Abstract
To investigate the effects of extraction solvents and drying methods on Plectranthus hadiensis var. tomentosus quality, eight compounds were isolated and the content of active compounds with their antioxidant activities were compared. Compounds 1 and 2 were known antioxidants, whereas the low-density lipoprotein (LDL)-antioxidant activities of compounds 3, 5, 6, and 7 are reported for the first time, with IC50 values of 2.5, 3.8, 22.8, and 53.7 μM, respectively. Our analysis of 30‒95% ethanol extracts from freeze- and air-dried leaves and stems revealed a relationship between extract composition and antioxidant activity. The 95% ethanol extracts of freeze-dried stems (FDS) exhibited highest phenolic and flavonoid content, which were 1.40 and 2.67 times, respectively, greater than those of air-dried stems (ADS), and very high LDL-antioxidant and DPPH radical scavenging activities, which may have resulted from the phenolic ester rosmarinic acid (2), a major component of FDS extracts and potent antioxidant. In contrast, the 95% ethanol extracts of ADS exhibited relatively low antioxidant activity, possibly owing to the low antioxidant activity of the main components ayanin (7) and (+)-plectranthone (8). These results are important for the development of P. hadiensis var. tomentosus as an effective natural antioxidant material.
Highlights
The excess production of oxidants, such as reactive oxygen species, and imbalances between antioxidative defence systems and active oxygen molecules can induce oxidative stress, and such oxidative stress has been linked to the pathogenesis of various chronic diseases, including cardiovascular disease [1], atherosclerosis [2], cancer [3], and inflammatory disorders [4]
The all three fractions were used for further isolation of active metabolites using repeated SiO2, octadecyl silica-gel (ODS), and Sephadex LH-20 column chromatographies: two sesquiterpenes (4 and 8) and one flavonoid (7) were isolated from the n-hexane fraction; three flavonoids (3, 5, and 6) were isolated from the CHCl3 fraction; and one flavonoid (1) and one phenolic acid (2) were isolated from the 10% aqueous MeOH fraction (Fig. 1)
The chemical structures of compounds 1–8 were identified as luteolin 7-O-glucuronide (1) [19], rosmarinic acid (2) [20], chrysosplenol D (3) [21], desacetyl plectranthone (4) [22], quercetin 3, 7-dimethyl ether (5) [23], casticin (6) [21], ayanin (7) [23], and (+)-plectranthone (8), the enantiomer of (−)-plectranthone [22], based on 1H and 13C NMR and ESI–MS data, and confirmed through comparison with published spectroscopic data (Fig. 1 and Additional file 1)
Summary
The excess production of oxidants, such as reactive oxygen species, and imbalances between antioxidative defence systems and active oxygen molecules can induce oxidative stress, and such oxidative stress has been linked to the pathogenesis of various chronic diseases, including cardiovascular disease [1], atherosclerosis [2], cancer [3], and inflammatory disorders [4]. We isolated eight compounds from the aerial parts of PHT, and analysed their antioxidant activity by evaluating their ability to inhibit Cu2+-mediated low-density lipoprotein (LDL) oxidation and apoB-100 fragmentation and scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals.
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