Protein glycation was viewed originally as a post-translational modification that accumulated mostly on extracellular proteins. AGEs are formed slowly throughout life and the concentrations of AGEs found represent a life-long accumulation of the glycation adduct. This applies to chemically stable AGE residues formed on long-lived proteins. For example, CML, CEL and pentosidine residue accumulation on skin collagen (1;2). FL and some AGEs (hydroimidazolones, for example) have relatively short chemical half-lives under physiological conditions (2 – 6 weeks), however, and their concentration depends on the balance of the rates of formation and decomposition. Moreover, protein glycation residues are also formed on cellular and short-lived extracellular proteins. The turnover of these proteins by cellular proteolysis releases glycated amino acids or glycation free glycation adducts (3). Protein damage by glycation is implicated in protein misfolding. Misfolded proteins are degraded by the proteasome to ensure the high quality of intracellular proteins (4); the median half-life of cellular proteins was 32 h (5). Protein glycation free glycation adducts are, therefore, the excreted debris of cellular proteolysis of proteins damaged by glycation. This appears to be an efficient process since there was little evidence of low molecular mass damaged peptides in peripheral venous plasma. Additional sources of free adducts are direct glycation of lysine and arginine. This probably plays a minor contribution because of the relatively low concentration of free amino acids compared to the concentration of corresponding amino acid residues in proteins (3).
Figure 1. Biodistribution scheme illustrating flows of formation and removal of protein glycation, oxidation and nitration free adducts.
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Saccharide-rich and thermally processed foods are a good source of FL, AGEs and oxidation adduct residues (6). There may be low bioavailability of AGE residues in proteins of ingested foods such that <10% is absorbed (7). Proteins glycated highly by FL and AGEs are resistant to proteolysis (8) and some AGEs inhibit intestinal proteases (9). The highest concentration of absorbed food AGEs is expected in portal venous plasma where we found the hydroimidazolone MG-H1 residues enriched by 10-fold in peptides (10). AGEs and oxidation adducts are therefore probably absorbed from food as both AGE free adducts and AGE-rich peptides; the latter appear to be degraded efficiently after absorption.
Proteins in tissues, plasma and extracellular matrix in vivo have high FL residue content (0.1 – 1.8 mmol/mol lys) and a wide range of AGE residue contents (0.001 – 15 mmol/mol amino acid modified) - depending on the location and type of AGE. Hydroimidazolones were the most important AGEs quantitatively both as protein residues and free adducts – particularly methylglyoxal derived hydroimidazolone MG-H1, CML and CEL were typically at levels 5 – 10-fold lower, and pentosidine and imidazolium crosslinks at levels 100-1000 fold lower than hydroimidazolones. Hydroimidazolone AGEs are found in highest concentrations in lens protein of elderly human subjects; MG-H1 residue content was 1 – 2% of total arginine residues). Stable AGEs such as CML accumulate on lens capsule, skin and cartilage collagen with age but reached <6 mmol/mol lys in cartilage in old age (1;11).
AGE free adducts are exported from cells, leak into plasma and are excreted in the urine. Major quantitative glycation adducts, FL, hydroimidazolones, CML and CEL, have high renal clearances 35 – 93 ml/min in normal human subjects but this declines in uremia (3). AGE free adducts are a major form by which glycation adducts are eliminated – taking into account AGEs in both urinary albumin and protein fragments (12).
The physiological importance of protein glycation remains under intensive investigation. Particularly damaging effects are produced by covalent crosslinking of proteins which confers resistance to proteolysis (13). Protein modification is also damaging when amino acid residues are located in sites of protein-protein interaction, enzyme-substrate interaction and protein-DNA interaction (for transcription factors). These and other protein modifications are expected to impair functional interactions and may be involved in biochemical dysfunction in disease.
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2. Wells-Knecht,MC, Lyons,TJ, McCance,DR, Thorpe,SR, Baynes,JW: Age-dependent increases in ortho-tyrosine and methionine sulfoxide in human skin collagen is not accelerated in diabetes. J Clin Invest 100:839-846, 1997
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