Читать книгу pH of the Skin: Issues and Challenges - Группа авторов - Страница 14

In the second part of the 19th century, sweat was subject to intense research and the question whether sweat was acid or alkaline was a central question that had controversy and was intensely debated upon. In a comprehensive paper in the journal Monatsheft für praktische Dermatologie, Ernst Heuss – a scholar of Prof. Dr. Paul Gerson Unna (Hamburg) – described the experimental procedures to determine the acid and alkaline character of sweat and skin colorimetrically [10]. He provided conclusive evidence that fresh sweat and the skin surface were acidic. He presumed that lower parts of the skin were less acidic, concurrently admitting that he had experimental difficulties to conclusively prove his assumption. In the years to come, the evidence for the acid character of the skin including a pH gradient within the skin (from acid toward neutral) was finally substantiated [11–14].

In 1928, Heinrich Schade and Alfred Marchionini reported results of their investigation of the reaction of the sweat. They confirmed the observations made by Heuss and others and added data from their own pH measurements from cadaver and living skin. It is interesting to note that they then discussed the experimental challenges of cleansing the electrode and skin and also the pH shifts in diluted and concentrated sweat and point out that the measurement of the pH is possible only when the skin is sufficiently moist. For dry skin, they suggested the use of a mist spray. They also mentioned that the use of acid skin preparations may influence skin pH. With the title of their paper “Der Säuremantel der Haut” (the acid mantle of the skin), they coined the term “the acid mantle” of the skin, which has become an easy-to-understand term for everybody [15]. In 1930, Scholtz confirmed and expanded Schade’s and Marchionini’s observations. He tested several acid topical treatments (e.g., sour mild) and recognized the significance of acid preparation in relapse prevention [16].

In the years 1938 and 1939, Marchionini et al. [17–21] published 5 scientific communications in the Klinische Wochenschrift on “Der Säuremantel der Haut und Bakterienabwehr” (acid mantle and defense against bacteria). They describe experimental details, give references related to their instrumentations, document age- and body site as well as skin disease dependent skin pH shifts, and discuss the significance of the pH and bacterial growth on the skin. In their fourth and fifth communication, they make a connection between the altered quantitative and qualitative bacterial growth in pathologically modified skin and the shifted skin pH and attribute it partly to a pathologic gap in the acid mantle (pathologische Lücke des Säuremantels). They also investigated the pH of several topical dermatologic preparations and concluded that their effect can at least partly be attributed to their acid character and recommend the systematic investigation of acid treatments in dermatology. It is interesting to note that Marchionini, Schade or Scholz did not explicitly explain why they coined the term “acid mantle” [15–21]. The designation “acid mantle” has become a plain metaphor to illustrate the protective value of the acid character of the skin and is nowadays used colloquially. However, Öhman and Vahlquist question the term and call it a misnomer because it implies something that can readily be removed from the skin (take off the coat/mantle), for example by cleansing [11].

After the pioneering work of Marchionini, Schade, and Scholz, the number of investigations on pH and its effect on skin has steadily grown to the present day [22–27].

It was also observed that skin partially resist acid/alkaline aggression. This led to the development of the alkali/acid resistance test – used in the 1960s – to detect workers who are likely to develop occupational diseases in certain chemical work environments. A mild variation of the alkali/acid resistance tests, also called the acid/alkali neutralization test, assesses how quickly the skin is able to buffer applied acids or bases without the occurrence of skin corrosion. Repetitive applications of acid or base demonstrated that the skin’s buffering capacity is limited and may be overcome. It has been postulated that amino acids and sweat are responsible for the buffering capacity. Data suggest that the buffering capacity of different skin layers differ substantially from each other. This observation could further improve our understanding related to issues in dermatopharmacology and toxicology. When the buffering capacity becomes nonexistent, the pH of skin becomes significantly altered, thereby initiating a series of processes within the skin with positive (wound healing) and negative (inflammation) effects [28–32].

Originally, the physiologic role of an acid skin surface was thought to be a defense mechanism against invading organisms. More recently, it has been demonstrated that several key processes involved in the synthesis and maintenance of a competent skin barrier are affected by pH and its gradient within the epidermis [13, 33–35]. For a number of biochemical processes within the skin, the compartmental pH is crucial, for example, in pigmentation, ion homeostasis, epidermal (stem) cell behavior, and so on. The often existing difference between the H⁺ concentration of extra- and intracellular as well as subcellular compartments establishes an ionic, electric, and/or osmotic driving force; hence, H⁺ concentration per se acts as an extra-, intra-, and subcellular signaling modality affecting and controlling many cellular functions. One may even consider pH a universal signal and effector [36–39]. It is therefore also no surprise that skin pH shifts have been observed in various skin pathologies. There are reports on the relationship between skin surface pH and prevalence of skin diseases [40–43] and the compromised barrier function and properties in aged skin have been attributed to pH-dependent alterations in the activity of key enzymes involved in the biochemical processing of the skin lipids, the formation of the lipid bilayers, and corneodesmosome degradation [35]. Therefore, it seems obvious that the exogenous acidification by means of topically applied formulations may offer preventive or therapeutic benefits.