Glycation

Glycation Suzanne R. Thorpe and John W. Baynes University of South Carolina, Columbia, South Carolina, USA

Glycation is a term describing the adduction of a carbohydrate to another biomolecule, such as a protein or lipid. Glycation may occur either enzymatically or nonenzymatically. The common term for enzymatic glycation is glycosylation, e.g., formation of a glycosidic bond using a sugar nucleotide donor during synthesis of glycoproteins. The terms nonenzymatic glycation, nonenzymatic glycosylation, or glycation (without a modifier) are commonly used in reference to direct chemical reactions of reducing sugars with proteins, illustrated by the reaction of glucose with lysine residues in protein to form a ketoamine (Amadori) adduct (Figure 1). Glucation, fructation, ribation, etc., are used in reference to glycation by specific sugars. Although glycation is a reversible reaction, it is considered a first step in the Maillard or browning reaction, which leads to irreversible chemical modification, browning, generation of fluorescence, and cross-linking of proteins during cooking, and in the body during aging and in disease. The irreversible adducts and cross-links in protein are known as advanced glycation end products (AGEs). Most AGEs are formed by a combination of glycation and oxidation reactions and are termed glycoxidation products. AGEs accumulate with age in long-lived extracellular proteins, such as collagen, and are also formed from glycolytic intermediates on shorter-lived intracellular proteins. Increased rates of formation of AGEs in tissue proteins during hyperglycemia and oxidative stress or inflammation are implicated in the pathophysiology of aging, and diabetes and other chronic diseases.

valine residues in the allosteric site regulating the oxygen affinity of Hb. The a-chain valines, which have similar exposure to solvent and the same low pKa, but are not in the allosteric site, are only , 10% as glycated as the b-chain valines. HbAII also has 66 lysine residues, but there are only about twice as many fructoselysine (FL) as fructosevaline residues in the protein. The more reactive lysines are adjacent to basic or acidic amino acid in the primary or three-dimensional structure of the protein, which either affect the nucleophilicity of the lysine amino group or catalyze the Amadori rearrangement. The primary sites of glycation of ribonuclease by glucose in vitro are at lysine residues in the active site. Glycation of some, but not all of these residues is catalyzed by phosphate and other anionic buffers that bind in the basic, RNA-binding region of the enzyme. Thus, glycation, although not as specific as enzymatic glycosylation of protein, is not a random modification of protein, but is affected by the pKa of the amino acid, amino acid sequence, protein conformation, and ligand binding.

CLINICAL SIGNIFICANCE OF GLYCATED HB

Interest in the Maillard reaction in biochemistry began with the identification of HbAIc, an anodic variant of adult hemoglobin (Hb), discovered during screening of diabetic patients. HbAIc contains glucose bound to the amino-terminal valine residues of the Hb b-chains. The glucose– valine adduct was characterized as an aminoketose or ketoamine (fructosevaline), formed by nonenzymatic reaction of glucose with protein (Figure 1). The high reactivity of the b-chain valine residues of Hb results from catalysis of the Amadori rearrangement by phosphate or 2,3-bisphosphoglycerate bound near these

Assays of glycated Hb (GlcHb) are a powerful tool for the clinical management of diabetes. Glycation is a slow reaction. The rate of formation of HbAIc is , 0.1% of HbAII per day at normal blood glucose concentration (5 mM), and is first order in glucose concentration. Since the red cell is long-lived (life span , 120 days) and is freely permeable to glucose, the mean extent of glycation of Hb varies with mean blood glucose concentration. The average percent glycation of Hb, ranges from 4 to 6% HbAIc in a control population, to 16% or higher in poorly controlled diabetic patients. Based on clinical studies, assays of percent HbAIc integrate the mean blood glucose concentration during the previous 4 –6 weeks. These assays complement acute blood glucose measurements for assessment of long-term glycemic control in diabetes. Assays for HbAIc use an anion exchange procedure that depends on the shift in pKa of the protein, which occurs on glycation of the valine residues. Alternatively,

Encyclopedia of Biological Chemistry, Volume 2. q 2004, Elsevier Inc. All Rights Reserved.

229

Glycation of Proteins in Blood GLYCATED HEMOGLOBIN

230

GLYCATION

O NH – CH – C O

(CH2)4

NH–CH–C (CH2)4 NH2 Lysine

C

NH

NH – CH – C

O

H C OH (CHOH)3

(CH2)4

COOH

[O2]

NH

Carboxymethllysine

C O

[O2] Arginine

(CHOH)3 CH2OH

Lysine N

HN

+

Fructoselysine

CH2OH Glucose

CML

CH2

CH2

+ H

O

HN Amadori adduct

N

Arginine Pentosidine [O2]

Glycation

AGEs

Glycoxidation FIGURE 1 Pathway for glycation of protein. Glucose reacts reversibly with amino groups in protein, e.g., lysine, forming a Schiff base (not shown), which rearranges to the Amadori compound (ketoamine), fructoselysine. Oxidation of the Amadori compound is one route to formation of advanced glycation or glycoxidation end products, such as CML and pentosidine.

total GlcHb may be measured by an affinity chromatography assay that depends on interaction of Amadori adducts on the surface of the protein with phenylboronate resins; this assay measures primarily the glycation of lysine residues. While assays of HbAIc and GlcHb actually measure different aspects of GlcHb, the two assays yield similar quantitative results and correlate strongly with one another. Both the ion-exchange and affinity chromatography assays are available as minicolumn kits and HPLC methods. Electrophoretic, nephelometric (immuno-turbidimetric), and ELISA assays are also used, but less commonly, for measurement of glycation of Hb and assessment of long-term glycemic control.

GLYCATION OF EXTRACELLULAR PLASMA PROTEINS

AND

Glucose is ubiquitous in body fluids, and therefore all tissue proteins are subject to glycation. The steadystate extent of glycation of a protein depends on the inherent reactivity of the protein with glucose under physiological conditions, the protein’s biological halflife or lifespan, and the ambient glucose concentration. The fractional glycation of lysine in lens proteins is , 40% that of skin collagen in humans, reflecting the lower glucose concentration in the lens, compared to extracellular fluids. Collagen accounts for about onethird of total body protein mass and is accessible by biopsy. Like glycation of Hb, the extent of glycation of skin collagen may be used as an index of very long-term hyperglycemia or glycative stress.

In addition to Hb, other intracellular proteins are also subject to glycation, e.g., superoxide dismutase in the lens, alcohol dehydrogenase in the liver, and the actomyosin complex in muscle. Assays for glycated plasma proteins, which have relatively short half-lives compared to hemoglobin or collagen, provide an index of intermediate-term blood glucose concentration, during the previous 7 – 10 days. In addition to ELISA and phenylboronate affinity chromatography assays for glycated albumin, total Amadori adducts on plasma proteins can be measured by the fructosamine assay. This assay measures the reduction of a tetrazolium dye by the Amadori compound on plasma proteins. The Amadori adduct has much stronger reducing activity than glucose at pH 10, so that the reducing activity of Amadori adducts on glycated plasma proteins can be measured without significant interference by blood glucose.

GLYCATION OF OTHER AMINO ACIDS BIOMOLECULES

AND

Glucose appears to react only with primary amino groups on protein. However, Amadori compounds undergo facile rearrangement and oxidation reactions. They may rearrange nonoxidatively to form deoxyglucosones, regenerating free lysine, or undergo autoxidative decomposition to form glucosone. These dicarbonyl compounds react with the side chains of lysine, arginine, cysteine, histidine, and probably tryptophan residues, expanding the range of glycative damage to protein. Amadori adducts to phosphatidylethanolamine and

GLYCATION

phosphatidylserine have also been measured in tissues. DNA undergoes glycation in vitro, although glycated DNA has not been identified in vivo, perhaps because of the involvement of the amino groups of nucleic acids in intrahelical hydrogen bonds and the shielding provided by histones and other nuclear proteins.

REACTIVITY

OF

GLUCOSE

The rate of reaction of a sugar with protein depends on the fraction of the sugar in the open-chain, aldehyde conformation. Glucose, among all sugars, exists to the greatest extent in the cyclic, hemiacetal conformation. It is therefore the least reactive in glycating proteins, in comparison to other hexoses, pentoses, ketoses, or phosphorylated metabolic intermediates. Another unique feature of glucose is that, although it is a reducing sugar and is readily oxidized, it is the least active among all sugars in oxidation or autoxidation (oxidation by molecular oxygen) reactions—again, because it exists largely in a hemiacetal rather than aldehyde conformation. In the Fehling and Benedict assays for reducing sugars, oxidation is promoted by strongly basic conditions that catalyze the enolization and isomerization of glucose. Its low reactivity with protein and its resistance to autoxidation was probably important in the evolutionary selection of glucose as blood sugar in mammals. Despite its chemical inertness, however, glucose does react with proteins at a measurable rate. Glycation occurs and advanced glycation end products (AGEs) accumulate on tissue proteins with age.

Advanced Glycation and Glycoxidation Reactions ADVANCED GLYCATION Food scientists learned early in the twenty-first century that glycation was one of the early steps in the Maillard or browning reaction of proteins during cooking, part of the process of caramelization that enhances the color, taste, aroma, and texture of cooked foods. The Maillard reaction also contributes to the loss in nutritional value of cooked foods, because of the chemical modification of the essential amino acids, lysine and arginine. When biomedical scientists discovered that hemoglobin was glycated in vivo, it was reasonable to speculate that advanced stages of the Maillard reaction should also proceed in vivo – the human body could be viewed, at one level, as a lowtemperature oven (98.68F) with a long cooking cycle (, 80 years). The Maillard reaction was seen as a possible source of the brown color and fluorescence that develops with age in long-lived proteins, such as crystallins and collagens, and as a contributing factor in

231

both the normal aging of tissue proteins and the acceleration of age-like pathology by hyperglycemia in diabetes. More than 20 AGEs have now been detected in tissue proteins (Figure 2), including amines, amides, pyrroles, imidazolones, imidazolium salts, and several bicyclic heteroaromatic compounds. The first two AGEs to be identified in vivo were N1-(carboxymethyl)lysine (CML) and pentosidine. Because of their stability during acid hydrolysis of proteins, these compounds are still the most frequently measured AGEs. CML, which is quantitatively the most abundant AGE, is measured by GC/MS or ELISA. Although present at much lower concentration, pentosidine is highly fluorescent and is measured by HPLC with fluorescence detection. Other AGEs are measured less frequently because they are either present at lower concentrations in tissues or are labile to acid or base hydrolysis and must be measured following enzymatic digestion of proteins. The known AGEs are only trace components of proteins in vivo. Even in skin collagen and lens proteins of elderly diabetic patients, CML and CEL (Figure 2) typically account for modification of less than 1% of the lysine residues in the protein. However, these biomarkers are thought to represent only a fraction of the total chemical modification of amino acids and cross-linking of proteins in vivo.

GLYCOXIDATION Although oxygen is not required for glycation, formation of CML and pentosidine from FL requires oxidation of the Amadori compound: cleavage between C-2 and C-3 of the carbohydrate to form the lysine adduct CML; and loss of one carbon from glucose during the formation of pentosidine. The development of brown color and fluorescence and formation of AGEs during incubation of proteins with glucose in vitro are also accelerated by oxygen. The oxidation reactions are more rapid in phosphate and carbonate buffers, compared to organic buffers, because of the presence of trace amounts of redox-active, metal ions, such as iron and copper in inorganic buffers, which catalyze oxidation and glycoxidation reactions. The term, glycoxidation, was coined in recognition of the importance of both glycation and oxidation in the formation of AGEs. Oxidation reactions often have high-negative free energies, and, especially in oxidative cleavage reactions, oxygen can be viewed as the fixative of irreversible damage to protein during the Maillard reaction. However, some AGEs, such as pyrraline and the cross-lines, may be formed from glucose under nonoxidative conditions. Likely precursors to these compounds include the dicarbonyl sugars 1- and 3-deoxygluosone, which are formed from Amadori compounds under nonoxidative conditions.

232

GLYCATION

R

CH2

C=O

NH

NH

(CH2)4 NH

Lysine

COOH

CH

HOH2C

(CH2)4

(CH2)4

O C

NH

CH

N

R

O

O

C

NH

GALA: R=CH2OH LOMA: R=COOH

CML: R=H CEL : R=CH3

N +

CHO

N

CH

Lysine

C

GOLD: R=H MOLD: R=CH3

Pyrraline

CH2OH

CH2OH Lysine

(HCOH)3

(HCOH)3

OH

O

N

OH N

+

N

N

Lysine

+

N

HO

N

Lysine

Lysine

Lysine

Lysine Cross-line

Fluorolink

Vesperlysine

Arginine

Arginine

Arginine OH N

N

N

N

N

OH HN

HN N

+ N

Lysine

Lysine

Glucosepane

R

O

HN

N

Pentosidine

R

HN N Lysine GODIC: R=H MODIC: R=CH3

OH CH3

CH3 N

NH

N NH

(CH2)3

(CH2)3

O NH CH

N

C

Hydroimidazolone MGO: R=CH3 3DG: R=THB

NH

CH

O C

Argpyrimidine

FIGURE 2 Structure of AGEs that have been detected in tissue proteins by chemical and/or immunological methods. Top: Nonfluorescent lysine adducts and cross-links, including amides, amines and imidazolium salts. Second row: Fluorescent lysine-lysine cross-links. Third row: Fluorescent arginine-lysine cross-links. Bottom: Arginine derivatives. CML, N1-(carboxymethyl)lysine; CEL, N1-(carboxyethyl)lysine; GALA, glycolic acid lysine monoamide; LOMA, lysine oxalic acid monoamide; GOLD and MOLD, GO and MGO lysine dimer (imidazolium salts); and GODIC and MODIC, GO and MGO dihydroimidazolylidene cross-links.

GLYCATION

233

ALTERNATIVE PATHWAYS TO AGES AND GLYCOXIDATION PRODUCTS

RECOGNITION AND TURNOVER OF AGE- PROTEINS

In the early 1980s, the Amadori adduct was considered an obligate intermediate in the formation of AGEs (Figure 1). However, it is now clear that there are multiple precursors and pathways to formation of the same AGE (Figure 3). CML, for example, may be formed from glyoxal or glycolaldehyde which are produced on autoxidation of many carbohydrates, or by autoxidation of Schiff base intermediates and Amadori adducts, or from ascorbate and dehydroascorbate and phosphorylated intermediates in glycolysis. To complicate the issue further, glycolaldehyde, formed on oxidation of serine by myeloperoxidase, and glyoxal formed during lipid peroxidation reactions, may also contribute to the formation of CML. Thus, the measurement of CML provides little information regarding its origin. Lipid-derived adducts to protein, such as malondialdehyde and hydroxynonenal adducts to protein, are known as advanced lipoxidation endproducts (ALEs), so that CML and GOLD, and the homologous compounds, CEL and MOLD, are now considered AGE/ALEs, reflecting the fact that they may be derived from a variety of carbohydrate and noncarbohydrate precursors in vivo. Some AGEs, such as pentosidine and cross-lines, which contain 5 – 6 carbon rings or side-chains (Figure 2), are clearly derived from carbohydrates, but may be formed from a variety of carbohydrate precursors, including reducing sugars, sugar phosphates, and ascorbate.

Nearly a dozen AGE-binding proteins have also been identified to date. The best characterized among the AGE receptors are RAGE (receptor for AGE), which is widely distributed among endothelial and parenchymal cells, and the macrophage scavenger receptors (MSRs). An unusual feature of these receptors is their range of ligand binding. In addition to binding of heterogeneously modified AGE-proteins, RAGE also recognizes the peptide amphoterin, which is involved in neuronal development, and the Ab-amyloid peptide found in Alzheimer’s plaque. MSRs were originally described as receptors for acetyl-LDL, later for their role in macrophage uptake of oxidized lipoproteins, then for their role in the recognition of AGE-proteins. The rapid uptake of injected AGE proteins (and oxidized LDL) in liver is consistent with a role for MSRs and the reticuloendothelial system in the removal of AGE-proteins from the circulation. A number of other candidate AGE receptor proteins have also been identified, based on interaction with AGE-proteins, but the determinants or motifs recognized by these receptors are, in most cases, unknown. Scavenger receptors recognize both AGE-proteins and oxidized lipoproteins, suggesting that common products are formed on chemical modification of proteins by both carbohydrates and lipids. CML has been proposed as a ligand for RAGE. Binding of AGE-proteins to AGE-receptors appears to induce oxidative stress to cells, possibly because of the activity of AGEs, a source of reactive oxygen, in much the same way that binding and uptake of oxidized lipoproteins causes oxidative stress to cells bearing MSR. Thus, there is some risk to the cell involved in recognition and catabolism of AGEs, and chronic increases in AGE-induced oxidative stress have been invoked as a source of damage to endothelial cells and development of vascular and renal disease in diabetes. Under normal conditions, oxidative stress and protein turnover induced by AGE-proteins may have a role in the remodeling and rejuvenation of tissues, however this process may become pathogenic in chronic disease.

Ascorbate Aldoses and ketoses

PUFA

[Protein]

[O]

Schiff base

[O]

[O]

Glyoxal Glycolaldehyde [Protein]

Amadori adduct

Metabolism

[HOCI]

[O] CML

Serine

FIGURE 3 Alternative sources of the AGE/ALEs. CML may be formed from reducing sugars, either directly by oxidative cleavage of Schiff base or Amadori adducts on protein, or indirectly through glyoxal or glycolaldehyde formed on oxidation of glucose or glucose adducts to protein. CML may also be formed from ascorbate, through glyoxal, glycolaldehyde, or tetroses formed on oxidative degradation of dehydroascorbate. Glyoxal or glycolaldehyde are also formed during peroxidation of polyunsaturated fatty acids, or during myeloperoxidase-mediated degradation of amino acids.

The Maillard Reaction in Aging and Disease AGING The age-dependent browning of tissue proteins is most apparent in the lens, which becomes visibly yellow with age. This change in color is accompanied by insolubilization, aggregation, and precipitation of lens crystallins, which may proceed to formation of cataracts.

234

GLYCATION

Collagens and other long-lived proteins in the body undergo similar changes with age. The increased crosslinking of collagen contributes to the decrease in elasticity and resiliency of the extracellular matrix with age. By interfering with turnover of matrix components by metalloproteinases, browning reactions may also contribute to the age-dependent thickening of basement membranes in the microvasculature of the kidney, retina, and other tissues, affecting the permeability and transport properties of the matrix. The accumulation of AGEs in long-lived proteins (Figure 4) is accompanied by an age-dependent increase in protein fluorescence and by a parallel increase in other chemical modifications of proteins, including products of both nonoxidative (e.g., aspartate racemization, spontaneous deamidation) and oxidative damage (o-tyrosine, nitrotyrosine, dityrosine, and methionine sulfoxide). Research on the Maillard hypothesis today is focused on increasing evidence that AGEs have effector functions, i.e., they activate or

DIABETES The glycation, AGE, or Maillard hypothesis identifies glucose as the culprit in diabetes, and proposes that diabetic complications result from accelerated Maillard reaction damage to tissue protein. Consistent with the hypothesis, both the extent of glycation of protein and the rate of accumulation of AGEs are accelerated by hyperglycemia in diabetes (Figure 4). The damage, like hyperglycemia, is systemic, and age-adjusted levels of AGEs in skin collagen are also correlated with the

diabetic nondiabetic

20 15 10 5 0 20

A

40

60

1.0

0.5

0

80 B

Age (years)

60

20

40

60

80

Age (years)

0.0

diabetic nondiabetic

diabetic D nondiabetic ND

2.0 mmol CML/mol lysine

mmol Pentosidine/mol lysine

1.5

0.0 0

40

20

ND

D

1.5

1.0

0.5

0.0

0 0 C

diabetic nondiabetic

2.0 mmol CML/mol lysine

25 mmol FL/mol lysine

participate in physiological responses to stress, including inflammation, hyperglycemia, and hyperlipidemia, and oxidative stress. Although proteins serve as the register or accumulator of Maillard reaction damage in vivo, glycoxidative and oxidative modifications of DNA are certain to have significant effects on the integrity of the genome during aging.

20

40

60

Age (years)

80

0 D

20 40 60 mmol Pentosidine/mol lysine

FIGURE 4 Influence of age and diabetes on glycation and glycoxidation of skin collagen. (A) Glycation of protein is reversible and relatively constant with age. Glycation of collagen increases with mean blood glucose concentration and correlates with GlcHb in diabetic patients. (B) and (C) The concentrations of the AGE/ALE, CML, and of the AGE, pentosidine, increase with age in the nondiabetic population and at an increased rate in diabetic patients. (D) Despite differences in origin and mechanism of formation and significant differences in their concentrations, the levels of CML and pentosidine in skin collagen correlate more closely with one another than with age in either control or diabetic populations. (Reproduced from Dyer, D. G., Dunn, J. A., Thorpe, S. R., Bailie, K. E., Lyons, T. J., McCance, D. R., and Baynes, J. W. (1993). Accumulation of Maillard reaction products in skin collagen in diabetes and aging. J. Clin. Invest. 91, 2463–2469, with permission.)

GLYCATION

severity of renal, retinal, and vascular complications of diabetes. The Maillard hypothesis is a chemical rather than metabolic theory on the origin of diabetic complications; it explains the development of similar complications in both type 1 (juvenile-onset, insulindependent) and type 2 (adult-onset, noninsulindependent) diabetes, despite the differences in etiology of these diseases (insulin deficiency versus insulin resistance). It also explains the development of complications in kidney, nerve, retina, and vasculature, since endothelial and parenchymal cells in these tissues are mostly independent of insulin for glucose transport. In these tissues, intracellular glucose tracks extracellular glucose concentration. These tissues are also rich in long-lived extracellular proteins, such as collagens, elastin, and myelin. In addition to the gradual accumulation of AGEs on extracellular proteins, AGEs are also formed on intracellular proteins in these tissues, probably from glycolytic intermediates, such as the triose phosphates. Research on intracellular glycation and AGE formation is still at an early stage, but is essential for a comprehensive understanding of the role of the Maillard reaction in the pathogenesis of diabetic complications.

AGE INHIBITORS Perhaps the most convincing evidence for the AGE hypothesis is the demonstrated efficacy of AGEinhibitors in retarding the development of a wide range of complications in animal models of diabetes. Both aminoguanidine (AG) and pyridoxamine (PM) are potent inhibitors of formation of AGEs by autoxidation of sugars and Schiff base adducts to proteins, while PM, described as an Amadorin, also inhibits formation of AGEs from Amadori compounds. AG and PM inhibit the formation of AGEs and cross-linking of collagen in vivo, and also limit the increase in immunohistochemically detectable AGEs in tissues in diabetes. Other activities of these inhibitors may contribute to their efficacy in vivo, since they inhibit amine oxidases, including some isoforms of nitric oxide synthase, have weak chelating activity, and also have antihypertensive and hypolipidemic effect in vivo.

OTHER DISEASES AGEs and ALEs are increased in tissues in a wide range of age-related, chronic diseases, including diabetes, atherosclerosis, dialysis-related amyloidosis, arthritis, and neurodegenerative diseases. The increase in glycoxidative and lipoxidative damage reflects, in part, uncontrolled chemistry occurring, either locally or systemically, in biological systems as a result of tissue damage, decreased antioxidant defenses and

235

increased decompartmentalization of metal ions. Although the AGE/ALEs are not the primary source of pathology in these diseases, they are macrophage chemoattractants and proinflammatory molecules, acting as both intermediaries in pathogenic processes and biomarkers of resultant tissue damage. AGE/ALE inhibitors may eventually prove useful for limiting damage or complications from a wide range of chronic, age-related diseases. Their effects on health and longevity are being explored in studies in animal models.

SEE ALSO

THE

FOLLOWING ARTICLES

Collagens † Diabetes † Glycogen Storage Diseases † Insulin- and Glucagon-Secreting Cells of the Pancreas

GLOSSARY advanced glycation end-product (AGE) Irreversible end-product of nonenzymatic reaction of carbohydrates with protein; includes fluorescent and nonfluorescent adducts and cross-links in protein. advanced lipoxidation end-product (ALE) Stable end-product of chemical modification of protein by reactive carbonyl intermediates formed during lipid peroxidation reactions. Amadori adduct or compound First stable product of glycation of protein; a ketoamine. glycation Enzymatic or nonenzymatic adduction of a carbohydrate to a biomolecule. glycoxidation product AGE formed by a combination of glycation and oxidation chemistry.

FURTHER READING Baynes, J. W., and Thorpe, S. R. (2000). Glycoxidation and lipoxidation in atherogenesis. Free Radic. Biol. Med. 28, 1708–1716. Bucciarelli, L. G., Wendt, T., Rong, L., Lalla, E., Hofmann, M. A., Goova, M. T., Taguchi, A., Yan, S. F., Yan, S. D., Stern, D. M., and Schmidt, A. M. (2002). RAGE is a multiligand receptor of the immunoglobulin superfamily: Implications for homeostasis and chronic disease. Cell Mol. Life Sci. 59, 1117–1128. Dyer, D. G., Dunn, J. A., Thorpe, S. R., Bailie, K. E., Lyons, T. J., McCance, D. R., and Baynes, J. W. (1993). Accumulation of Maillard reaction products in skin collagen in diabetes and aging. J. Clin. Invest. 91, 2463–2469. Miyata, T., Devuyst, O., Kurokawa, K., and van Ypersele de Strihou, C. (2002). Toward better dialysis compatibility: Advances in the biochemistry and pathophysiology of the peritoneal membranes. Kidney Int. 61, 375– 386. Schleicher, E. D., Bierhaus, A., Haringm, H. U., Nawroth, P. P., and Lehmann, R. (2001). Chemistry and pathobiology of advanced glycation end products. Contrib. Nephrol. 131, 1 –9. Stitt, A. W., Jenkins, A. J., and Cooper, M. E. (2002). Advanced glycation end products and diabetic complications. Expert Opin. Invest. Drugs 11, 1205–1223.

236

GLYCATION

Thorpe, S. R., and Baynes, J. W. (2003). Maillard reaction products in tissue proteins: New products and new perspectives. Amino Acids 25, 275–281. Ulrich, P., and Cerami, A. (2001). Protein glycation, diabetes, and aging. Recent Prog. Horm. Res. 56, 1–21. Vlassara, H. (2001). The AGE-receptor in the pathogenesis of diabetic complications. Diabetes Metab. Res. Rev. 17, 436–443. Vlassara, H., and Palace, M. R. (2002). Diabetes and advanced glycation endproducts. J. Intern. Med. 251, 87–101.

BIOGRAPHY Suzanne Thorpe and John Baynes are a husband-and-wife team who have worked together for over 25 years in teaching and research at the University of South Carolina. They study chemical modifications of proteins by carbohydrates, lipids, and oxidation reactions, focusing on the role of nonenzymatic chemistry in regulatory biology and in the pathogenesis of aging and age-related, chronic diseases.