Enzyme that, when applied topically on skin, appears to be a very good antioxidant. Taken internally, alpha lipoic acid is a water- and fat-soluble antioxidant that is capable of regenerating other antioxidants, such as vitamins C and E. It is also believed to exert numerous soothing effects.
While studies of alpha lipoic acid do exist, few of them were carried out on people, and none were double-blind in an attempt to evaluate its effects on signs of aging. [1, 2]
It’s clear from the research that alpha lipoic acid is a potent antioxidant, but it isn’t the only one; there are lots of great antioxidants, whether in the form of food, supplements, or applied topically to skin. Note that alpha lipoic acid is extremely vulnerable to degradation by sunlight. Last, higher concentrations of alpha lipoic acid (5% or greater) are capable of causing a burning or stinging sensation on skin. 
- Beitner H. Randomized; placebo-controlled; double blind study on the clinical efficacy of a cream containing 5% alpha-lipoic acid related to photoageing of facial skin. Br J Dermatol. 2003;149(4):841-9.
- Podda M, Grundmann-Kollmann M. Low molecular weight antioxidants and their role in skin ageing. Clin Exp Dermatol. 2001;26(7):578-82.
- Wada T, Wakami H, Konishi T, Matsugo S. The Degradation and Regeneration of a-Lipoic Acid under the Irradiation of UV Light in the Existence of Homocysteine. J Clin Biochem Nutr. 2009;44(3):218–222.