Effect of Diabetes and Aging on Carboxymethyllysine Levels in Human Urine

Carboxymethyllysine (CML) has been identified as a modified amino acid that accumulates with age in human lens proteins and collagen. CML may be formed by oxidation of fructoselysine (FL), the Amadori adduct formed on nonenzymatic glycosylation of lysine residues in protein, or by reaction of ascorbate with protein under autoxidizing conditions. We proposed that measurements of tissue and urinary CML may be useful as indices of oxidative stress or damage to proteins in vivo. To determine the extent to which oxidation of nonenzymatically glycosylated proteins contributes to urinary CML, we measured the urinary concentrations of FL and CML in diabetic (n = 26) and control (n = 28) patients. The urinary concentration of FL correlated strongly with HbA₁ measurements and was significantly higher in diabetic compared with control samples (9.2 ± 6.5 and 4.0 ± 2.8 μg/mg creatinine, respectively; P < 0.0001). There was also a strong correlation between the concentrations of CML and FL in both diabetic and control urine (r = 0.67, P < 0.0001) but only a weakly significant increase in the CML concentration in diabetic compared with control urine (1.2 ± 0.5 and 1.0 ± 0.3 μg/mg creatinine, respectively; P = 0.05). The molar ratio of CML to FL was significantly lower in diabetic compared with control patients (0.25 ± 0.12 and 0.43 ± 0.16, respectively; P < 0.0001). The correlation between urinary FL and CML and the increase in CML in diabetic urine suggests that urinary CML is derived in part from autoxidation of FL in nonenzymatically glycosylated proteins, whereas the decreased ratio of CML to FL in diabetic urine suggests that oxidative stress is decreased in diabetes. The ratio of CML to FL showed no correlation with age in either diabetic or control populations, arguing against an increase in oxidative stress to proteins with age.