Rapid determination of flavonoids in plumules of sacred lotus cultivars and assessment of their antioxidant activities
Introduction
Flavonoids are important polyphenolic compounds occurring in plant kingdom and have received much attention for their potential role in human health in recent years. Usually, there are two ways that sugars are linked to flavonoid skeletons resulting in C- or O-glycosides (Abad-García et al., 2008). In C-glycosides, sugars are attached directly to the nucleuses through a CC bond that is stable towards acid hydrolysis, while the sugars of O-glycosides are connected to aglycones by an acid hydrolyzable OC bond (Du et al., 2010). Generally, C-glycosides are less common than O-glycosides (Brazier-Hicks et al., 2009). They are only present in some specific plant groups with substitution occurring singly or doubly at the C-8 and/or C-6 position (Brazier-Hicks et al., 2009 Harborne, 1993). These flavonoid C-glycosides have various biological properties, such as antimicrobial (Dinda et al., 2006), antifungal (McNally et al., 2003), antioxidant (Kitta et al., 1992, Ramarathnam et al., 1989), and UV-protective (Les and Sheridan, 1990). In medicinal perspective, some activities also have been confirmed, such as counteracting inflammation and cancer development (Manthey et al., 2001), acting as antihypertensive (Prabhakar et al., 1981) and anti-obesity agent (Kim et al., 2010).
Sacred lotus, an ancient dicotyledonous plant, belonging to Nelumbo genus, consists of only two species all over the world, namely Nelumbo nucifera Gaertn. and Nelumbo lutea Pers. In China, it has a history of cultivation over 2000 years and has been widely cultivated in the majority of provinces (Guo, 2008). As one of the top ten famous Chinese traditional flowers, lotus is highly appreciated for its ornamental value and unique cultural and religious significance (Li et al., 2010). As recorded in “Chinese Pharmacopoeia”, plumule, stamen, leaf and other parts of lotus can be used as medicines. Virtually, it is employed as not only foodstuff but also herbal medicine (Shen-Miller et al., 2002).
Lotus plumule is the green germ of a mature lotus seed with bitter taste, located between the two cotyledons (Kato et al., 2015). With so many kinds of bioactive compounds including alkaloids, flavonoids, polysaccharides, sitosterols, volatile oils and microelements in it, lotus plumule can be used in the treatment of hypertension, arrhythmia, platelet aggregation, lipid peroxidation, cancer and radicals scavenging (Zeng et al., 2005). Liensinine, isoliensinine and neferine are thought to be the three major biologically active compounds in lotus plumule (Yu et al., 2013). And the pharmacological properties of these bisbenzylisoquinoline alkaloids including antioxidant, antidepressant, antiarrhythmic, and anti-HIV have been demonstrated (Dong et al., 2012, Itoh et al., 2011, Kashiwada et al., 2005, Zhou et al., 2007). In previous studies, more attention has been given to the investigation of alkaloids and little is known about the flavonoids. Recently, accumulated C-glycosyl flavonoids have been found in lotus plumule (Li et al., 2014). To comprehend the influence of the newly discovered compounds on medicinal value of lotus plumule, C-glycosyl flavonoids in lotus cultivars were assessed in this study. A fast and efficient method was established to evaluate the flavonoid profiles in lotus plumules by UPLC and two complementary methods were used to estimate the antioxidant activity. This work will also be helpful in clarifying flavonoids metabolic pathways in lotus plumule.
Section snippets
Plant materials
Seeds at their full maturity of 38 lotus cultivars were collected at the lotus germplasm resources garden (latitude 39°48′N, longitude 116°28′E, and altitude 76 m) in Beijing Botanical Garden, Institute of Botany, the Chinese Academy of Sciences (IBCAS) in mid-August, 2014. These cultivars were all planted in the same size cylinders (diameter 40 cm, height 30 cm) and had been cultivated under the same conditions for more than 3 years. Plumules were peeled from seeds and dried at room temperature,
Identification of flavonoid O-glycosides
Chen et al. (2012) separated and identified five compounds in lotus plumule by HPLC in 53 min. With HPLC, nineteen compositions were separated and identified in 70 min (Li et al., 2014). But the protocol was time-consuming and the mobile phase with high acidity (10% formic acid aqueous solution) would do harm to the apparatus. In this study, twenty compounds were separated by UPLC (Fig. 1), the elution time was greatly shortened (7 min) and the acidity of mobile phase was decreased notably (0.1%
Conclusions
In this research, abundant flavonoids in plumules of lotus cultivars were determined by UPLC–MS/MS for the first time. In all, twenty flavonoids were identified, fourteen of which were flavonoid C-glycosides. These detected flavonoid C-glycosides will lay solid scientific foundation to the healthcare and medicinal values of lotus plumule. And also, these compounds may provide help to the controversies on the taxonomy of lotus. The 38 cultivars were clustered into three clusters according to the
Acknowledgement
This study was financially supported by National Natural Science Foundation of China (Grant No. 31471910).
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