Background
Flavonoids play a key role in the treatment of various diseases. Compounds with the ability to protect against DNA damage caused by reactive oxygen species (ROS) and inhibit tyrosinase may be useful for the treatment and prevention from ROS-related diseases. Flavonoids are a large type of compounds in natural products. Flavonoids already have been used widely as lead compounds or drugs.
Some studies showed that the number and location of phenolic hydroxyl on the flavonoids will significantly influence the inhibition of tyrosinase activity [
1‐
3]. The number of phenolic hydroxyl on the B ring of flavonoids or catechins structure or resorcinol structure, can greatly enhance the inhibition of tyrosinase activity. At present, 4-hexyl resorcinol have been used as commodity in shrimp preservation [
4]. The number and position of phenolic hydroxyl on the 1,2-diphenylethene derivatives can greatly effect the inhibition of tyrosinase activity. Two phenol hydroxyls compared to one hydroxyl and phenol hydroxyl replaced methoxyl will significantly enhance the inhibition of tyrosinase activity [
5‐
7].
The tyrosinase inhibition mechanism of phenol hydroxyl compounds was analysed. Because the activity center of tyrosinase is hydrophobic, H
+, combined with Eoxy double oxygen, only come from the hydroxyl of tyrosine and dopamine. Phenol hydroxyl compounds, similar to tyrosine and dopamine, can inhibit the activity of tyrosinase [
8].
Phenolic hydroxyl is indispensable to the antioxidant activity of flavonoids. Many studies showed that the antioxidant activity increased with the phenol hydroxyl number in B ring of flavonoids. Seyoum [
9] studied the activity of scavenging free radicals of 52 kinds of flavonoids. The result showed that two or three phenol hydroxyls compared to one hydroxyl in A ring or B ring, will greatly enhance the antioxidant activity.
The relationship between phenolic hydroxyl number and antioxidant activity of flavonoids is very significant. The reason may be: (1) the more phenolic hydroxyl number, the more H
+ combined with free radicals; (2) the phenolic hydroxyl has strongly denounce electronic effect, which result to the free radicals reaction; (3) the more phenolic hydroxyl number, the more hydrogen bonding, antioxidant activity is also enhanced obviously [
10].
The number and location of phenolic hydroxyl of the flavonoids will significantly influence the inhibition of tyrosinase activity. Phenolic hydroxyl is indispensable to the antioxidant activity of flavonoids. Isoeugenol, shikonin, baicalein, rosmarinic acid, and dihydromyricetin have respectively one, two, three, four, or five phenolic hydroxyls. The different molecular structures with the similar structure to l-3,4-dihydroxyphenylalanine (l-DOPA) were expected to the different antityrosinase and antioxidant activities.
Tyrosinase (EC 1.14.18.1) plays a key role in the biosynthesis of melanin pigment [
11]. Under normal physiological conditions, melanin plays a key role in the protection against UV injury, animal mimicry and camouflage [
12]. Thus, it has attracted researchers to find efficient tyrosinase inhibitors. Recently, molecular docking for modeling the antityrosinase activity of compounds had been used widely in drug design [
13].
Isoeugenol is the major constituent of
Eugenia caryophyllata Thunb., which has extensive pharmacological activities, such as antimicrobe, stomach-invigorating. The result of Jin [
14] indicated that isoeugenol analogs exhibited the cytotoxic activity against A549, KB, and KB-VCR cell lines.
Shikonin is the major constituent of
Arnebia euchroma(
Royle)
Johnst, which has extensive pharmacological activities. Shikonin has good antioxidant activities, which supports the use of shikonin as the new anti-aging candidate drug, cosmetic materials and food additives. The results of Chen [
15] revealed that SK-Hep-1 cells apoptosis induced by shikonin proceeds by involvement of reactive oxygen species and an oxidative stress-mediated pathway.
Baicalein, a kind of oriental medicine, exhibits antioxidant and anti-inflammatory activities. The results of Li-Weber [
16] revealed that baicalein can inhibit several genes of the cell cycle, attenuate NF-κB activity, and scavenge many kinds of oxidative radicals.
Rosmarinic acid, isolated from
Perilla frutescens (L.) or
Rosmarinus officinalis, exhibits many potent biological activities. The result of Zhu [
17] indicated that rosmarinic acid extract exhibits the high activity of inhibiting á-glucosidase for allergy treatments and diabetes mellitus.
Dihydromyricetin can be used to scavenge the free radicals. It also has the effects of anti-oxidation and anti-tumour. Based on the results of Xin [
18], dihydromyricetin was less toxic and highly effective as a good, natural antioxidant for polypropylene.
This investigation tested the antityrosinase activity, the inhibition constant, and inhibition type of compounds. Molecular docking can simulate the binding mode and binding affinity of the tyrosinase and compounds. This investigation also tested the antioxidant effects of isoeugenol, shikonin, baicalein, rosmarinic acid, and dihydromyricetin on supercoiled pBR322 plasmid DNA, lipid peroxidation, and DPPH, ABTS, hydroxyl, or superoxide free radical scavenging activity in vitro.
Discussion
Isoeugenol is the major constituent of
E. caryophyllata Thunb. The result of Hubungan [
33] indicated that antioxidant activities in the following orders: butylated hydroxytoluene (BHT) > mannich product of isoeugenol > isoeugenol > mannich product of eugenol > eugenol. The result of Ko [
34] indicated that demethyldi-isoeugenol inhibited Fe
2+-induced lipid peroxidation. It also scavenged superoxide anion generated by peroxyl radical (ROO.) derived from AAPH.
Shikonin is the major constituent of Arnebia euchroma(Royle)Johnst. The observed results revealed that shikonin demonstrated higher reducing ability (0.431%), and deoxy-shikonin showed maximum inhibition (0.440%) to DPPH-radical scavenging assay.
Baicalein is the major constituent of
Rheum officinale. The results of Nishioka [
35] revealed that baicalein can inhibit the express of human intestinal sucrase in the Caco-2 cells. The results of Tsai [
36] revealed that baicalein can protect against the acute lung injury induced by lipopolysaccharide in rats. The results of Jeli [
37] revealed that baicalein exhibit good inhibitory activities of both production of cytokine IL-6 and tyrosine kinase.
Rosmarinic acid can inhibit the enzymatic browning of fruits and vegetables. The result of Ha [
38] showed that rosmarinic acid possess mushroom tyrosinase inhibitory activities (IC
50 of 4.0 μM). The result of Ding [
39] showed that rosmarinic acid methyl ester can inhibit tyrosinase, and reduce the melanin contents in B16 cells. The result of Fujimoto [
40] showed that rosmarinic acid afforded a highly tyrosinase-inhibitory active product. Rosmarinic acid has antioxidant and prooxidant activities. The result of Sánchez-Campillo [
41] indicated that rosmarinic acid can be used as a good photo-protective agent.
Zhao et al. [
42] evaluated the antioxidant properties of Citri Exocarpium Rubrum based on its DPPH free radical scavenging activity, ferric ion reducing antioxidant power (FRAP) and trolox equivalent antioxidant capacity (TEAC) assays. Bivariate correlation analysis revealed correlations between the characteristic peaks and the antioxidant activities of the samples.
Sambucus williamsii Hance (
Jiegumu) is traditionally used in Chinese medicine to treat bone and joint diseases. The major phytochemicals are phenolic acids, lignans, and terpenoids. This compounds may have the antioxidant, anti-inflammatory, bone fracture healing, and anti-osteoporotic effects [
43].
Tyrosinase (EC 1.14.18.1) play a key role in melanin biosynthesis [
44]. Due to the over expression of tyrosinase, excessive melanin leads to melasma and age spots [
45]. Tyrosinase is responsible for the browning of vegetables and fruits in the food industry, which results in reduced market value and shorter product shelf life [
46]. Increased attention has also drawn to the applications of antioxidants and tyrosinase inhibitors as preservatives in skin-protective ingredients in cosmetics and in the food industry. On the other hand, ROS could induce oxidative damage of proteins and DNA, and peroxidation of membrane lipids. Lipid peroxidation will generate malondialdehyde (MDA), and do harm to cells [
47]. It may be useful in diets to obtain properly antioxidants.
Conclusion
In conclusion, isoeugenol, shikonin, baicalein, rosmarinic acid, and dihydromyricetin exhibited good antityrosinase activities. These compounds also exhibited good antioxidant effects on lipid peroxidation, supercoiled pBR322 plasmid DNA, and DPPH, ABTS, hydroxyl, or superoxide free radical scavenging activity. The different molecular structures lead to the different antityrosinase and antioxidant activities. The activity order is isoeugenol < shikonin < baicalein < rosmarinic acid < dihydromyricetin. The results showed the compounds with more phenolic hydroxyls have more antioxidant and antityrosinase activities. This was the first study of molecular docking for modeling the antityrosinase activity of compounds. This was also the first study of the lipid peroxidation inhibition activity of compounds in liver mitochondria induced by Fe2+/vitamin C(Vc) system in vitro, the protective effects on supercoiled pBR322 plasmid DNA. In a word, the results support the use of compounds as the new anti-aging candidate drugs, cosmetic materials and food additives.
Authors’ contributions
ARZ, SWC, and QLS proposed and designed the experiment. HHD, YYY, LXZ, and XYY were involved in the experiment. SWC and QLS revised the manuscript and were responsible for the overall research. All authors read and approveed the final manuscript.