The Natural Moisturizing Factor in Human Skin. Hypo-AllergenicTwinlab Na-PCA is a pH-balanced, concentrated solution of the sodium salt of pyrrolidone carboxylic acid, the natural moisturizing factor found in human skin. Na-PCA is synthesized from glutamic acid, a nonessential amino acid. The skin contains natural moisturizers, the most important being Na-PCA. Old skin contains only about half the amount of Na-PCA as that found in young skin. The ability of skin to hold moisture is directly related to its Na-PCA content. It is water and not oil, that keeps skin soft and supple. Na-PCA pulls water out of the air, moisturizing the skin, improving its appearance and giving it a moist youthful glow.