Evidence for the existence of a self-regulated enzymatic process within the human stratum corneum -an unexpected role for urocanic acid.

The existence of a flux of proton donors from skin (inner part of the forearm) to the electrode was observed in 12 male and female volunteers. This flux was used to collect and identify the ionic species responsible for skin acidity. It was then found that: (i) pK of these proton donors (pK = 6.13 +/- 0.07) was quasi-identical to that of trans-urocanic acid (6.10), and (ii) the amount of urocanic acid present in stratum corneum was sufficient in itself to explain the acidic level as measured with pH meter (R = 0.8484, n = 10, p = 0.00136). As a result, the contribution of other ionic species can be considered as negligible in normal human skin. The data recorded led us to identify three groups (Fast, Medium, and Slow) characterized by different skin surface pH values (low, medium, and close to neutral) and showing a pH gradient in the outer layers of the stratum corneum, or not. Data analysis suggests that these characteristics depend on urocanic acid production rate within the stratum corneum and that this production rate is self-regulated by its urocanic acid content.