Hyperpigmentation is considered by many to be a beauty problem and is responsible for photoaging. To treat this skin condition, medicinal cosmetics containing tyrosinase inhibitors are used, resulting in skin whitening. In this study, taraxerol methyl ether (1), spinasterol (2), 6-hydroxyflavanone (3), (+)-dihydrokaempferol (4), 3,4-dihydroxybenzoic acid (5), taraxerol (6), taraxerone (7), and lupeol acetate (8) were isolated from Manilkara zapota bark. Their chemical structures were elucidated by analysis of their nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) data, and by comparing them with data found in the literature. The in vitro antityrosinase, antioxidant, and cytotoxic activities of the isolated compounds (1–8) were evaluated. (+)-Dihydrokaempferol (4) exhibited higher monophenolase inhibitory activity than both kojic acid and α-arbutin. However, it showed diphenolase inhibitory activity similar to kojic acid. (+)-Dihydrokaempferol (4) was a competitive inhibitor of both monophenolase and diphenolase activities. It exhibited the strongest 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS), and ferric reducing antioxidant power (FRAP) activities of the isolated compounds. Furthermore, (+)-dihydrokaempferol (4) also demonstrated potent cytotoxicity in breast carcinoma cell line (BT474), lung bronchus carcinoma cell line (Chago-K1), liver carcinoma cell line (HepG2), gastric carcinoma cell line (KATO-III), and colon carcinoma cell line (SW620). These results suggest that M. zapota bark might be a good potential source of antioxidants and tyrosinase inhibitors for applications in cosmeceutical products.

Chunhakant et al.: Quantification of (+)-Dihydrokaempferol in Manilkara zapotaFree radicals can cause oxidative damage in biomolecules and lead to pathological diseases. Antioxidants can protect against the oxidation of living cells. The aim of this study was to evaluate the total phenolic and flavonoid contents and the antioxidant and antityrosinase activities of different parts of Manilkara zapota and measure the quantity of (+)-dihydrokaempferol in different plant parts. The bark, flowers, fruit, leaves, roots, seeds and wood of Manilkara zapota were extracted with methanol and water. All the crude extracts were evaluated for biological activities and measured the quantity of (+)-dihydrokaempferol using high performance liquid chromatography. The methanol crude extract of the flowers showed the highest total phenolic (743±29 milligrams of gallic acid equivalent per gram crude extract) and flavonoid contents (133.8±3.1 milligrams of quercetin per gram of crude extract). Moreover, it exhibited the strongest antioxidant activity with 2,2-diphenyl-1-picrylhydrazyl (half maximal inhibitory concentration of 22.74±0.67 μg/ml), 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (half maximal inhibitory concentration of 20.89±0.17 μg/ml) and ferric reducing antioxidant power values (790.22±0.81 milligrams of trolox equivalent per gram of crude extract). The methanol crude extract of the bark displayed the strongest monophenolase inhibitory activity (half maximal inhibitory concentration of 85.2±2.1 μg/ml), while the methanol crude extract of the roots exhibited the highest diphenolase inhibitory activity (half maximal inhibitory concentration of 33.52±0.68 μg/ml). (+)-Dihydrokaempferol was found in the bark, flowers, leaves, roots and wood. The highest content of (+)-dihydrokaempferol was detected in the methanol crude extract of the bark. These results suggested that the flowers of Manilkara zapota may be used as a source of natural antioxidants and the bark and roots may be beneficial sources of tyrosinase inhibitors.