Tyrosinase inhibitors from sapodilla plum Manilkara zapota L.

Abstract

Six different parts of Manilkara zapota which consisted of barks, flowers, fruits, leaves, roots and seeds were investigated for total phenolic content, total flavonoid content, antioxidant and antityrosinase activities. Methanol crude extract of flowers showed the highest total phenolic content (368.73 ± 0.65 mg GAE/g), while methanol crude extracts of seeds and roots showed high total flavonoid content (90.21 ± 0.57 and 89.03 ± 1.00 mg QE/g, respectively). Methanol crude extract of seeds showed the strongest DPPH (IC50 282.05 ± 0.60 μg/mL) and ABTS (IC50 205.11 ± 0.89 μg/mL) radical scavenging activities and showed the highest FRAP value of 296.46 ± 0.08 mg TEAC/mg. Methanol crude extract of roots showed the highest tyrosinase inhibitory activities on both monophenolase (IC50 0.81 ± 0.92 mg/mL) and diphenolase inhibitory activities (IC50 0.55 ± 0.50 mg/mL). Total phenolic content and antioxidant radical activities showed high correlation by ABTS, DPPH and FRAP assays. Correlations between total phenolic content with antityrosinase activities and with total flavonoid content were very low. A low correlation was found between total total flavonoid content and antioxidant activities by DPPH, ABTS and FRAP assays. There were no correlation between total flavonoid content with antityrosinase activities and no correlation between antioxidant and antityrosinase activities. Moreover, bioassay-guided fractionation on tyrosinase inhibitory activity was used to isolate tyrosinase inhibitors from M. zapota barks. Separation of n-hexane and ethyl acetate crude extracts of M. zapota barks afforded seven isolated compounds; taraxerol methyl ether (I), 6-hydroxyflavanone (II), (+)-dihydrokaempferol (III), 3,4-dihydroxybenzoic acid (IV), taraxerol (V), taraxerone (VI) and lupeol acetate (VII). (+)-Dihydrokaempferol (III) displayed significant tyrosinase inhibition (IC50 32.17 ± 0.32 μM) against monophenolase activity which was more potent than kojic acid (IC50 40.21 ± 0.63 μM). Furthermore, (+)-dihydrokaempferol (III) (IC50 31.60 ± 0.73 μM) showed similar activity on diphenolase inhibitory activity when compared with kojic acid (IC50 30.07 ± 0.32 μM). Furthermore, isolated compounds I-VII were evaluated antioxidant and cytotoxic activities. (+)-Dihydrokaempferol (III) exhibited the strongest scavenging activities on DPPH (IC50 2.21 ± 0.77 μM) and ABTS (IC50 214.83 ± 0.51 μM) and showed the highest FRAP value of 6.23 ± 0.10 μM. It displayed strong cytotoxic activity against human cancer cell lines; BT474, ChaGo-K-1, HepG2, KATO-III and SW620 with IC50 values of 11.66 ± 0.42, 12.32 ± 0.73, 13.67 ± 0.38, 39.79 ± 0.38 and 41.11 ± 1.08 μM, respectively. This study indicated that M. zapota might constitute a rich source of total phenolic contents and natural antioxidants. Moreover, (+)-dihydrokaempferol (III) could be developed as a natural tyrosinase inhibitor.