Читать книгу pH of the Skin: Issues and Challenges - Группа авторов - Страница 29

Skin surface acidity can be measured according to 2 criteria – its value given by pH and its strength determined by the ability of the skin to resist an acidic/alkaline aggression, called buffer capacity or acid/alkali neutralization capacity [15].

As already mentioned, a buffer system is formed by a weak acid and its salt or a weak base and its salt. Early studies focused on sweat and on sebaceous glands as the source of pH and buffer capacity in the skin. However, the contribution of these glands seems to be minor. Instead, the SC buffer system results from differentiated keratinocytes in the SC and is formed by weak organic acids, fatty acids, urocanic acid, lactic acid, carbonic acid, and the bases with the anions OH⁻, COO⁻ or amines R-NH₂. Also, amino acids and peptides are part of the buffer system [18]. In addition, the role for keratin as buffer substance has been proposed; however, the data are weak [11, 19].

The filaggrin-histidine-urocanic acid pathway contributes to SC acidification. Calculated pKa of urocanic acid 3.85–6.13 [20], should result in the normal pH of 4.9 in the optimal 1:1 ratio. Free fatty acids are required for the formation of lamellar membranes and also for the acidification of the SC. pKa of most fatty acids is around 4.5 and buffers from fatty acids and its salt in the 1:1 ratio should result in pH close to 4.9. The SC contains lactic acid [18, 21]. The relatively low pKa of 3.9 would stand against an important role of lactic acid in skin surface pH because the lactic acid/lactate buffer would significantly deviate from the 1:1 ratio at the average skin pH 4.9 resulting in low buffer capacity. A participation of on pH and in skin’s buffering capacity has been described. The relatively high pKs 6.4 argues against an important role of buffer for the normal skin pH [16]. Pigmented skin has a lower pH than fair skin. Melanin-containing granules from the outer epidermis contribute to the reduced pH of the lower SC, likely through the release of protons from the acidic milieu of phagolysosomes [22, 23]. Indole-5,6-chinone carboxylic acid is the end product of Eumelanin production; the calculated pKa 3.6–6.3 should result in a pH of about 4.9 in the 1:1 ratio [24].

The buffer capacity is lower than normal in the skin of babies, aged people, and in diseased skin. Reduced buffering capacity has been documented for the skin of the elderly [11]. Free fatty acids and amino acids work best as buffer at pH that is equally their pKa which is the pH at which 50% of the acid is associated and 50% is disassociated. With increasing pH, more free fatty acids are in the polar stage and that affects lipid lamellar formation and buffer capacity [16, 25, 26].