A possible mechanism of action for azelaic acid in the human epidermis

Summary

Azelaic acid, and other saturated dicarboxylic acids (C₉-C₁₂), are shown to be competitive inhibitors of tyrosinase (K _I azelaic acid = 2.73×10⁻³ M) and of membrane-associated thioredoxin reductase (K _I azelaic acid = 1.25×10⁻⁵ M). The monomethyl ester of azelaic acid does not inhibit thioredoxin reductase, but it does inhibit tyrosinase, although double the concentration is necessary compared with azelaic acid (K _I azelaic acid monomethyl ester = 5.24×10⁻³ M). Neither azelaic acid nor its monomethyl ester inhibit tyrosinase when catechol is used as a substrate instead of l-tyrosine. Therefore, the weak inhibitory action of azelaic acid on tyrosinase appears to be due to the competition of a single carboxylate group on this inhibitor for the α-carboxylate binding site of the l-tyrosine substrate on the enzyme active site. Based on the inhibitor constant on tyrosinase, at least cytotoxic levels of azelaic acid would be required for the direct inhibition of melanin biosynthesis in melanosomes if this mechanism is responsible for depigmentation in the hyperpigmentation disorders lentigo maligna and melasma. Alternatively only 10⁻⁵ M azelaic acid is required to inhibit thioredoxin reductase. This enzyme is shown to regulate tyrosinase through a feedback mechanism involving electron transfer to intra-cellular thioredoxin, followed by a specific interaction between reduced thioredoxin and tyrosinase. Furthermore, the thioredoxin reductase/thioredoxin system is shown to be a principal electron donor for the ribonucleotide reductases which regulates DNA synthesis. Inhibition of thioredoxin reductase by azelaic acid provides a rationale for both its depigmenting property and the reversible inhibition of DNA synthesis observed in cultured epidermal cells and also in some of the bacteria associated with acne vulgaris.