Oxidative degradation of glucose adducts to protein. Formation of 3-(N epsilon-lysino)-lactic acid from model compounds and glycated proteins

Mechanism of the degradation of non-enzymatically glycated proteins under physiological conditions. Studies with the model fructosamine, N epsilon-(1-deoxy-D-fructos-1-yl)hippuryl-lysine

The degradation of fructosamines, formed from the non-enzymic glycation of proteins under physiological conditions, to advanced glycation end products was investigated by studying the model peptide fructosamine N epsilon-(1-deoxy-D-fructos-1-yl)hippuryl-lysine (DHL). At pH 7.4 and 37 degrees C in aerobic phosphate buffer, DHL degraded to form N epsilon-carboxymethyl-hippuryl-lysine, and hippuryl-lysine over a 29-day incubation period. The expected N epsilon-(3-lactato)hippuryl-lysine and 'hippuryl-lysylpyrraline' derivatives were not found. Superoxide radicals and hydrogen peroxide were formed during the degradation of DHL but were also both consumed during the degradation reaction. Reversal of the Amadori rearrangement was not a major fate of the fructosamine. The formation of N epsilon-carboxymethyl-hippuryl-lysine was decreased by desferrioxamine, catalase, superoxide dismutase, catalase with superoxide dismutase, anaerobic conditions and aminoguanidine. The formation of hippuryl-lysine was decreased by desferrioxamine, catalase and catalase with superoxide dismutase, but was increased by the addition of aminoguanidine. N epsilon-Carboxymethyl-serine and unmodified lysine residues are major peptide-based end products in the degradation of lysyl-fructosamine under physiological conditions. Oxygen, redox-active metal ions, catalase, superoxide dismutase and the pharmacological agent aminoguanidine are expected to be influential on the rate and fate of fructosamine degradation.

In vivo glycation of bovine lens crystallins

To investigate the possible role of glycation in the aging of lens proteins, we used bovine lenses as a model. We studied crystallins isolated from prenatal bovine lenses, calf lenses and lenses from mature animals (up to 20 years old). The experiments show an increase in glycation levels with age in all crystallin fractions. Regarding the lysine content of the different crystallins, gamma-crystallin showed relatively high levels of early glycation products. The results also revealed high levels of early glycation products for the HM material (containing mainly alpha-crystallin). In alpha-crystallin, alpha A-subunits were glycated to a higher extent compared with the alpha B-subunits. There is an age-related increase in advanced glycation products, measured as specific fluorescence (excitation/emission wavelengths 370/440 nm), mainly present in the HM and alpha-crystallin fraction.

Direct evidence for the alterations in protein structure and conformation upon in vitro nonenzymatic glycosylation

The formation of nonenzymatic glycosylation products appears to be a link between chronic hyperglycaemia and long-term diabetic complications. However, little is known concerning the glycation-induced modifications in the structure and conformation of proteins, which possibly underlie their altered functional characteristics. This study conveys a direct evidence for and compares the glucose-induced modifications in the conformation of three proteins with various half-lives: bovine serum albumin, human haemoglobin and bovine tendon collagen. These proteins incubated in vitro with glucose in various media containing optionally EDTA and Fe2+ ions contained up to 4-10 times as much attached glucose as did their relevant controls, and the extent of glycation was the highest in the samples incubated under air or in the absence of EDTA. The fluorescence and ESR data indicate that the Trp in albumin molecule, given albumin glycation-induced structural modifications, became more exposed to water surrounding solution whereas the Trp residues of haemoglobin remained shielded from water; also collagen fluorescence derived from the supposedly newly formed covalent crosslinks is vastly increased, and particularly when collagen was glycated under air or in the presence of Fe2+ ions. Possible mechanisms underlying the increased mobility of selected protein domains and glycation-mediated alterations in protein conformation are considered and discussed.

Age-related increase of collagen fluorescence in human subcutaneous tissue

Nonenzymatic glycation produces compounds with characteristic fluorescence in long-lived proteins. We recently described the influence of age in rat collagen-linked fluorescence. To examine the effect of age in humans, we studied the subcutaneous collagen-linked fluorescence in samples from 26 subjects of both sexes (age range, 42 to 78 years) who were undergoing vascular surgery. Intensity of fluorescence at 385 nm (upon excitation at 335 nm) and 440 nm (upon excitation at 370 nm) increased exponentially with age (r = .827, y = 114 + e0.038x, P less than .001; and r = .905, y = 36 + e0.039x, P less than 0.001, respectively). The two sets of data exhibited a high degree of correlation (r = .980, P less than .001, n = 26). Age-adjusted fluorescence data did not correlate with sex, body weight, or type of vascular pathology. The collagen fluorescence accumulation rate was 3.7% per year, and the characteristic time (CT) was 26 to 27 years. We conclude that the fluorescence measurement is a reliable methodology that can be used as a marker for biological age until new, more-specific tools are available.

Glycation alters collagen fibril organization

Incubation of rat tail tendon in 0.2M ribose results in accelerated non-enzymatic glycosylation of collagen, with the formation of fluorescent cross-links between molecules and decreased solubility. Electron micrographs of tendon cross-sections show an increased fibril packing density with increasing degrees of glycation. After a one-week incubation in ribose, every fibril appears in close contact with all of its neighbors, and the packing density has increased to 76%, from a value of 62% in controls. Irregular diameters and fusion of fibrils also are seen. All of the fibrils in a bundle appear to become cross-linked together, creating a larger stress bearing unit. This model is consistent with stress-strain curves showing a large increase in tensile stress and stiffness after a one-week incubation period in ribose. The diameters of the collagen fibrils increase in size in glycated tendon. We hypothesize larger diameters result from an increased resistance to shrinkage during the specimen preparation process, as a result of the rigid sugar derived cross-links. Closer fibril packing, increased fibril diameters, and irregular diameters have been reported in diabetic tissues, and may result from decades of glycation induced cross-link accumulation.

The significance of the products of the Maillard (browning) reaction in diabetes

The accumulation of the products of the Maillard reaction leads to structural and functional modifications of tissue proteins. In normoglycaemia, these modifications result in slow age-related accumulation of AGE-proteins. Hyperglycaemia accelerates formation of the Maillard products. The increased rate of Amadori products formation in poorly controlled diabetes leads to the impairment of the function of susceptible short-lived proteins and accelerates the formation of AGE on proteins with a long half-life. AGE accumulation increases protein crosslinking and leads to changes in the mechanical and biological properties of the affected proteins. AGE-modified proteins covalently bind other molecules. This may contribute to the formation of pathological tissue deposits and to the in situ formation of immune complexes. AGE-modified proteins also induce changes in biosynthetic/secretory patterns of macrophages, endothelial cells, and mesangial cells. These data led to the formulation of hypotheses which propose a central role for the Maillard products both in the process of ageing and in the development of the late complications of diabetes. More clinical studies are required to further substantiate these attractive hypotheses.

Collagen in the ageing human intervertebral disc: an increase in covalently bound fluorophores and chromophores

Human disc tissue gradually changes in colour from white in the young to yellow brown in the elderly. It was investigated to what degree this colouration and an associated fluorescence (which are characteristic of the non-enzymic reaction products of sugars or oxidized lipids with proteins), were the result of covalent derivatives on the collagen and other extracellular matrix proteins. Annulus fibrosus was obtained from four subjects aged 19 to 92 years. Papain-solubilized samples of tissue showed increasing yellow colour and the glycation-related fluorescence with age. On chromatography of CNBr-digests of tissue, the yellow colour and fluorescence remained bound to the CNBr-peptide fragments of the collagen and other matrix proteins. One peptide, alpha 1(II)CB12 (from type II collagen), was selected for purification and shown to contain increasing amounts of the characteristic fluorescence with age. Sequencing by Edman degradation over 24 cycles confirmed the identity of the peptide, and by analysis of a portion of the PTH-derivatives showed fluorescence at cycle 11, a lysine residue. The results imply that much of the yellow colour and characteristic fluorescence that accumulate in ageing human discs are contributed by covalent adducts (possibly derived from non-enzymic reactions with carbohydrates or lipids) linked to the collagen and probably to other long-lived matrix proteins. The disc is perhaps particularly susceptible to such protein modifications because, being large and avascular, of its tendency to a low oxygen tension. Such modifications to structural proteins may contribute to the commonly observed degeneration and impaired material function of ageing disc tissue.

Age-dependent accumulation of N epsilon-(carboxymethyl)lysine and N epsilon-(carboxymethyl)hydroxylysine in human skin collagen

N epsilon-(Carboxymethyl)lysine (CML) is formed on oxidative cleavage of carbohydrate adducts to lysine residues in glycated proteins in vitro [Ahmed et al. (1988) J. Biol. Chem. 263, 8816-8821; Dunn et al. (1990) Biochemistry 29, 10964-10970]. We have shown that, in human lens proteins in vivo, the concentration of fructose-lysine (FL), the Amadori adduct of glucose to lysine, is constant with age, while the concentration of the oxidation product, CML, increases significantly with age [Dunn et al. (1989) Biochemistry 28, 9464-9468]. In this work we extend our studies to the analysis of human skin collagen. The extent of glycation of insoluble skin collagen was greater than that of lens proteins (4-6 mmol of FL/mol of lysine in collagen versus 1-2 mmol of FL/mol of lysine in lens proteins), consistent with the lower concentration of glucose in lens, compared to plasma. In contrast to lens, there was a slight but significant age-dependent increase in glycation of skin collagen, 33% between ages 20 and 80. As in lens protein, CML, present at only trace levels in neonatal collagen, increased significantly with age, although the amount of CML in collagen at 80 years of age, approximately 1.5 mmol of CML/mol of lysine, was less than that found in lens protein, approximately 7 mmol of CML/mol of lysine. The concentration of N epsilon-(carboxymethyl)hydroxylysine (CMhL), the product of oxidation of glycated hydroxylysine, also increased with age in collagen, in parallel with the increase in CML, from trace levels at infancy to approximately 5 mmol of CMhL/mol of hydroxylysine at age 80.(ABSTRACT TRUNCATED AT 250 WORDS)

The Maillard reaction in vivo

The Maillard or browning reaction between reducing sugars and protein contributes to the chemical deterioration and loss of nutritional value of proteins during food processing and storage. This article presents and discusses evidence that the Maillard reaction is also involved in the chemical aging of long-lived proteins in human tissues. While the concentration of the Amadori adduct of glucose to lens protein and skin collagen is relatively constant with age, products of sequential glycation and oxidation of protein, termed glycoxidation products, accumulate in these long-lived proteins with advancing age and at an accelerated rate in diabetes. Among these products are the chemically modified amino acids, N epsilon-(carboxymethyl)lysine (CML), N epsilon-(carboxymethyl)hydroxylysine (CMhL), and the fluorescent crosslink, pentosidine. While these glycoxidation products are present at only trace levels in tissue proteins, there is strong evidence for the presence of other browning products which remain to be characterized. Mechanisms for detoxifying reactive intermediates in the Maillard reaction and catabolism of extensively browned proteins are also discussed, along with recent approaches for therapeutic modulation of advanced stages of the Maillard reaction.

Reaction of ascorbate with lysine and protein under autoxidizing conditions: formation of N epsilon-(carboxymethyl)lysine by reaction between lysine and products of autoxidation of ascorbate

N epsilon-(Carboxymethyl)lysine (CML) has been identified as a product of oxidation of glucose adducts to protein in vitro and has been detected in human tissue proteins and urine [Ahmed, M. U., Thorpe, S. R., & Baynes, J. W. (1986) J. Biol. Chem. 261, 4889-4894; Dunn, J. A., Patrick, J. S., Thorpe, S. R., & Baynes, J. W. (1989) Biochemistry 28, 9464-9468]. In the present study we show that CML is also formed in reactions between ascorbate and lysine residues in model compounds and protein in vitro. The formation of CML from ascorbate and lysine proceeds spontaneously at physiological pH and temperature under air. Kinetic studies indicate that oxidation of ascorbic acid to dehydroascorbate is required. Threose and N epsilon-threuloselysine, the Amadori adduct of threose to lysine, were identified in the ascorbate reaction mixtures, suggesting that CML was formed by oxidative cleavage of N epsilon-threuloselysine. Support for this mechanism was obtained by identifying CML as a product of reaction between threose and lysine and by analysis of the relative rates of formation of threuloselysine and CML in reactions of ascorbate or threose with lysine. The detection of CML as a product of reaction of ascorbate and threose with lysine suggests that other sugars, in addition to glucose, may be sources of CML in proteins in vivo. The proposed mechanism for formation of CML from ascorbate is an example of autoxidative glycosylation of protein and suggests that CML may also be an indicator of autoxidative glycosylation of proteins in vivo.