epsilon-(gamma-Glutamyl)lysine cross-links in human stratum corneum

epsilon-(gamma-Glutamyl)lysine has been found in human stratum corneum in the fraction containing the alpha helical fibrous proteins (keratins) and other high molecular weight proteins. The S-carboxymethylated fractions were enzymatically digested with pronase, carboxypeptidase A and B, leucine aminopeptidase and prolidase, and epsilon-(gamma-glutamyl)lysine isolated from digests by gel filtration and cation ion exchange chromatography. Acid hydrolysis of the purified epsilon-(gamma-glutamyl)lysine yielded equimolar amounts of lysine and glutamic acid, and end group analysis of the peptide by dansylation (application of 5-dimethylaminonaphthalene-1-sulfonyl) confirmed the isomer assignment to be epsilon-(gamma-glutamyl)lysine. About 9 nmol of the peptide per mg of protein were found in the fraction by isotope dilution after the enzymatic digestion. These results suggest that proteins in stratum corneum may be covalently cross-linked through epsilon-(gamma-glutamyl)lysine bonds.