Glycation of nail proteins: from basic biochemical findings to a representative marker for diabetic glycation-associated target organ damage

Abstract

Background: Although assessment of glycated nail proteins may be a useful marker for monitoring of diabetes, their nature and formation are still poorly understood. Besides a detailed anatomical analysis of keratin glycation, the usefulness of glycated nail protein assessment for monitoring diabetic complications was investigated.

Methods: 216 patients (94 males, 122 females; mean age ± standard deviation: 75.0 ± 8.7 years) were enrolled. Glycation of nail and eye lens proteins was assessed using a photometric nitroblue tetrazolium-based assay. Following chromatographic separation of extracted nail proteins, binding and nonbinding fractions were analyzed using one-dimensional gel electrophoresis. Using a hand piece containing a latch-type-bur, a meticulous cutting of the nail plate into superficial and deep layers was performed, followed by a differential analysis of fructosamine.

Results: Using SDS PAGE, four and two bands were identified among the nonglycated and glycated nail fraction respectively. Significantly lower fructosamine concentrations were found in the superficial nail layer (mean: 2.16 ± 1.37 μmol/g nails) in comparison with the deep layer (mean: 4.36 ± 2.55 μmol/g nails) (P<0.05). A significant higher amount of glycated eye lens proteins was found in diabetes mellitus patients (mean: 3.80 ± 1.57 μmol/g eye lens) in comparison with nondiabetics (mean: 3.35 ± 1.34 μmol/g eye lens) (P<0.05). A marked correlation was found between glycated nail and glycated eye lens proteins [y (glycated nail proteins) = 0.39 + 0.99 x (eye lens glycated proteins); r2 = 0.58, P<0.001]. The concentration of glycated eye lens proteins and the HbA1c level were found to be predictors of the concentration of glycated nail proteins.

Conclusions: Glycation of nail proteins takes place in the deep layer of finger nails, which is in close contact with blood vessels and interstitial fluid. Glycation of nail proteins can be regarded as a representative marker for diabetic glycation-associated target organ damage.

Fig 1. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of nail proteins after boronate affinity chromatography (extracted keratins with different molecular mass, ranging from 46 to 67 kDa).

Fig 2. Glycated nail protein concentration between deep and superficial layers of the human finger nail (n = 12).

Fig 3. Correlation between glycated nail proteins and glycated eye lens proteins in the combined group of diabetes patients and nondiabetics (n = 51).

The equation of linear regression is y (glycated nail proteins) = 0.39 + 0.99 x (glycated eye lens proteins) (r2 = 0.58; P<0.001).

Fig 4. Correlation between glycated nail proteins and glycated eye lens proteins in the group of diabetes patients (n = 35).

The equation of linear regression is y (glycated nail proteins) = 0.26 + 1.12 x (glycated eye lens proteins) (r2 = 0.71; P<0.001).

Fig 5. Illustration of the diffusion process of glucose from the deep part to the superficial part of the nail, explaining the differential concentration of glycated proteins in both layers of the human finger nail.