Studies on Tyrosinase Inhibitory and Antioxidant Activities of Benzoic Acid Derivatives Containing Kojic Acid Moiety

Abstract

Tyrosinase 1 is widely distributed in microorganisms, animals, and plants and is responsible for melanization in animals and browning in plants. Melanin plays a crucial protective role against the photodamage of skin. However, the production of abnormal melanin pigmentation is a serious esthetic problem for human beings. In the food industry, tyrosinase is responsible for enzymatic browning reactions in damaged fruit during post-harvest handling and processing. Therefore, the development of tyrosinase inhibitors that can be used as skin whitening agents 2 or preservatives 3 for fresh food has been pursued. Kojic acid 4 (1) is produced by various fungi and bacteria and is widely used as a skin whitening agent because of its tyrosinase inhibitory activity. Kojic acid has also been shown to prevent photodamage due to its free radical scavenging activity. 5 However, its inhibitory activity and storage properties are insufficient for use in cosmetics and as an anti-browning agent for food. To increase its activity, many semi-synthetic kojic acid derivatives have been synthesized. Many of these compounds were formed by modifying the C-2 hydroxyl group to form esters, 6 ethers, 7 sulfides 8 and peptide 9 derivatives because C-5 enolic hydroxyl group is considered as a pharmacophore for tyrosinase inhibition. Recently, we synthesized a 2,4-dihydroxy benzoate ester of kojic acid containing an adamantane moiety (2) and evaluated its tyrosinase activity. 10 Although compound 2 has a kojic acid moiety for chelation to copper in the active site of tyrosinase, tyrosinase inhibitory activity is not found. We also synthesized a benzoate ester of kojic acid without an adamantane moiety and investigated its tyrosinase inhibitory and radical scavenging activities. Scheme 1 shows the synthetic pathways for the benzoate ester of kojic acid. The reaction of kojic acid with thionyl chloride produces compound 6, which is conveniently converted to compound 7 using dimethylsulfate and potassium carbonate in acetone under reflux conditions. Compounds 6 and 7, which are both chlorides, react with potassium salts of benzoic acids in dimethylformamide at 110 o C - 120 o C to give the corresponding ester derivatives 4a-4m and 5a-5c. First, we evaluated the tyrosinase inhibitory activities of the kojic acid and benzoate ester derivatives. Tyrosinase is known to be the enzyme responsible for the oxidation of tyrosine. The inhibitory activities were evaluated by measuring the transformation rate of L-tyrosine to L-dopaquinone. Compounds 4a-4m exhibited potent inhibitory activities against tyrosinase. The hydrophobic benzoate increased the inhibitory activity of kojic acid. However, the numbers and