Antityrosinase and antioxidant properties of mung bean seed proanthocyanidins: Novel insights into the inhibitory mechanism
Introduction
Tyrosinase (EC 1.14.18.1) is a rate-limiting enzyme of melanogenesis (Seo, Sharma, & Sharma, 2003). It catalyzes tyrosine to dihydroxyphenylalanine (l-DOPA), and l-DOPA to o-quinones (Korner & Pawelek, 1982). Its abnormal expression brings about various dermatological diseases (Kim & Uyama, 2005). In the food industry, tyrosinase induces enzymatic browning, which generally leads to significant losses of nutritional quality and market value of many fruits and vegetables (Loizzo, Tundis, & Menichini, 2012). These phenomena have encouraged researchers to search new potent tyrosinase inhibitors for their use in the medical, food, and agricultural industries.
Autoxidation in food and biological systems results in a multitude of adverse effects in food preservation as well as in human health (Shahidi & Zhong, 2010). Antioxidants can stop the oxidization of polyphenolic compounds in the fruit and vegetables. They play a crucial role in preventing or delaying autoxidation and have attracted much attention as food preservative, dietary supplements and natural health products (Kaur and Kapoor, 2001, Shahidi and Zhong, 2010). In addition, the intake of food containing high amounts of antioxidative nutraceuticals helps to reduce the risks of various diseases (Kaur and Kapoor, 2001, Shahidi and Ambigaipalan, 2015). Therefore, there is an increasing interest in searching natural antioxidants (e.g., flavonols, flavonoids, procyanidins, tannins, anthocyanins) existed in dietary plants (Chai et al., 2014, Kaur and Kapoor, 2001).
Proanthocyanidins are polyphenol compounds naturally present in legume seeds. They have attracted significant research and practical attention because of their various biological activities (Chai et al., 2014, Li et al., 2016). These compounds are formed of flavan-3-ol monomers, which are polymerized into B-type proanthocyanidins (mainly) and A-type proanthocyanidins (Santos-Buelga & Scalbert, 2000) (Fig. 1 in supplementary materials). The proanthocyanidins have a high structural diversity that is based on the monomer units, substituent, interflavan linkage, and degree of polymerization (DP) (Dixon, Xie, & Sharma, 2005). Thus, there is still difficult in the characterization of polymerized proanthocyanidins.
Mung bean [Vigna radiata (L.) Wilczek] is one of the most significant edible legumes grown widely throughout the world. Many bioactive substances including flavonoids (eg. quercetin-3-glucoside, quercetin, myricetin, and kaempferol), phenolic acids (eg. p-hydroxybenzoic, protocatechuic, and gallic acid), organic acids, and polysaccharides have been identified from it in the past decades (Tang, Dong, Ren, Li, & He, 2014). However, there is no relevant report about the mung bean proanthocyanidins, including their structure and bioactivity. This research therefore aimed to study the structure, antityrosinase and antioxidant activities of these compounds and provided a scientific foundation for their uses in food, nutritional, cosmetic and medical industries.
Section snippets
Chemicals and materials
We purchased mushroom tyrosinase, l-tyrosine, L-3,4-dihydroxyphenylalanine (l-DOPA), Sephadex LH-20, high performance liquid chromatography (HPLC) standards, benzyl mercaptan, formic acid, caesium chloride (CsCl), 2,5-dihydroxybenzoic acid, gallic acid, 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,4,6-tripyridyl-S-triazine (TPTZ), Trolox, ascorbic acid (Vc), and butylated hydroxyanisole (BHA) from Sigma-Aldrich (St. Louis,
Yield, total phenolics and extractable proanthocyanidins content
According to the procedure of extraction and purification, we obtained 0.14 g of purified proanthocyanidins from 5 g dry mung bean seed powders. Therefore, the yield was 2.8%. In addition, high level of total phenolics (260 ± 10 mg gallic acid equivalents/g dry weight) and extractable proanthocyanidins (87 ± 8 mg polymeric proanthocyanidins equivalents/g dry weight) in mung bean seed showed that this material could be selected as the source of these valuable constituents.
MALDI-TOF MS analysis of the proanthocyanidins extracted from mung bean seed
The MALDI-TOF mass
Conclusions
In conclusion, this study demonstrated that procyanidins, prodelphinidins and their rhamnosides were the constitutional units of mung bean seed proanthocyanidins. These proanthocyanidin polymers have structural diversity derived from monomer units, substituent, interflavan linkage, and degree of polymerization. They were efficient, reversible, and mixed competitive inhibitors of tyrosinase. Interactions of mung bean seed proanthocyanidins with the enzyme (hydrogen bond, hydrophobic interaction,
Acknowledgements
The present investigation was supported by the Natural Science Foundation of China (No. 31501414), Natural Science Fund of Jiangxi Province, China (No. 20171BAB214019) and the Education department project of Jiangxi province, China (GJJ150302).
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