Thermal transformation of polar into less-polar ginsenosides through demalonylation and deglycosylation in extracts from ginseng pulp

Abstract

The present study was conducted to qualitatively and quantitatively elucidate dynamic changes of ginsenosides in ginseng pulp steamed under different temperatures (100 or 120 °C) for different durations (1-6 h) through UPLC-QTOF-MS/MS and HPLC with the aid of as numerous as 18 authentic standards of ginsenosides. Results show that levels of eight polar ginsenosides (i.e., Rg₁, Re, Rb₁, Rc, Rb₂, Rb₃, F₁, and Rd) declined but those of 10 less-polar ginsenosides [i.e., Rf, Rg₂, 20(S)-Rh₁, 20(R)-Rg₂, F₄, 20(S)-Rg₃, 20(R)-Rg₃, PPT, Rg₅, and 20(R)-Rh₂] elevated with increases of both steaming temperature and duration; the optimum steaming conditions for achieving the highest total ginsenosides were 100 °C for 1 h. Particular, 20(R)-Rg₃, a representative less-polar ginsenoside with high bioactivity such as potent anti-cancer effect, increased sharply but Re, the most abundant polar ginsenoside in fresh ginseng pulp, decreased dramatically. More importantly, ginsenoside species enhanced from 18 to 42 after steaming, mainly due to transformation of polar into less-polar ginsenosides. Furthermore, four malonyl-ginsenosides were detected in fresh ginseng pulps and ten acetyl-ginsenosides were formed during steaming, demonstrating that demalonylation and acetylation of ginsenosides were the dominant underling mechanisms for transformation of polar into less-polar ginsenosides.

Figure 1

HPLC analyses of ginsenosides in extracts from raw and steamed ginseng pulps or ginseng pulp soaking supernatant. Shown here are chromatograms of (A) raw ginseng pulp extract (GPE); (B) ginseng pulp soaking supernatant extract (GPS); (C) GPS steamed for 6 h at 100 °C (S1006) and (D) for 1 h at 120 °C (S1201) and (E) GPS steamed for 6 h at 120 °C (S1206); and (F) standards of the 18 ginsenosides. (F) Ginsenosides marked are: (1) Rg₁, (2) Re, (3) Rf, (4) Rb₁, (5) 20(S)-Rg₂, (6) 20(S)-Rh₁, (7) 20(R)-Rg₂, (8) Rc, (9) Rb₂, (10) Rb₃, (11) F₁, (12) Rd, (13) F₄, (14) 20(S)-Rg₃, (15) 20(R)-Rg₃, (16) 20(S)-PPT, (17) Rg₅, and (18) Rh_2. Peak numbers of the 18 standard ginsenosides that are not shown were not detected in the corresponding samples. An inset within (F) is an amplification of the partial profile between 40 and 85 min as indicated by the dashed box.

Figure 2

Changes in total contents of the 18 ginsenosides in raw and steamed ginseng pulps extracts (GPE) or extracts from ginseng pulp soaking supernatant (GPS) during steaming for 6 h at 100 °C (S100 or E100) and 120 °C (S120 or E100). Data at 0 h represent those of GPE or GPS. Values are denoted as the mean ± standard deviation (n = 3).

Figure 3

Changes in contents of the 18 ginsenosides in raw and steamed ginseng pulps extracts (GPE) or extracts from ginseng pulp soaking supernatant (GPS). Shown are contents of polar (A,B) and less-polar (C,D) ginsenosides, as indicated by differently-colored columns, in GPS (A,C) and GPE (B,D) steamed for 1–6 h at 100 °C (S100 and E100) and 120 °C (S120 and E120). Values marked by different letters at same temperatures are significantly different (p < 0.05) to those of GPE or GPS.

Figure 4

Chemical structures and possible transformation mechanisms of ginsenosides identified from raw and steamed ginseng pulps extracts (GPE) and/or extracts from ginseng pulp soaking supernatant (GPS). (A) Protopanaxadiols; (B) Protopanaxatriols. ara(f), α-l-arabinofuranosyl; ara(p), α-l-arabinopyranosyl; glc, β-d-glucopyranosyl; xyl, β-l-xylopyranosyl; rha, α-l-rhamnopyranosyl. Chemical links between C-20 and C-22 of F₄ and Rg₅, and between C-20 and C-21 of Rg₆ and Rk₁ are double bonds.