Antioxidant and antityrosinase activity of mulberry (Morus alba L.) twigs and root bark
Introduction
Lipid components in food are susceptible to oxidation, which results in certain annoying effects in the course of food storage. They not only play a destructive role, thus deteriorating food quality, but also contaminate the food with harmful substances. At the same time, lipid oxidation induced by reactive oxygen radicals will lead to cellular damage and promote the pathological progression of atherosclerosis, carcinogenesis, and diabetes (Wang et al., 2007). In view of the above, various antioxidants for reducing lipid oxidation have been investigated. Moreover, there has been increasing interest in replacing synthetic antioxidants with natural ones for safety concern. As many of the natural phytochemicals present in plants have been found to exhibit antioxidative capacity in vivo they have attracted much attention as natural inhibitors of oxidation in foods (Heim et al., 2002). In addition to lipid oxidation, the browning reaction in foods is another unfavorable effect which decreases its appeal and nutritional value during storage. Reports have indicated that tyrosinase had a primary role in the enzymatic browning reaction in food processing by catalyzing the oxidation of phenol and regulating the initial step of melanin production (Roh et al., 2004). Therefore, many studies have focused on using natural additives to decrease the browning progression in food production (Germanas et al., 2007). In addition to food browning, tyrosinase activation and its accompanying melanin generation would eliminate the glutathione levels and decrease the antioxidant capacity in foods (Penalver et al., 2002, Nappi and Vass, 1996), as well as promote the progression of melanin hyperpigmentation in different skin diseases (Lee et al., 2002, Takara et al., 2007).
The mulberry fruit is widely regarded as a nutritious food in China. Yet, the mulberry (Morus alba L.) leaves, root bark, and twigs have long been used in Chinese medicine to treat fever, protect the liver, improve eyesight, strengthen joints, facilitate discharge of urine, and lower blood pressure (Zhishen et al., 1999). Different parts of the mulberry, from the root bark to the leaves, have been extensively investigated for their various health benefits, including antioxidative, hypolipidemic, antihyperglycemic, and antiatherogenic effects (Harauma et al., 2007). Previous studies have indicated that mulberry fruits and leaves exhibited significant scavenging effects on free radicals and protected low-density lipoprotein against oxidative damage. The reports suggested that polyphenols and flavonoids present in mulberry contribute to these health benefits (Yen et al., 1996, Doi et al., 2000, Katsube et al., 2006, Bae and Suh, 2007). Additionally, only a small part of the mulberry twig is used as the medicine in China, most of which acts as agricultural wastes and create greater environmental problems (Zhang and Shi, 2010). Therefore, more rational uses of mulberry twigs as a source of natural antioxidants to provide a means for recycling have attracted much attention. However, despite the large number of studies on the fruits or leaves of mulberry, the effects of mulberry twigs on tyrosinase and quantitative antioxidant activity have rarely been examined. In view of the importance of decreasing both tyrosinase activity and oxidative reaction in foods to protect the nutritional components against oxidative damage, the aims of this study were to investigate the modulation capacity of mulberry twigs on tyrosinase activity and oxidative damage.
Section snippets
Materials
1,1-Diphenyl-2-picrylhydrazyl (DPPH), lecithin, phenazine methosulphate (PMS), nitroblue tetrazolium (NBT), potassium ferricyanide, ferrozine, thiobarbituric acid (TBA), phosphoric acid and mushroom tyrosinase were purchased from Sigma (St. Louis, MO, USA). Isoquercitrin, maclurin, morin, resveratrol and rutin were purchased from Xtrasynthese (Genay, France). The purity of these standards was assessed with a photodiode array detector (Hitachi L-7455, Japan). Methanol and acetonitrile were of
Results
Fig. 1A shows the DPPH radical scavenging activity of EEMT and EEMR. As can be seen, the EEMT and EEMR, in the range of 0–100 μg/ml, exhibited 0–86% and 0–37% scavenging effects on DPPH radicals, respectively. The DPPH scavenging activity of EEMT at the concentration of 100 μg/ml was about 2.32-fold greater than that of EEMR. Since the total polyphenol content and total flavonoid content of natural products were regular indexes of their antioxidant activity, the gallic acid and rutin equivalents
Discussion
The destructive oxidative stress derived from reactive oxygen species will induce oxidation in biomolecules (e.g. lipids, proteins, and DNA) and aggravate the progression of cells to a pathological status (Halliwell, 1999). On the other hand, tyrosinase not only catalyzes unfavorable reactions, which deteriorates the appearance and nutritional value of foods, but also increases oxidative risk in different physiological systems (Sanchez-Ferrer et al., 1995). Recent reports have suggested that
Conflict of Interest
The authors declare that there is no conflict of interest.
Acknowledgements
The origins of the plant material were authenticated by Dr. J.J. Yang (Department of Pharmacy, Chia-Nan University of Pharmacy and Science, Taiwan) and financial support from the Chia-Nan University of Pharmacy and Science were gratefully acknowledged.
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