Luteolin and Vernodalol as Bioactive Compounds of Leaf and Root Vernonia amygdalina Extracts: Effects on α-Glucosidase, Glycation, ROS, Cell Viability, and In Silico ADMET Parameters

Abstract

The aqueous decoctions of Vernonia amygdalina (VA) leaves and roots are widely used in traditional African medicine as an antidiabetic remedy. The amount of luteolin and vernodalol in leaf and root extracts was detected, and their role was studied regarding α-glucosidase activity, bovine serum albumin glycation (BSA), reactive oxygen species (ROS) formation, and cell viability, together with in silico absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties. Vernodalol did not affect α-glucosidase activity, whereas luteolin did. Furthermore, luteolin inhibited the formation of advanced glycation end products (AGEs) in a concentration-dependent manner, whereas vernodalol did not reduce it. Additionally, luteolin exhibited high antiradical activity, while vernodalol demonstrated a lower scavenger effect, although similar to that of ascorbic acid. Both luteolin and vernodalol inhibited HT-29 cell viability, showing a half-maximum inhibitory concentration (IC₅₀) of 22.2 µM (-Log IC₅₀ = 4.65 ± 0.05) and 5.7 µM (-Log IC₅₀ = 5.24 ± 0.16), respectively. Finally, an in silico ADMET study showed that both compounds are suitable candidates as drugs, with appropriate pharmacokinetics. This research underlines for the first time the greater presence of vernodalol in VA roots compared to leaves, while luteolin is prevalent in the latter, suggesting that the former could be used as a natural source of vernodalol. Consequently, root extracts could be proposed for vernodalol-dependent antiproliferative activity, while leaf extracts could be suggested for luteolin-dependent effects, such as antioxidant and antidiabetic effects.

Keywords: MTT assay; ORAC assay; antioxidants; flavonoids; phytotherapy; terpenes.

Figure 1

Chemical structures of two characteristic compounds detected in Vernonia amygdalina extracts.

Figure 2

Exemplificative HPLC-DAD chromatograms: (A) 1 mg/mL macerate aqueous root extract (AquRE, clear blue), AquRE + 0.125 mg/mL vernodalol (black), and 0.125 mg/mL vernodalol alone (blue); (B) 1 mg/mL Soxhlet aqueous root extract (SoxRE, green), SoxRE + 0.125 mg/mL vernodalol (black), and 0.125 mg/mL vernodalol alone (blue); and (C) 1 mg/mL macerate ethanol root extract (EthRE, blue), 1 mg/mL SoxRE + 0.125 mg/mL vernodalol (black), and 0.125 mg/mL vernodalol alone (green). (D) The insert shows the UV absorption spectrum of vernodalol.

Figure 3

Quantitative detection of luteolin (A) and vernodalol (B) in leaf and root extracts of Vernonia amygdalina. Data are expressed as mean ± SEM of 4–5 experiments. AquLE, aqueous leaf extract; AquRE, aqueous root extract; SoxLE, Soxhlet aqueous leaf extract; SoxRE, Soxhlet aqueous root extract; EthLE, ethanol leaf extract and EthRE; ethanol root extract. * p < 0.05, *** p < 0.005, **** p < 0.0001 versus macerate aqueous extracts (AquLE or AquRE).

Figure 4

Antiradical activity of luteolin and vernodalol detected by ORAC assay. Positive control: ascorbic acid. Data are the mean ± SEM of 3–6 experiments. **** p < 0.0001 versus positive control (ascorbic acid). TEAC: trolox equivalent antioxidant capacity.

Figure 5

Effects of luteolin (A) and vernodalol (B) on α-glucosidase activity. The activity is expressed in percentage of the enzymatic action without inhibitor. Vernodalol was tested only up to 25 µM because no inhibition was observed. Data are the mean ± SEM of 4–6 experiments. **: p < 0.01; **** p < 0.0001 versus α-glucosidase activity.

Figure 6

Effects of luteolin (yellow bars) and vernodalol (red bars) on AGE formation after 7 (A,D) or 14 (B,C,E,F) days of incubation of 10 mg/mL BSA with 0.05 M ribose, 0.5 M fructose (B), or 1.0 M glucose (C). Aminoguanidine (2.5 mM, AG) was the positive control. Data are the mean ± SEM of 3–6 experiments. ** p < 0.01, *** p < 0.001, **** p < 0.0001 versus BSA glycation (BSA + glycation agent).

Figure 7

Effects of luteolin and vernodalol on the viability of human colon adenocarcinoma (HT-29) cells expressed as percentage of control. Data are the mean ± SEM of 3–6 experiments.