Low-density lipoprotein-antioxidant flavonoids and a phenolic ester from Plectranthus hadiensis var. tomentosus

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.