Tyrosinase inhibitor activity of coumarin-resveratrol hybrids

Article abstract of DOI:10.3390/molecules14072514

In the present work we report on the contribution of the coumarin moiety to tyrosinase inhibition. Coumarin-resveratrol hybrids 1-8 have been resynthesized to investigate the structure-activity relationships and the IC⁵⁰ values of these compoun

Full text of DOI:10.3390/molecules14072514

Molecules 2009, 14, 2514-2520; doi:10.3390/molecules14072514
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molecules
ISSN 1420-3049
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Article
Tyrosinase Inhibitor Activity of Coumarin-Resveratrol Hybrids
Antonella Fais 1,*, Marcella Corda 1, Benedetta Era 1, M. Benedetta Fadda 1, Maria Joao
Matos 2,3, Elias Quezada 2,3, Lourdes Santana 2, Carmen Picciau 3, Gianni Podda 3 and
Giovanna Delogu 3,*
1
Dipartimento Scienze Applicate ai Biosistemi, Università degli Studi di Cagliari, Cittadella
Universitaria di Monserrato, S.S. 554 Km 0.700 bivio per Sestu, 09042 Monserrato, CA, Italy;
E-mails: corda@unica.it (M.C.), era@unica.it (B.E.), faddam@unica.it (M-B.F.)
2
Departamento de Química Orgánica, Facultade de Farmacia, Universidade de Santiago de
Compostela, 15782 Santiago de Compostela, Spain; E-mail: lourdes.santana@usc.es (L.S.)
3
Dipartimento Farmaco Chimico Tecnologico, Università degli Studi di Cagliari, Via Ospedale 72,
09124 Cagliari, CA, Italy; E-mails: carmenpicciau@alice.it (C.P.), gpodda@unica.it (G.P.),
mariacmatos@gmail.com (M-J.M), elias.quezada@usc.es (E.Q.)
* Authors to whom correspondence should be addressed; E-mails: delogug@unica.it (G.D.),
fais@unica.it (A.F.); Tel.: +390706758566 (G.D.), +390706754506 (A.F.); Fax: +390706758553
(G.D.), +390706754523 (A.F.)
Received: 11 May 2009; in revised form: 9 July 2009 / Accepted: 10 July 2009 /
Published: 13 July 2009
Abstract: In the present work we report on the contribution of the coumarin moiety to
tyrosinase inhibition. Coumarin-resveratrol hybrids 1-8 have been resynthesized to
investigate the structure-activity relationships and the IC50 values of these compounds
were measured. The results showed that these compounds exhibited tyrosinase inhibitory
activity. Compound 3-(3’,4’,5’-trihydroxyphenyl)-6,8-dihydroxycoumarin (8) is the most
potent compound (0.27 mM), more so than umbelliferone (0.42 mM), used as reference
compound. The kinetic studies revealed that compound 8 caused non-competitive
tyrosinase inhibition.
Keywords: tyrosinase inhibitors; resveratrol; coumarins

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Introduction
Tyrosinase (EC 1.14.18.1) is an enzyme with a dinuclear copper centre, widely distributed in
Nature and mainly involved in the formation of pigments such as melanins and other polyphenolic
compounds [1]. Roughly speaking the tyrosinases consist of three domains, of which the central
domain contains the Cu-binding sites, called CuA and CuB. In this way their overall design is similar
to that of arthropod hemocyanins. Matoba and coworkers [2] established the crystallographic structure
of tyrosinase, enabling a better understanding of its mechanism of action. This enzyme uses molecular
oxygen to catalyze the oxidation of monophenols to their corresponding o-diphenols (monophenolase
or cresolase activity) and their subsequent oxidation to o-quinones (diphenolase or cathecolase
activity). The active centre of tyrosinases , formed by dinuclear coppers, is flexible during catalysis.
Within different taxa, the sequence homology of tyrosinases is high and conserved domains can be
identified [3]. The enzyme seems to be almost universally distributed in animals, plants, fungi and
bacteria but much is still unknown about its biological function. In higher plants, it protects the plant
against insects and microorganisms, catalyzing the formation of a melanin scab impervious to further
attack [4,5]. Moreover, tyrosinase is responsible of browning of certain fruits and vegetable during
their handling, processing and storage after harvest, and both browning of fruit and skin darkening can
be suppressed, at least partially, by deactivation of tyrosinase. In mammals, the enzyme is responsible
for skin pigmentation [6,7] and is linked to Parkinson disease and other neurodegenerative
diseases [8,9].
The study of tyrosinase inhibitory activity became of interest in recent years because of the
significant industrial and economic impact of the inhibitors of this protein. Recently, different
inhibitory compounds derived from natural sources or partly/fully synthetic have been tested [10].
Studies on tyrosinases, their substrates and inhibitors, are needed to better understand the details of its
biological activity and to know how to control it [11].
Our research aimed to study compounds that hypothetically would be good tyrosinase inhibitors, by
using all available structural information. Our experience on both coumarin and resveratrol structures,
it has encouraged us to examine coumarin-resveratrol hybrids in this context.
Resveratrol (3,5,4’-trihydroxy-trans-stilbene) is a polyphenolic phytoalexin produced in plants via
a metabolic sequence induced in response to biotic or abiotic stress factors. It has been found in a
multitude of dietary plants and derived products, such grapes, peanuts, legumes and in red wine. It has
been shown to be an antimutagen, antioxidant, and inducer of quinone reductase. In addition, many
biological effects have been attributed to resveratrol, such as modulation of hepatic apolipoprotein,
activation of the estrogens’ receptor, inhibition of platelet aggregation and in vitro eicosanoid
synthesis, inhibition of transcription and enzyme activity of cyclooxygenase-2 [12]. All these effects
point toward the possibility that resveratrol may contribute to the cardioprotective effects offered by
red wine consumption [13-15].
Inhibitory effects of hydroxystilbene compounds on mushroom tyrosinase activity have been
evaluated [16] and some of these have significant inhibitory activity. In this group, resveratrol, better
known for its cancer-chemopreventive potential [17], represents the simplest structure, with free
hydroxyl groups at positions 3, 5 and 4’. This compound showed stronger DOPA oxidase inhibitory
activity than kojic acid [18].

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On the other hand, coumarins represent a class of compounds of interest for a long time due to their
biological activities: they have been shown to be useful as antitumoural and anti-HIV agents [19]. In
addition, they have show to possess cardioprotective properties: many of them are selective coronary
vasodilators, an effect that may be related to a Ca2+-antagonistic activity [20]. In addition, Masamoto
et al. [21] investigated the structure-activity relationship of 18 coumarins for their inhibitory activity
on mushroom tyrosinase, and they find that esculetin exhibited the strongest inhibitory activity.
Recently, in contrast with the findings of Masamoto, Sollai et al. [22] have shown that esculetin is
nevertheless to be considered a tyrosinase substrate rather than an inhibitor, whereas umbelliferone
appears to be an inhibitor of the mentioned oxidase.
Based on this information, and considering the importance of hydroxyl groups in tyrosinase
inhibitory activity [23,24], we synthesized the hydroxylated coumarin-resveratrol hybrids 1-8 in order
to study their inhibition mechanisms to provide a basis for the development of new effective tyrosinase
inhibitors.
Results and Discussion
The studied compounds 1-8 were prepared by the reaction of methoxy substituted o-hydroxy-
benzaldehydes with the corresponding arylacetic acids, according to the Perkin conditions
(Scheme 1) [25-28].
Scheme 1. General synthetic route to obtain compounds 1-8.
R3´
R4´
R5´
H(OMe)
R2
R1
CHO
OH
a)
H(MeO)
b)
O
O
HO
O
R
A
B
1-8
(1) R= R2=R3´=R4´=R5´=H, R1=OH
(2) R= R1=R2=R3´=R5´=H, R4´=OH
(3) R= R2=R3´=R5´=H, R1=R4´=OH
(4) R= R1=R3´=R5´=H, R2=R4´=OH
(5) R= R1=R4´=H, R2=R3´=R5´=OH
(6) R= R2=R4´=H, R1=R3´=R5´=OH
(7) R1=R4´=H, R=R2=R3´=R5´=OH
(8) R1=H, R=R2=R3´=R4´=R5´=OH
Reagents and conditions: a) DCC, DMSO, reflux; b) HI, Ac2O, AcOH, reflux.
Inhibitory effects on L-3,4-dihydroxyphenylalanine (L-DOPA) oxidizing diphenolase activity of
tyrosinase of the hydroxyphenyl coumarin derivates and umbelliferone are summarized in Table 1.
Compound 1 is an umbelliferone analogue with one phenyl group at C3. The phenyl substituent led to
a dramatic loss of potency, and compound 1 did not inhibit any tyrosinase activity. This result
indicates that 3-phenyl substituent of umbelliferone may be an important factor for the absence of
tyrosinase inhibition activity.
Having identified the importance of the 4’-hydroxyl group of flavonoid compounds in the
pharmacological activity, we studied the role of hydroxyl group at position 4’ of the coumarin-

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resveratrol hybrid 2. Furthermore, we decided to examine the effect of adding hydroxyl groups at the
4’ position of compound 1 (thus obtaining compound 3). As shown in Table 1, these compounds have
no demonstrable inhibitory effects. On the other hand, the inhibitory potency was enhanced in 6-
hydroxy-3-(4’-hydroxyphenyl)coumarin (4), suggesting that a different position of hydroxyl group
substituent of the A ring might be advantageous.
Table 1. Inhibitory effect of hydroxyphenylcoumarins derivate on mushroom tyrosinase
activity (substrate: L-DOPA).
Compound
% inhibition (at 0.8 mM)
IC50 (mM)
>0.1
0
0
1
2
>0.1
9.6
26.7
0
>0.1
1.56
>0.1
3
4
5
19.3
0
3.68
>0.1
6
7
68.3
-
0.27
0.42
8
Umbelliferone a
a Obtained from data in ref. [20].
The coumarin-resveratrol hybrids 5-8 with hydroxyl groups in positions 3’ and 5’ were found to
exhibit different inhibitory potencies, depending on the position and number of hydroxyl groups in the
A and B rings. The activities of the compounds 5-7 did not provide interesting results as none of them
showed any significant inhibitory activity (Table 1), but compound 8 showed the highest tyrosinase
inhibitory activity (IC50=0.27 mM). This could be due to a synergistic effect of the hydroxyl groups on
the aromatic rings. This implies that the number of free OH groups plays an important role in
determining the activity. It is unclear, however, why the additive effect of the 4’-hydroxyl is different
from that of the other hydroxyl substitution patterns, although Gao and co-workers have reported the
inhibitory activity of 3’,4’,5,5’,7-pentahydroxyflavone that has a similar structure to compound 8 [29].
Furthermore, under similar experimental conditions, these molecules have shown similar rates of
inhibition.
Figure 1. Dixon plot for the inhibition of tyrosinase by compound 8 with respect L-DOPA
as substrate at concentrations: ( ) 0.5 mM;(▲) 0.25 mM; (◯ ) and 0.1 mM.
20
15
10
5
20
15
10
5
0
0
-0.4 -0.2
0
0.2
0.4
0.6
0.8
1
I (mM)

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To better understand tyrosinase inhibition by compound 8, we studied its effect on the diphenolase
activities of the enzyme. A plot of 1/V vs inhibitor concentration (Figure 1) shows that 8 produced a
non-competitive inhibition. In the non–competitive type of inhibition, the inhibitor does not affect the
enzyme-substrate combination but affects only the velocity of reaction. The inhibitory activity exerted
by 8 is different from that shown from pentahydroxyflavone, which was previously reported as
competitive inhibitor [29].
Experimental
Chemistry
The studied compounds are all known and were prepared by a traditional Perkin reaction carried out
in refluxing dimethylsulfoxide (DMSO) between o-hydroxybenzaldehydes (or their methoxy
substituted derivatives) A, and the corresponding arylacetic acids B, using dicyclohexylcarbodiimide
(DCC) as dehydrating agent (Scheme 1) [25-27]. Hydrolysis of methoxy groups, by treatment with HI
in acetic acid/acetic anhydride, gives the desired hydroxyl derivatives 1-8 [25-28].
Tyrosinase inhibition assay
Tyrosinase activity assays were performed with L-DOPA as substrate, as previously described [30]
with slight modifications. The pre-incubation with enzyme consisted of a 1/15 M phosphoric acid
buffer solution (pH 6.8, 1.8 mL), an aqueous solution of mushroom tyrosinase (1000 U/mL, Sigma
Chemical Co., 0.1 mL) and DMSO (0.1 mL) with or without an added sample. The mixture was pre-
incubated at 25°C for 10 minutes. Then, a 1.5 mM L-DOPA solution (1 mL) was added and the
reaction was monitored at 475 nm for 5 min. The detected absorbance increases accompanying the
oxidation of the substrate (L-DOPA). The final concentration of dopaquinone was measured using the
molar absorption coefficient of 3,700 M-1 cm-1 [31]. One unit of enzymatic activity was defined as the
amount of enzyme transforming 1 μmole of L-DOPA per minute. The percentage of inhibition of
tyrosinase activity was calculated as inhibition (%) = (A-B)/A×100, where A represents the difference
in the absorbance of control sample between an incubation time of 0.5 and 1.0 min, and B represents
the difference in absorbance of the test sample between an incubation time of 0.5 and 1.0 min. The
activity of mushroom tyrosinase was determinate by spectrophotometric techniques (Varian Cary 50).
DMSO inhibitory activity, in the same experimental conditions, was evaluated as 10.7%.
Conclusions
Inhibitory effects of hydroxylated coumarin-resveratrol hybrids 1-8 on mushroom tyrosinase
activity have been evaluated. The experimental data showed that compound 8 possesses the highest
activity among the tested hybrids, and it was a non-competitive inhibition with respect to L-DOPA.
This result indicates that the number and the position of free OH groups play an important role in
determining the activity. Further experiments are in progress in order to know how these compounds
interact with the enzyme on molecular basis. Moreover, since the initial step in melanin synthesis is
also the same in catecholamine synthesis, we are beginning to study a possible close relationship of
inhibitory tyrosinase activity and vasorelaxant activity.

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Acknowledgements
We thank Progetto di Ricerca Scientifica 2007-Università di Cagliari and Fondazione del Banco di
Sardegna, Xunta de Galicia (PGIDIT05PXIB20304PR) and Ministerio de Sanidad y Consumo (FIS
PI061537 and PI061457) for financial support.
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