Tyrosinase inhibitory activity of flavonoids from Artocarpus heterophyllous

Background Tyrosinase is an oxidoreductase that is very important in medicine and cosmetics because the excessive production of melanin causes hyperpigmentation. The development of novel, effective tyrosinase inhibitors has long been pursued. In preliminary tests, we found that an extract of the wood of Artocarpus heterophyllous (AH) potently inhibited tyrosinase activity. Results Two new flavonoids, artocaepin E (1) and artocaepin F (2), were isolated from the wood of AH, together with norartocarpetin (3), artocarpanone (4), liquiritigenin (5), steppogenin (6), and dihydromorin (7). Their structures were elucidated using one-dimensional (1D) and two-dimensional (2D) nuclear magnetic resonance (NMR) and mass spectrometry. The absolute configuration of 2 was determined from the circular dichroism (CD) spectrum. Artocarpanone (4) had the most potent tyrosinase inhibitory effect, with an IC50 of 2.0 ± 0.1 μM, followed by artocaepin E (1) and steppogenin (6), with IC50 values of 6.7 ± 0.8 and 7.5 ± 0.5 μM, respectively. A kinetic investigation indicated that 1 showed competitive inhibition, with an inhibition constant (Ki) of 6.23 μM. Conclusions These results demonstrate that extracts of the wood of AH and its phytochemical constituents are potential sources for skin-whitening agents.Graphical abstract Artocarmin E (1) and artocarmin F (2) were isolated from the wood of Artocarpus heterophyllous. Their structures were elucidated using nuclear magnetic resonance analysis and mass spectrometric methods Electronic supplementary material The online version of this article (doi:10.1186/s13065-016-0150-7) contains supplementary material, which is available to authorized users.


Background
Tyrosinase is a key enzyme in mammalian melanin synthesis. It catalyzes the first step in two reactions of melanin synthesis: the hydroxylation of l-tyrosine to 3,4-dihydroxyphenylalanine (l-DOPA) and the oxidation of l-DOPA to dopaquinone. This o-quinone is a highly reactive compound that can polymerize spontaneously to form melanin [1]. In mammals, melanin protects the skin from ultraviolet (UV) damage by absorbing UV sunlight and removing reactive oxygen species. However, the production of abnormal melanin pigmentation is an esthetic problem in humans [2]. Therefore, the development of high-performance tyrosinase inhibitors is much needed.
Artocarpus heterophyllous Lam belongs to the family Moraceae and is popularly known as jackfruit; it is distributed widely in tropical and subtropical regions of Asia. In Vietnam, this plant is known as "Mit, " and is cultivated for its edible fruits, while the wood has been used for its anti-inflammatory, antioxidant, and antiaging effects [3]. In preliminary tests, we found that an extract of the wood of AH strongly inhibited tyrosinase activity. This plant is a rich source of prenylated flavonoids and their derivatives. Numerous compounds have been isolated from AH, including prenylated flavonoids, chalcones, and simple polyphenols. These possess various biological activities, including cytotoxic, tyrosinase inhibitory, anti-inflammatory, and antioxidant effects [4][5][6]. The present phytochemical investigation of the wood of AH led to the isolation of two new flavonoids (1, 2) and five known compounds (3−7). In this paper, we report the isolation and structure elucidation of these isolated compounds, as well as their tyrosinase inhibitory activities.
Compound 1 was obtained as a yellow amorphous solid. High-resolution electrospray ionization mass spectrometry (HR-ESI-MS) gave a pseudo-molecular peak at m/z 517.1487 [M + H] + (calcd. for 517.1499), consistent with the molecular formula C 29 H 24 O 9 . The infrared (IR) spectrum suggested the presence of hydroxy (3410 cm −1 ), phenyl (1600, 1455 cm −1 ), and carbonyl (1710, 1700 cm −1 ) groups. The 1 H NMR spectrum of 1 included signals due to two sets of ortho-coupled aromatic protons at δ  Table 1). In comparison, the [13] C NMR and distortionless enhancement by polarization transfer (DEPT) spectrum of 1 contained 29 carbon signals, including a ketone carbonyl, an ester carbonyl, six olefinic, a methyl, a methylene, an oxymethylene, and 18 aromatic carbons (Additional file 1). The data were similar to that of artocaepin D, which was isolated from the same plant [11], except for the presence of signals of a set of resonances due to a trans-p-coumaroyl unit, which was confirmed by the 1 H-1 H correlation spectroscopy (COSY) and heteronuclear multiple bond correlation (HMBC) spectra (Fig. 2). This moiety was located at C-4″ by the HMBC correlation of H-4″ (δ H 4.51, 2H, s) with the C-9′″ (δ C 166.5) ester carbonyl carbon of the transp-coumaroyl group. The nuclear Overhauser effect spectroscopy (NOESY) correlations of H-1″ with H-5″ and of H-2″ with H-4″ indicated an E-configuration for the C-2″/C-3″ double bond of 1. Therefore, the structure of artocaepin E was concluded to be 1.

Biological assay
The tyrosinase inhibitory activity of all isolated compounds (1-7) was tested [11]. Kojic acid, a well-known tyrosinase inhibitor currently used as a cosmetic skinwhitening agent, was used as a positive control. Of the tested compounds, artocarpanone (4) had the most potent inhibitory effect against tyrosinase, with an IC 50 of 2.0 ± 0.1 μM, followed by artocaepin E (1) and steppogenin (6), with IC 50 values of 6.7 ± 0.1 and 7.5 ± 0.5 μM, respectively ( Table 2). Liquiritigenin (5) also showed significant concentration-dependent inhibition, with an IC 50 of 22.0 ± 2.5 μM; this compound showed moderate inhibitory activity compared to the above compounds. However, it showed more potent inhibitory activity than kojic acid, which inhibited tyrosinase with an IC 50 of 44.6 ± 0.4 μM ( Table 2). The other compounds, artocaepin F (2), norartocarpetin (3), and dihydromorin (7), showed very weak inhibitory activity, with IC 50 values over 50 μM. Further study examined the inhibitory mechanism of artocaepin E (1), which strongly inhibited tyrosinase activity. To determine the type of enzyme inhibition and the inhibition constant for an enzyme-inhibitor complex, the mechanism was analyzed by Lineweaver-Burk plots. The results indicated that 1 displayed competitive inhibition, with an inhibition constant (K i ) of 6.23 μM (Fig. 3).
On close inspection of the inhibitory activity exerted by these compounds, the following biological profile of the structure-activity relationship was deduced. In terms of the flavone skeleton, compounds 1 and 3 are derivative of apigenin, a common flavone in plants; however, the presence of one hydroxyl group at C-2′, a trans-p-coumaroyl unit connected to the hydroxyprenyl through an ester linkage at C-6 of the apigenin skeleton in 1, led to significantly stronger inhibitory activity than that of 3 (6.7 vs. >50 μM). This suggests that the absence of the side-chain at C-6 of the B-ring leads to a significant loss of activity, and the presence of a side-chain such as trans-p-coumaroyl connected to the hydroxyprenyl may positively influence the tyrosinase inhibitory activity. Regarding the flavanone skeleton, artocarpanone (4), which possesses a methoxyl group at C-7 of ring A, had the strongest inhibitory activity. Steppogenin (6) shares the same structure as 4, except for the hydroxyl group at C-7; however, it had 3.75-fold higher inhibitory activity than 4. In comparison, dihydromorin (7), which has four hydroxyl groups at C-2′, C-3, C-5, and C-7, had weak activity. These results imply that the methoxyl and hydroxyl groups in the main flavanone skeleton play an important role in tyrosinase inhibition.

General procedure
Optical rotations were recorded on a JASCO DIP-140 digital polarimeter. CD measurements were carried out on a JASCO J-805 spectropolarimeter. IR spectra were measured with a Shimadzu IR-408 spectrophotometer in CHCl 3 solution. NMR spectra were taken on a Bruker Advance III 500 spectrometer (Brucker Biospin) with tetramethylsilane (TMS) as an internal standard, and chemical shifts are expressed in δ values. HR-ESI-MS measurements were carried out on a Bruker microTOF-QII spectrometer. Column chromatography was performed with BW-820MH Si gel (Fuji Silisia, Aichi, Japan). Analytical and preparative TLC was carried out on precoated Merk Kiesegel 60F 254 or RP-18F 254 plates (0.25 or 0.5 mm thickness).

Plant material
The wood of A. heterophyllous was collected at the Seven-Mountain area, An Giang province, Vietnam in August 2010. The plant was identified by Ms. Hoang Viet, Faculty of Biology, University of Science, Vietnam National University-Hochiminh City. The voucher sample of the wood part (AN-2985) is preserved at Department of Analytical Chemistry, Faculty of Chemistry, University of Science, Vietnam National University-Hochiminh City.

Tyrosinase inhibitory assay
All the samples were first dissolved in DMSO and used for the actual experiment at concentrations of 100 -1 µg/ mL (or µM for pure compounds). The tyrosinase inhibitory activity assay was performed as previously described by Arung et al. [13]. The assay mixtures consisting of 1900 µL of test solution in 0.1 M phosphate buffer pH 6.8 and 100 µL of enzyme solution (15 U/mL in 0.1 M phosphate buffer pH 6.8) was prepared immediately before use. After preincubation at room temperature for 30 min, the reaction was initiated by the addition of 1000 µL of substrate solution (1.5 mM l-DOPA in 0.1 M phosphate buffer pH 6.8). The assay mixture was incubated at room temperature for 7 min, and the absorbance at 475 nm was measured with a Shimadzu UV-1800 spectrophotometer. Kojic acid, a known tyrosinase inhibitor, was used as positive control. Tyrosinase inhibitory activity was expressed as the percentage inhibitory of enzyme tyrosinase in the above assay system, calculated as (1 − B/A) × 100, where A and B are the activities of the enzyme without and with test material. IC50 values were calculated from the mean values of data from four determinations.

Inhibition mechanism
The procedure for determination of the inhibition mechanism was similar to that for determination of IC 50 , except that uninhibited and inhibited reactions were observed for three different concentrations of l-DOPA (0.2, 0.3, 0.4, 0.5, and 0.6 mM) at 30 °C in 0.1 M phosphate buffer pH 6.8. The dependence of absorbance (475 nm) on time was measured, and the reaction rate was calculated for all reactions (uninhibited and inhibited). Then, a Lineweaver-Burk plot was constructed, and K m and V m values were calculated. Each measurement was performed in duplicate.