Oxidation of glycated proteins: age-dependent accumulation of N.epsilon.-(carboxymethyl)lysine in lens proteins

Abstract

N epsilon-(Carboxymethyl)lysine (CML) has been identified as a product of oxidation of fructoselysine (FL) in glycated (nonenzymatically glycosylated) proteins in vitro and has also been detected in human tissues and urine [Ahmed et al. (1986) J. Biol. Chem. 261, 4889-4894]. In this study, we compare the amounts of CML and FL in normal human lens proteins, aged 0-79 years, using specific and sensitive assays based on selected ion monitoring gas chromatography-mass spectrometry. Our results indicate that the lens content of FL increases significantly between infancy and about age 5 but that there is only a slight, statistically insignificant increase in FL between age 5 and 80 (mean +/- SD = 1.4 +/- 0.4 mmol of FL/mol of Lys). In contrast, the lens content of the oxidation product, CML, increased linearly with age, ranging from trace levels at infancy up to 8 mmol of CML/mol of lysine at age 79. The ratio of CML to FL also increased linearly from 0.5 to 5 mol of CML/mol of FL between age 1 and 79, respectively. These results indicate that CML, rather than FL, is the major product of glycation detectable in adult human lens protein. The age-dependent accumulation of CML in lens protein indicates that products of both glycation and oxidation accumulate in the lens with age, while the constant rate of accumulation of CML in lens with age argues against an age-dependent decline in free radical defense mechanisms in this tissue.

Age-related muscle dysfunction

“…For instance, CML accumulates in aging human skin collagen (Dunn et al, 1991), lens crystallins (Dunn et al, 1989), and arterial walls (Schleicher et al, 1997). These findings suggest glycoxidation reactions and oxidative stress may be important mechanisms in age-related degenerative processes.…”

Age-related accumulation of Maillard reaction products in human articular cartilage collagen

“…The major initial product is fructose-lysine (FL) [1,2], which results from glycation of ε-amino groups on lysine residues. In subsequent Maillard or browning reactions, products known as advanced glycation end products (AGEs) are formed from FL [3,4], and accumulate with age in long-lived proteins [1,2,[5][6][7][8].…”

“…FL, CML, CEL and pentosidine were measured in cartilage collagen, and their levels were compared with those previously reported for skin collagen and lens protein [1,2,21]. In addition, general measures of glycation-derived cross-links, i.e.…”

“…In subsequent Maillard or browning reactions, products known as advanced glycation end products (AGEs) are formed from FL [3,4], and accumulate with age in long-lived proteins [1,2,[5][6][7][8]. These AGEs include structurally characterized adducts, such as N ε -(carboxymethyl)lysine (CML) [1][2][3] and N ε -(carboxyethyl)-lysine (CEL) [9], fluorescent cross-links, such as pentosidine formed between lysine and arginine residues [5,10], as well as chemically unidentified compounds which result in proteinbound browning or fluorescence, and cross-linking [11][12][13][14].In comparison with other collagen-rich tissues such as skin, cartilage contains relatively large amounts of pentosidine [5,15]. Pentosidine levels in articular cartilage increase linearly with age [6-8], as was previously described for skin collagen [16] and lens proteins [10].…”

“…The FL, CML and CEL content of the collagen samples is expressed as mmol per mol of lysine residues. Historical data for lens protein and skin collagen have been determined using the same methodology as described above [1,2,9,21].Pentosidine content and amino acid composition were determined by HPLC as described previously [24,25]. In short, dried hydrolysates were dissolved in internal standard solution containing 10 µM pyridoxine (Sigma) and 2.4 mM homoarginine (Sigma).…”