The presence of N(ε)-(Carboxymethyl) lysine in the human epidermis

It is well known that advanced glycation end products (AGEs) are formed in long-lived dermal proteins such as collagen, and that their formation is related to skin aging. To examine the distribution of AGEs in skin tissue, we performed immunofluorescence studies on the human skin using an anti-AGEs antibody. Interestingly, AGEs signals were observed not only in the dermis but also in the epidermis. The objectives of this study were to confirm the presence of N(ε)-(Carboxymethyl) lysine (CML), an AGE structure, in the epidermis and to characterize the CML-modified proteins. The presence of CML in the stratum corneum (SC) was examined using liquid chromatography-electrospray ionization time-of-flight mass spectrometry. Concordance between the retention times of a compound in the SC hydrolysate and authentic CML, as well as with the specific mass transition of CML, was detected. This result showed that CML is present in the epidermis. In order to characterize the CML-modified proteins in the epidermis, protein samples extracted from the SC were analyzed using two-dimensional electrophoresis followed by an amino acid sequence analysis. The clarified peptide sequences covered approximately 27% of the amino acid sequences of cytokeratin 10 (K10). In the immunoblotting experiment following the two-dimensional electrophoresis, where protein samples extracted from whole epidermis were used, the position of the major CML-positive spots corresponded to those of K10. Taken together these results showed that CML is present in the human epidermis, and suggest that K10 is one of the target molecules for CML modification in the epidermis.