as an AGE formed from glyceraldehyde and lysine residue of protein. In the present study, we investigated detection and determination of GLAP from glycated protein using fluorescence HPLC method. Albumin (BSA) and carbonyls (glyceraldehyde, glycolaldehyde, methylglyoxal, glyoxal, three pentoses or three hexoses) were dissolved in phosphate buffed solution (pH 7.4), and incubated at 37 degrees C for a week. GLAP was formed only in the glyceraldehyde-modified BSA. It is suggested that GLAP was specific AGE derived from glyceraldehyde. In addition, GLAP depressed the intracellular glutathione level and induced the reactive oxygen species (ROS) in HL-60 cells. GLAP caused the oxidative stress. Therefore, GLAP will be a biomarker in the AGE related disease such as diabetic complications or chronic renal failure.     

The pecking order of skin Advanced Glycation Endproducts (AGEs) as long-term markers of glycemic damage and risk factors for micro- and subclinical macrovascular disease progression in Type 1 diabetes

To date more than 20 glycation products were identified, of which ~15 in the insoluble human skin collagen fraction. The goal of this review is to streamline 30 years of research and ask a set of important questions: in Type 1 diabetes which glycation products correlate best with 1) past mean glycemia 2) reversibility with improved glycemic control, 2) cross-sectional severity of retinopathy, nephropathy and neuropathy and 3) the future long-term risk of progression of micro- and subclinical macrovascular disease. The trio of glycemia related glycation markers furosine (FUR)/fructose-lysine (FL), glucosepane and methylglyoxal hydroimidazolone (MG-H1) emerges as extraordinarily strong predictors of existing and future microvascular disease progression risk despite adjustment for both past and prospective A1c levels. X² values are up to 25.1, p values generally less than 0.0001, and significance remains after adjustment for various factors such as A1c, former treatment group, log albumin excretion rate, abnormal autonomic nerve function and LDL levels at baseline. In contrast, subclinical cardiovascular progression is more weakly correlated with AGEs/glycemia with X² values?<?5.0 and p values generally <?0.05 after all adjustments. Except for future carotid intima-media thickness, which correlates with total AGE burden (MG-H1, pentosidine, fluorophore LW-1 and decreased collagen solubility), adjusted FUR and Collagen Fluorescence (CLF) are the strongest markers for future coronary artery calcium deposition, while cardiac hypertrophy is associated with LW-1 and CLF adjusted for A1c. We conclude that a robust clinical skin biopsy AGE risk panel for microvascular disease should include at least FUR/FL, glucosepane and MG-H1, while a macrovascular disease risk panel should include at least FL/FUR, MG-H1, LW-1 and CLF.     

Angiotensin-converting enzyme inhibitors attenuated advanced glycation end products-induced renal tubular hypertrophy via enhancing nitric oxide signaling

Advanced glycation end products (AGE) and angiotensin II were closely correlated with the progression of diabetic nephopathy (DN). Nitric oxide (NO) is a protective mediator of renal tubular hypertrophy in DN. Here, we examined the molecular mechanisms of angiotensin-converting enzyme inhibitor (ACEI) and NO signaling responsible for diminishing AGE-induced renal tubular hypertrophy. In human renal proximal tubular cells, AGE decreased NO production, inducible NOS activity, guanosine 3′,5′-cyclic monophosphate (cGMP) synthesis, and cGMP-dependent protein kinase (PKG) activation. All theses effects of AGE were reversed by treatment with ACEIs (captopril and enalapril), the NO donor S-nitroso-N-acetylpenicillamine (SNAP), and the PKG activator 8-para-chlorophenylthio-cGMPs (8-pCPT-cGMPs). In addition, AGE-enhanced activation of extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK) were clearly reduced by captopril, enalapril, SNAP, and 8-pCPT-cGMPs. The abilities of ACEIs and NO/PKG activation to inhibit AGE-induced hypertrophic growth were verified by the observation that captopril, enalapril, SNAP, and 8-pCPT-cGMPs decreased protein levels of fibronectin, p21 ^Waf1/Cip1, and receptor for AGE. The results of the present study suggest that ACEIs significantly reduced AGE-increased ERK/JNK/p38 MAPK activation and renal tubular hypertrophy partly through enhancement of the NO/PKG pathway.     

The degradation of l-threose at Maillard reaction conditions

l-Threose, a comparatively unstable aldose, is produced from l-ascorbic acid in the presence of oxygen and participates vigorously in Maillard reactions, even at comparatively mild conditions. In the present study, the degradation of l-threose at pH 7.0 alone, in the presence of N-α-acetyl-l-lysine, and at pH 2.0 alone at 37°C was investigated by identification of some of the products produced in the reactions by means of GLC and GLC-MS. Among the compounds identified were3-deoxy-tetros-2-ulose (1), the predicted alkaline rearrangement product derived from 1 (2,4-dihydroxybutyrate, the 4-carbon metasaccharinic acid), as well as glyceraldehyde. Isotopic tracer studies clearly show that the glyceraldehyde is produced by loss of C-1 from the starting l-threose molecule. The presence of N-acetyl lysine in incubation solutions appears to accelerate the production of 1, but the formation of glyceraldehyde appears to be independent of the lysine derivative.     

Molecular characteristics of methylglyoxal-modified bovine and human serum albumins. Comparison with glucose-derived advanced glycation endproduct-modified serum albumins

The amino acid modification, gel filtration chromatographic, and electrophoretic characteristics of bovine and human serum albumins irreversibly modified by methylglyoxal (MG-SA) and by glucose-derived advanced glycation endproducts (AGE-SA) were investigated. Methylglyoxal selectively modified arginine residues at low concentration (1 mM); at high methylglyoxal concentration (100 mM), the extent of arginine modification increased and lysine residues were also modified. Both arginine and lysine residues were modified in AGE-SA. Analytical gel filtration HPLC of serum albumin derivatives suggested that the proportion of dimers and oligomers increased with modification in both low and highly modified MG-SA and AGE-SA derivatives relative to unmodified serum albumins. In SDS-PAGE analysis, dimers and oligomers of low-modified MG-SA were dissociated into monomers, but not in highly modified MG-SA. MG-SA had increased anodic electrophoretic mobility under nondenaturing conditions atpH 8.6, indicating an increased net negative charge, which increased with extent of modification; highly modified MG-SA and AGE-SA had similar high electrophoretic mobilities. MG-SA derivatives were fluorescent: the fluorescence was characteristic of the arginine-derived imidazoloneN -(5-methyl-4-imidazolon-2-yl)ornithine, but other fluorophores were also present. AGE-SA had similar fluorescence, attributed, in part, to glucose-derived imidazolones. AGE formed from glucose-modified proteins and AGE-like compounds formed from methylglyoxal-modified proteins may both be signals for recognition and degradation of senescent macromolecules.Abbreviations AGE advanced glycation endproduct - BSA bovine serum albumin - HSA human serum albumin - MG-SA methylglyoxal-modified serum albumin - MG-BSA methylglyoxal-modified bovine serum albumin - MG-HSA methylglyoxal-modified human serum albumin - AGE-SA AGE-modified serum albumin - AGE-BSA AGE-modified bovine serum albumin - AGE-HSA AGE-modified human serum albumin - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - HPLC high-performance liquid chromatography - FFI 2-(2-furoyl)-4(5)-(2-furanyl)-1H-imidazole     

Substituent Effect on Photosynthesis of N2-α-Substituted Benzyl-N4-alkyl-2,4-diamino-6-chloro-s-triazines and Their Phytotoxic Property

The influence of substituents on the activities of a series of N²-α-substituted benzyl-N⁴-alkyl-2,4-diamino-6-chloro-s-triazines as inhibitors of photosystem II (PSII) was examined, and the phytotoxic differences between them and atrazine, as to the photosynthesis in leaf disks, mesophyll cells, intact chloroplasts and broken chloroplasts of spinach, and as to seedling-growth, were discussed. The inhibitory activity of the N²-α,α-dimethylbenzyl-N⁴-ethyl derivative (6), which was comparable on that of atrazine, was lower than those of the N²-α-alkylbenzyl analogues (1 ~5). The N⁴-?-alkyl-N²-α- methylbenzyl derivatives, in spite of the carbon length of the alkyl group, exhibited more potent activity than atrazine, but an a α β substitution of the N⁴-n-alkyl group caused a decrease in the activity with a few exceptions. These data may imply that the space of the binding site on PSII surrounding both the N² and N⁴ amino groups is relatively large. The binding between the receptor site and the N⁴ amino group, however, is easily influenced by a slight structural change in an inhibitor. The herbicidal compounds, N²-α-methylbenzyl-A^⁴-ethyl (1), A^²-α,α-dimethylbenzyl-N⁴-1-methylpropyl (30) and N²-α-methylbenzyl-N⁴,N⁴-diethyl (42) derivatives, exhibited potent inhibitory activity in the seedling growth test under dark/light conditions, whereas atrazine was very poor. The inhibitory activity of compound (1) toward photosynthesis was poor with leaf disks, compared to atrazine, whereas, the order of their activities was the reverse for plant preparations such as abaxial epidermis peeled leaf disks, mesophyll cells, intact chloroplasts and broken chloroplasts. It was indicated that a change in the phytotoxic symptom in the whole plant assay would be correlated to the permeability of the compound through the plant membrane(s).     

An autoantibody against N-(carboxyethyl)lysine (CEL): Possible involvement in the removal of CEL-modified proteins by macrophages

Advanced glycation end products (AGEs) are believed to play a significant role in the development of diabetic complications. In this study, we measured the levels of autoantibodies against several AGE structures in healthy human plasma and investigated the physiological role of the autoantibodies. A high titer of the autoantibody against N^ε-(carboxyethyl)lysine (CEL) was detected in human plasma compared with other AGE structures such as CML and pentosidine. The purified human anti-CEL autoantibody reacted with CEL-modified human serum albumin (CEL-HSA), but not CML-HSA. A rabbit polyclonal anti-CEL antibody, used as a model autoantibody against CEL, accelerated the uptake of CEL-HSA by macrophages, but did not enhance the uptake of native HSA. Furthermore, when ¹²⁵I-labeled CEL-HSA was injected into the tail vein of mice, accumulation of ¹²⁵I-CEL-HSA in the liver was accelerated by co-injection of the rabbit anti-CEL antibody. These results demonstrate that the autoantibody against CEL in plasma may play a role in the macrophage uptake of CEL-modified proteins.     

Screening for glucose‐triggered modifications of glutathione

Nonenzymatic protein glycation is caused by a Schiff's base reaction between the aldehyde groups of reducing sugars and the primary amines of proteins. These structures may undergo further Amadori rearrangement and free radical‐mediated oxidation to finally generate irreversible advanced glycation end products (AGEs). One of the factors known to modulate the glycation of proteins is glutathione, the most abundant nonprotein thiol tripeptide with the γ‐linkage, H‐Glu(Cys‐Gly‐OH)‐OH (GSH). Screening for products formed by GSH with D ‐glucose is an essential step in understanding the participation of GSH in glycation (the Maillard) reaction. Under the conditions used in these studies we observed N‐(1‐deoxy‐D ‐fructos‐1‐yl)‐pyroglutamic acid as the major glycation product formed in the mixtures of GSH and glucose in vitro. A RP HPLC/MS and tandem MS analyses of the GSH/glucose mixtures revealed that cleavage of the N‐terminal glutamic acid and the formation of pyroglutamic acid‐related Amadori product were accompanied by generation of Cys‐Gly‐derived Amadori and thiazolidine compounds. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Determination of Glyceraldehyde Formed in Glucose Degradation and Glycation

Glyceraldehyde (GLA) was determined in glucose degradation and glycation. GLA was detected as a decahydroacridine-1,8-dione derivative on reversed phase HPLC using cyclohexane-1,3-dione derivatizing reagent. The glucose-derived GLA level was higher than the glycation-derived GLA level, because GLA was converted to intermediates and advanced glycation end products (AGE) in glycation. GLA was also generated from 3-deoxyglucosone and glucosone as intermediates of glucose degradation and glycation. This study suggests that glyceraldehyde is generated by hyperglycemia in diabetes, and that it is also formed in medicines such as peritoneal dialysis solution.     

Pathway Engineering of Bacillus subtilis for Enhanced N‐Acetylneuraminic Acid Production via Whole‐Cell Biocatalysis

N‐acetylneuraminic acid (NeuAc) is a common sialic acid that has a wide range of applications in nutraceuticals and pharmaceuticals. However, low production efficiency and high environmental pollution associated with traditional extraction and chemical synthesis methods constrain the supply of NeuAc. Here, a biological approach is developed for food‐grade NeuAc production via whole‐cell biocatalysis by the generally regarded as safe (GRAS) bacterium Bacillus subtilis (B. subtilis). Promoters for controlling N‐acetylglucosamine 2‐epimerase (AGE) and NeuAc adolase (NanA) are optimized, yielding 32.84 g L^?1 NeuAc production with a molar conversion rate of 26.55% from N‐acetylglucosamine (GlcNAc). Next, NeuAc production is further enhanced to 46.04 g L^?1, which is 40.2% higher than that of the strain with promoter optimization, by expressing NanA from Staphylococcus hominis instead of NanA from Escherichia coli. To enhance the expression level of ShNanA, the N‐terminal coding sequences of genes with high expression levels are fused to the 5′‐end of the ShNanA gene, resulting in 56.82 g L^?1 NeuAc production. Finally, formation of the by‐product acetoin from pyruvate is blocked by deleting the alsS and alsD genes, resulting in 68.75 g L^?1 NeuAc production with a molar conversion rate of 55.57% from GlcNAc. Overall, a GRAS B. subtilis strain is demonstrated as a whole‐cell biocatalyst for efficient NeuAc production.     

Methylglyoxal-induced modification of arginine residues decreases the activity of NADPH-generating enzymes

Inadequate control of plasma and cellular glucose and ketone levels in diabetes is associated with increased generation of reactive aldehydes, including methylglyoxal (MGO). These aldehydes react with protein side chains to form advanced glycation end-products (AGEs). Arg residues are particularly susceptible to MGO glycation and are essential for binding NADP⁺ in several enzymes that generate NADPH, a coenzyme for many critical metabolic and antioxidant enzymes. In most animal cells, NADPH is produced predominantly by glucose-6-phosphate dehydrogenase (G6PD) in the oxidative phase of the pentose phosphate pathway and, to a lesser extent, by isocitrate dehydrogenase (IDH) and malic enzyme (ME). In this study, the activities of isolated G6PD, IDH, and ME were inhibited by MGO (0–2.5 mM, 2–3 h, 37 °C), in a dose- and time-dependent manner, with G6PD and IDH more sensitive to modification than ME. Significant inhibition of these two enzymes occurred with MGO levels ≥500 μM. Incubation with radiolabeled MGO (0–500 µM, 0–3 h, 37 °C) demonstrated dose- and time-dependent adduction to G6PD and IDH. HPLC analysis provided evidence for AGE formation and particularly the hydroimidazolones MG-H1 and MG-H2 from Arg residues, with corresponding loss of parent Arg residues. Peptide mass mapping studies confirmed hydroimidazolone formation on multiple peptides in G6PD and IDH, including those critical for NADP⁺ binding, and substrate binding, in the case of IDH. These results suggest that modification of NADPH-producing enzymes by reactive aldehydes may result in alterations to the cellular redox environment, potentially predisposing cells to further damage by oxidants and reactive aldehydes.     

The combined effect of acetylation and glycation on the chaperone and anti-apoptotic functions of human α-crystallin

N^ε-acetylation occurs on select lysine residues in α-crystallin of the human lens and alters its chaperone function. In this study, we investigated the effect of N^ε-acetylation on advanced glycation end product (AGE) formation and consequences of the combined N^ε-acetylation and AGE formation on the function of α-crystallin. Immunoprecipitation experiments revealed that N^ε-acetylation of lysine residues and AGE formation co-occurs in both αA- and αB-crystallin of the human lens. Prior acetylation of αA- and αB-crystallin with acetic anhydride (Ac₂O) before glycation with methylglyoxal (MGO) resulted in significant inhibition of the synthesis of two AGEs, hydroimidazolone (HI) and argpyrimidine. Similarly, synthesis of ascorbate-derived AGEs, pentosidine and N^ε-carboxymethyl lysine (CML), was inhibited in both proteins by prior acetylation. In all cases, inhibition of AGE synthesis was positively related to the degree of acetylation. While prior acetylation further increased the chaperone activity of MGO-glycated αA-crystallin, it inhibited the loss of chaperone activity by ascorbate-glycation in both proteins. BioPORTER-mediated transfer of αA- and αB-crystallin into CHO cells resulted in significant protection against hyperthermia-induced apoptosis. This effect was enhanced in acetylated and MGO-modified αA- and αB-crystallin. Caspase-3 activity was reduced in α-crystallin transferred cells. Glycation of acetylated proteins with either MGO or ascorbate produced no significant change in the anti-apoptotic function. Collectively, these data demonstrate that lysine acetylation and AGE formation can occur concurrently in α-crystallin of human lens, and that lysine acetylation improves anti-apoptotic function of α-crystallin and prevents ascorbate-mediated loss of chaperone function.     

Methylglyoxal-modified arginine residues — a signal for receptor-mediated endocytosis and degradation of proteins by monocytic THP-1 cells

Non-enzymatic glycosylation or glycation of proteins to form advanced glycation endproducts (AGE) has been proposed as a process which provides a signal for the degradation of proteins. Despite this, the AGE which act a recognition factor for receptor-mediated endocytosis and degradation of glycated proteins by monocytes and macrophages has not been identified. Methylglyoxal, a reactive α-oxoaldehyde and physiological metabolite, reacted irreversibly with arginine residues in proteins to form N_δ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine and N_δ-(5-methyl-4-imidazolon-2-yl)ornithine residues. Human serum albumin minimally-modified with methylglyoxal (MG_min-HSA) was bound by cell surface receptors of human monocytic THP-1 cells in vitro at 4°C: the binding constant K_d value was 377±35 nM and the number of receptors per cell was 5.9±0.2×10⁵ (n=12). N_α-Acetyl-N_δ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine displaced MG_min-HSA from THP-1 cells, suggesting that the N_δ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine residue was the receptor recognition factor. At 37°C, MG_min-HSA was internalised by THP-1 cells and degraded. Similar binding and degradation of human serum albumin modified by glucose-derived AGE was found but only when highly modified. MG_min-HSA, therefore, is the first example of a protein minimally-modified by AGE-like compounds that binds specifically to monocyte receptors. The irreversible modification of proteins by methylglyoxal is a potent signal for the degradation of proteins by monocytic cells in which the arginine derivative, N_δ-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine, is the receptor recognition factor. This factor is not present in glucose-modified proteins.