J. Soc. Cosmetic Chemists, 18,351--360 (May 27, 1967) Identification of a Natural Moisturizing Agent in Skin KARL LADEN, Ph.D.,* and ROBERT SPITZER, Ph.D.} Preset, ted September 20-21, 1066, _qeminar, New York City Synopsis--Sodium-2-pyrrolidone-5-carboxylate has been identified as a naturally occurring humectant in skin. A significant relationship was established between the moisture-binding ability and the PCA content of samples of stratum corneum. INTRODUCTION In 1952 Blank (1, 2) demonstrated that the lowered moisture con- tent of the skin is probably the prime factor in causing the condition commonly called "dry skin." Contrary to older beliefs, the amount of oil in the stratum corneum is not the essential factor in determining the softness and flexibility of the skin. Thus, pieces of hardened stratum corneum immersed in various oils do not regain their flexibility, whereas immersion in water increases their flexibility. The factors which influence the state of hydration of the stratum corneum may be classified into three general categories: 1. The rate at which water reaches the stratum corneum from layers beneath it. 2. The rate at which water leaves the skin surface by evaporation. 3. The ability of the stratum corneum to hold moisture. The rate at which water reaches the skin surface is governed by the supply of water from the eccrine glands and moisture obtained by trans- * Gillette Medical Research Institute, 6220 Kansas Ave., N. E., Washington D.C. 20011 • University of Illinois Medical School, Chicago, Ill. 351

352 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS epidermal transport. The latter source appears to be limited by the low water vapor permeability of the stratum corneum itself (3). The removal of water from the skin by evaporation is governed by climatic factors such as temperature, humidity, and wind velocity and perhaps by the presence of sebum. The factors affecting the ability of skin to hold moisture have been the object of many investigations (1, 2, 4-14). There is much evidence that the skin contains hydrophilic nitrogenous substances as well as other hydrophilic substances which enhance the ability of the skin to hold water. When these substances are extracted from the skin, its ability to hold moisture is greatly diminished. In addition, in various skin conditions associated with scaling the scales appear to have lesser amounts of these substances as well as a low capacity to bind moisture (15-19). It was the purpose of this study to investigate the chemical nature of these natural moisturing agents present in skin. A second study to be reported at a future date will establish on a more quantitative basis the role played by water and these moisturizing agents on the physical properties of skin. ANALYSIS OF THE WATER SOLUBLE COMPONENTS OF SKIN Whereas many differences are known to exist between stratum cor- ncure and callus, both of these forms of keratin show the presence of natural moisturizing agents, and both show the same general physical properties with regard to softening with water. In these studies there- fore, both callus and stratum corneum have been used as substrates for investigation. In the initial studies, the effect of removing the water- soluble components from callus on its ability to bind moisture was eval- uated. Sixty grams of callus was extracted with 750 ml of ether in a soxhlet extractor for seven hours. The ether-extracted callus was air dried and then placed in an Erlenmeyer flask with 500 ml of water and shaken for three and one-half hours. The mixture was suction filtered, an additional 300 ml of water was added to the callus, and the mixture shaken overnight. The latter process was repeated one more time. The tiltrates were combined and freeze-dried to yield 9 g of water-ex- tractable material. The water-extracted callus was air-dried. The humectancy of samples was determined by drying the samples to constant weight in a vacuum desiccator and then equilibrating the