Evidence for the Existence of a Self-Regulated Enzymatic Process Within the Human Stratum Corneum –An Unexpected Role for Urocanic Acid  Paul M. Krien, Michel Kermici  Journal of Investigative Dermatology  Volume 115, Issue 3, Pages 414-420 (September 2000) DOI: 10.1046/j.1523-1747.2000.00083.x Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions

Figure 1 Time-dependent evolution of the proton concentration in the electrode compartment. Proton concentration is related to the pH value using the equation: C = 10–pH. A significant increase in the concentration is only observed when the tip of the electrode is in contact with skin surface. Journal of Investigative Dermatology 2000 115, 414-420DOI: (10.1046/j.1523-1747.2000.00083.x) Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions

Figure 2 Influence of 15 successive tape stripping on the flux of protons in the electrode chamber. Three groups of response were observed: A partial abrasion of stratum corneum had a negligible influence on the magnitude of flux in group Slow, induced a slight decrease in group Medium, and a high decrease in group Fast. Journal of Investigative Dermatology 2000 115, 414-420DOI: (10.1046/j.1523-1747.2000.00083.x) Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions

Figure 3 Evolution of skin surface pH of two volunteers after usual cleansing operation (skin washed with soap and tap water). A significant increase in the acidity of skin surface was observed with type Slow volunteers (see text for details). Journal of Investigative Dermatology 2000 115, 414-420DOI: (10.1046/j.1523-1747.2000.00083.x) Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions

Figure 4 Stratum corneum pK determination via a titration curve. Titration curve was obtained after 1 h contact of water with skin surface of volunteer 21 (see Table 2). Similar titration curves were observed with all the other volunteers. Journal of Investigative Dermatology 2000 115, 414-420DOI: (10.1046/j.1523-1747.2000.00083.x) Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions

Figure 5 In vivo pH gradient in human stratum corneum. Over a total of 17 volunteers, six (male) were observed in group Fast, nine (female) in group Medium, and two (male) in group Slow. The data are presented as mean ± SEM. Where no SEM is shown, the SEM was smaller than the sign of the symbol. The linear fit of data indicates a significant increase in the proton concentration with tape strip number in type Slow volunteers (R = 0.9798, n = 5, p =0.00345) and in type Medium volunteers (R = 0.99637, n = 5, p =0.00026). No significant increase was observed in type Fast volunteers (R = -0.09812, n = 5, p =0.8753). Journal of Investigative Dermatology 2000 115, 414-420DOI: (10.1046/j.1523-1747.2000.00083.x) Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions

Figure 6 In vivo distribution of UA in human stratum corneum. The data are presented as mean ± SEM. Where no SEM is shown, the SEM was smaller than the sign of the symbol. Over a total of six volunteers, two were observed in group 1, and four in group 2. A significant increase in the UA distribution was observed with group 2 only (R = 0.9912, n = 5, p <0.001). Journal of Investigative Dermatology 2000 115, 414-420DOI: (10.1046/j.1523-1747.2000.00083.x) Copyright © 2000 The Society for Investigative Dermatology, Inc Terms and Conditions