Abstract
Malva verticillata (Cluster mallow), a leafy vegetable that has been popular in East Asia for a long time, has also been used in herbal teas and medicines. The aqueous fraction of the aerial parts of Malva verticillata, exhibiting a very high quantity of flavonoids compared to the EtOAc and n-BuOH fractions, exhibited significant recovery effects on pancreatic islets damaged by alloxan in zebrafish larvae. Thus, the bioactive components responsible for this anti-diabetic activity were investigated. A new flavonoid glucuronide (1) and five known flavonoids were isolated from the aqueous fraction. Based on several spectroscopic methods, compound 1 was identified to be nortangeretin-8-O-β-d-glucuronide, and was named malvaflavone A. The A-ring of compound 1 had a 5,6,7,8-tetrahydroxy moiety, which rarely occurs in plant systems. Also 8-O-glucuronide attached to the flavonoid moiety was rarely occurred in plant system. Compounds 1, 3, 4, and 6 significantly improved the pancreatic islet size in zebrafish at 0.1 μM, and compounds 1 and 6 were found to block β-cell K+ channels in experiments with diazoxide. In ABTS, ORAC, and SOD assays, compounds 1–5 exhibited high anti-oxidant activities compared with quercetin and BHA (positive controls), indicating that the 8-O-glucuronide attached to the flavonoid moiety is a key structure for the expression of anti-oxidant activity. This is the first report of the isolation of compounds 1–6 from M. verticillata as well evaluated for anti-diabetic and anti-oxidant ativities.
Highlights
Diabetes is characterized by variations in the metabolism of carbohydrates, proteins and fats.It results from insulin secretion disorders or reduced insulin sensitivity to blood sugar levels [1].In diabetic patients and animal models, the pancreatic islet (PI) size and β-cell number are smaller than normal [2]
Epidemiological studies have consistently shown that regular consumption of foods such as vegetables is strongly related to a lower risk of developing chronic diseases such as diabetes [5]
A considerable amount of evidence has indicated that increased oxidative damage contributes to the development of chronic diseases such as diabetes, and several epidemiologic studies have demonstrated that the consumption of polyphenol-rich vegetables is associated with a reduced risk of oxidative stress-related diseases [9,10]
Summary
Diabetes is characterized by variations in the metabolism of carbohydrates, proteins and fats. Epidemiological studies have consistently shown that regular consumption of foods such as vegetables is strongly related to a lower risk of developing chronic diseases such as diabetes [5]. A considerable amount of evidence has indicated that increased oxidative damage contributes to the development of chronic diseases such as diabetes, and several epidemiologic studies have demonstrated that the consumption of polyphenol-rich vegetables is associated with a reduced risk of oxidative stress-related diseases [9,10]. There is a need to search for anti-diabetic food components that prevent chronic diseases and promote health. In this study, flavonoids with anti-diabetic activity were isolated and identified from the aqueous Fr of the aerial parts of M. verticillata. The anti-oxidant potential of these flavonoids was measured with DPPH, ABTS, ORAC radical scavenging (RS), and superoxide dismutase (SOD) assays
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