Histidine‐rich Protein as a Possible Origin of Free Amino Acids of Stratum Corneum

Pulse‐chase labeling experiments with 3H‐histidine and 3H‐arginine were performed in hairless mice to investigate the origin of free amino acids in the stratum corneum. Time‐course labeling of epidermal proteins was examined by sodium dodecyl sulfate‐polyacryl amide gel electrophoresis (SDS‐PAGE). The radioactivity was also measured in the following three epidermal fractions; 0.1 N HClO4 soluble‐ethanol soluble fraction (Fr. I), 0.1 N HClO4 soluble‐ethanol insoluble fraction (Fr. II, histidine‐rich protein fraction) and 0.1 N HClO4 insoluble‐8M urea soluble fraction (Fr. III). Labeled amino acids were rapidly incorporated into Fr. III, especially into a particular protein which showed little mobility on SDS‐PAGE. As the radioactivity of this protein decreased, an increase in radioactivity was observed in a 32,000 MW histidine‐rich protein, the only major protein in Fr. II. The radioactivity of 32,000 MW protein then fell, accompanied by an increase in radioactivity of Fr. I. The major radioactive substances in Fr. I were identified as 3H‐histidine and 3H‐urocanic acid after 3H‐histidine injection and as 3H‐arginine after 3H‐arginine injection. The amino acid composition of the histidine‐rich protein was very similar to the composition of free amino acids of the stratum corneum, in which amino acid metabolites were considered as their precursor amino acids. These results suggested that the free amino acids of the stratum corneum might be a final product of the degradation of histidine‐rich protein.