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.

The trans to cis photoisomerization of urocanic acid (UCA) in skin is considered to play an important role in the mechanism of immunosuppression. We have investigated the effects of skin type and various sunscreens with low sun protection factor (SPF) on the UV-induced cis-UCA formation in human skin after exposure to artificial UV light. The rate of cis-UCA formation depends little on the skin type and is reduced by topical application of sunscreens. The rate of cis-UCA formation decreases with increasing SPF and only broad-spectrum, highly protective sunscreens offer protection against the UV-induced formation of cis-UCA, which accumulates in the stratum corneum after multiple UV exposures. A theoretical approach to estimate the distribution of cis-UCA after irradiation indicates that this compound may diffuse into the deeper layers of the epidermis with D approximately 10(-17) m2/s, and that its elimination from the stratum corneum is mainly due to desquamation.

Percutaneous absorption of five compounds was studied in the hairless rat in vivo: benzoic acid, caffeine, hydrocortisone, inulin and thiourea. The results clearly demonstrate that, as with in vitro experiments, a steady-state flux can be achieved in vivo. This steady-state flux is strongly molecule dependent. Thus, the values for inulin and benzoic acid differ by a factor of about 40. In contrast, although the physicochemical properties of the studied compounds vary widely, their lag times were not significantly different. The mean lag time was 11 +/- 2 min. Different compounds could be considered to have approximately the same apparent diffusion coefficient with regard to their percutaneous absorption in vivo. Thus, for a given thickness of stratum corneum and a given anatomical site, the penetration flux value of a substance depends only on its stratum corneum/vehicle partition coefficient. Using a classical model, we have demonstrated that the amount of substance present in the stratum corneum (Qsc) at equilibrium (30 min) is related to this partition coefficient. There is also a linear relationship between steady-state flux and Qsc. In practice, the in vivo steady-state flux of penetration of a compound can be predicted from the simple measurement of the amount present in the stratum corneum after a contact time of 30 min.

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