Antioxidant and tyrosinase inhibitory activities of methanol extracts from Magnolia denudata and Magnolia denudata var. purpurascens flowers

Abstract

Methanolic extracts of white colored (Magnolia denudata, WME) and violet colored (M. denudata var. purpurascens, VME) flowers were prepared, and their antioxidant activities were evaluated by total phenolic contents (TPC), 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity (RSA), 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) RSA, and reducing power (RP). TPC of WME and VME were 85.80 and 80.96 mg/g gallic acid equivalents, respectively. DPPH RSA and ABTS RSA of WME and VME at 1 mg/mL concentration were 87.74 and 75.05, and 88.24 and 85.98%, respectively. In the case of RP, there was no significant (P < 0.05) difference between WME (1.094) and VME (1.081). Tyrosinase inhibitory activities of WME and VME were 17.48% and 8.63%, respectively.

Introduction

Reactive oxygen species (ROS) in the forms of superoxide anion radical (·O₂⁻), hydroxyl radical (·OH), hydrogen peroxide (H₂O₂) are generated by normal metabolic processes or exogenous factors and agents, and these can occur oxidative damage to human cells, causing diseases such as cancer, cardiovascular disease, osteoporosis, and degenerative diseases (Halliwell & Gutteridge, 1984). Antioxidants can delay or inhibit the initiation or propagation of oxidative chain reaction and thus prevent or repair oxidative damage done to the body's cells by oxygen (Velioglu, Mazza, Gao, & Oomah, 1998). Thus synthetic antioxidants such as butylated hydroxyanisol (BHA), butylated hydroxy toluene (BHT), and tertiary butylhydroquinone (TBHQ) have been widely used in food industry. Synthetic antioxidants, however, are known to induce carcinogenesis by mutagenicity and toxicity against human enzymes and lipids (Buxiang and Fukuhara, 1997, Hirose et al., 1998). For this reason, studies of natural antioxidants are attractive to researchers for use in foods or medicinal materials to replace synthetic antioxidants. Particularly, much study is focused on natural products including vegetables and wild plant sources (Chan, Decher, & Means, 1993).

Tyrosinase (monophenol monooxygenase, E:C:1.14. 18.1), also known as polyphenol oxidase (PPO) is a copper containing enzyme, is involved in the first two steps of melanin biosynthesis (Karioti, Protopappa, Megoulas, & Skaltsa, 2007). This enzyme is responsible for enzymatic browning in plants, producing undesirable changes in color, flavor and nutritive values of plant-derived foods and beverages (Friedman, 1996). Tyrosinase inhibitors are chemical agents capable of reducing enzymatic reactions, such as food browning and melanisation of human skin. Therefore, tyrosinase inhibitors have become increasingly important in cosmetic and medical products in relation to hyperpigmentation (Kim et al., 2002, Lim et al., 2009). The tyrosinase inhibitors such as kaempferol of saffron flower (Kubo & Kinst-Hori, 1999) and catechins (No et al., 1999) have been discovered from various plants.

The genus Magnolia (Magnoliaceae), consisting of around 230 species of trees and shrubs, one of the most primitive among angiosperms, is mainly distributed throughout East and Southeast Asia, southeast North America, Central America, and South America (Xu & Kirchoff, 2008). Magnolia denudata and Magnolia denudata var. purpurascens flowers are wildly distributed in southern China and it offers flowers with white or violet color. The overall floral display of this tree is showy because individual flower is large and very numerous, and flowering occurs in early spring before leaf flushing (Wang, Jia, Wang, & Zhang, 2010). Flower of M. denudata used as traditional medicine and flower buds called Flos Magnoliae (also called Shin-I or Xin-yi) are commonly used as medicinal herbs with a long history of clinical application in many Asian countries for symptomatic management of allergic rhinitis, sinusitis and headache. Various biologically active compounds such as eudesmin, magnolin, epimagnoli, neolignans, lignans, phenyl propanoids, sesquiterpenes, and alkaloids have been also isolated from Magnolia spp. (Kelm and Nair, 2000, Seo, 2010, Shen et al., 2008).

Although there were several researches on antioxidant activities of M. denudata and Magnolia liliflora, such as DPPH free radical scavenging, hydroxyl free radical scavenging and superoxide free radical scavenging (Kang et al., 2003, Seo, 2010, Zeng et al., 2008), to our knowledge, no information is available on comparative antioxidant properties of M. denudata and M. denudata var. purpurascens flowers, as well as tyrosinase inhibitory activity. Therefore, the objective of this study was to evaluate the antioxidant activities of methanolic extracts from M. denudata spp. flowers with white and violet colors by total phenolic contents (TPC), 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity (RSA), 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) RSA, and reducing power (RP). In addition, tyrosinase inhibitory activities of methanolic extracts were also measured to see if these extracts can be used as the ingredients of cosmetics.