Modulating effects of a novel skin-lightening agent, alpha-lipoic acid derivative, on melanin production by the formation of DOPA conjugate products.

Sodium zinc dihydrolipoylhistidinate (DHLHZn) is a compound of Zn(2+)/dihydrolipoic acid derivate complex, which was developed for cosmetic/medical use. To characterize DHLHZn as a novel skin-lightening agent, inhibitory actions of DHLHZn on tyrosinase (including its reaction pathway) have been elucidated in this study. In a B16 melanoma cell system, DHLHZn was active in suppressing the synthesis of melanins as well as alpha-arbutin, well known as a depigmenting drug. Furthermore, in a tyrosinase assay, DHLHZn showed stronger inhibitory effect on DOPAchrome formation than other tyrosinase inhibitors such as kojic acid. Our previous report demonstrated that the sulfhydryl groups of lipoyl motif react with DOPAquinone to form lipoyl DOPA conjugates. We therefore postulated that conjugated products between DHLHZn and DOPAquinone might be formed. Upon reaction of DHLHZn with L-DOPA following tyrosinase-catalyzed oxidation, the formation of DHLH DOPA conjugated products was confirmed by HPLC-tandem mass spectrometry using reserpine as the internal standard. In addition, the inhibitory kinetics analyzed by a Lineweaver-Burk plot exhibited the reversibility of DHLHZn as a competitive inhibitor with a KI value of 0.35 microM. These results indicate that this covalent reaction might contribute to alternating DOPAquinone, which is a tyrosinase reaction product, and result in the competitive inhibitory effect of DHLHZn on DOPAchrome formation. DHLHZn may thus serve as a potentially effective skin-lightening agent, an effectiveness that is based on the compound's covalent scavenging of DOPAquinone resulting in depigmentation.