Synthesis and Biological Evaluation of Kojic acid Derivatives as Tyrosinase Inhibitors

Abstract

Tyrosinase 1 is a copper-containing enzyme which is widely distributed in nature. This enzyme is associated with the production of melanin for the protection of the skin from solar irradiation. However, overproduction of melanin in the skin is of particular concern to woman, for example, in the conditions melasma and lentigo. 2 Tyrosinase catalyzes the first two steps of melanin biosynthesis, namely the hydroxylation of L-tyrosine to L-dopa and the subsequent oxidation of L-dopa to dopaquinone. Tyrosinase is contained in vegetables and fruits and is responsible for the undesirable enzymatic browning that occurs upon long term storage of these. 3 Therefore, tyrosinase inhibitors have attracted attention as important depigmenting agents for the treatment of hyperpigmentation and as anti-browning agents in vegetables and fruits. Kojic acid 4 (1) is a well known tyrosinase inhibitor. However, its inhibitory activity is not potent enough for the above purposes. To overcome this drawback, many semisynthetic kojic acid derivatives have been synthesized, usually by modification of the C-2 hydroxyl group. 5 Recently, we synthesized kojic acid derivatives containing thioether 6 and ester 7 linkers and evaluated their tyrosinase inhibitory activities. Among them, 2-((cyclohexylthio)methyl)-5-hydroxy-4H-pyran-4-one (2b), showed the most potent inhibitory activity. Compound 2b is composed of three parts, including the kojic acid moiety, a thioether linker, and a hydrophobic cyclohexane group. We synthesized and evaluated additional compounds to get more structural insight into the tyrosinase inhibitory activity of the kojic acid derivatives. The crystallographic structure of tyrosinase has recently been established, enabling closer examination of its three-dimensional structure, which revealed the presence of a hydrophobic protein pocket adjoining the binuclear copper active site. 8 We also conducted a molecular docking study to elucidate the structural importance of the kojic acid derivatives in binding with the active site of tyrosinase. The synthetic pathways of the kojic acid derivatives are shown in Scheme 1. Thioether derivatives (2a-2e) were synthesized by the condensation of kojyl chloride 4 with the potassium salts of thiols. Ester derivatives (3a-3e) were synthesized by the condensation of kojyl chloride 4 with the potassium salts of acids. The inhibitory activities of the kojic acid derivatives on mushroom tyrosinase were investigated using kojic acid as a positive control. The results are summarized in Table 1.