Alkaloids of Phaedranassa dubia (Kunth) J.F. Macbr. and Phaedranassa brevifolia Meerow (Amaryllidaceae) from Ecuador and its cholinesterase-inhibitory activity

Abstract

Alzheimer's disease is considered the most common cause of dementia and, in an increasingly aging population worldwide, the quest for treatment is a priority. Amaryllidaceae alkaloids are of main interest because of their cholinesterase inhibition potential, which is the main palliative treatment available for this disease. We evaluated the alkaloidal profile and the in vitro inhibitory activity on acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) of bulb alkaloid extract of Phaedranassa dubia and Phaedranassa brevifolia collected in Ecuador. Using gas chromatography coupled to mass spectrometry (GC-MS), we identified typical Amaryllidaceae alkaloids in these species, highlighting the presence of lycorine-type alkaloids in P. dubia and haemanthamine/crinine-type in P. brevifolia. The species P. dubia and P. brevifolia showed inhibitory activities against AChE (IC₅₀ values of 25.48 ± 0.39 and 3.45 ± 0.29 μg.mL^-1, respectively) and BuChE (IC₅₀ values of 114.96 ± 4.94 and 58.89 ± 0.55 μg.mL^-1, respectively). Computational experiments allowed us to understand the interactions of the alkaloids identified in these samples toward the active sites of AChE and BuChE. In silico, some alkaloids detected in these Amaryllidaceae species presented higher estimated binding free energy toward BuChE than galanthamine. This is the first study about the alkaloid profile and biological potential of P. brevifolia species.

Keywords: AChE; AChE, Acetylcholinesterase; AE, alkaloid extract; ATCI, acetylthiocholine iodide; Alkaloids; Alzheimer's disease; Amaryllidaceae; BTCI, butyrylthiocholine iodide; BuChE; BuChE, butyrylcholinesterase; CD, circular dichroism; DTNB, (5,5′-dithio-bis-[2-nitrobenzoic acid]); Et2O, diethyl ether; EtOAc, ethyl acetate; GAL, galanthamine; GC-MS, gas chromatography coupled to mass spectrometry; IUCN, International Union for Conservation of Nature; MS, mass spectrometry; MeOH, methanol; Molecular docking; NMR, nuclear magnetic resonance; Phaedranassa; UV, ultraviolet.

Fig. 1

(a) Phaedranassa dubia flowers; (b) P. brevifolia flowers; (c) P. dubia bulb and leaves; (d) P. brevifolia bulb and leaves. Photos: Karen Acosta León and Nora Oleas.

Fig. 2

Alkaloids identified in P. dubia and P. brevifolia collected in Ecuador by GC-MS.

Fig. 3

Number of alkaloids detected in P. dubia (a) and P. brevifolia (b) separated by alkaloid-type. LY: lycorine-type; H/C: Haemanthamine/crinine-type; GA: galanthamine-type; HO: homolycorine-type; MO: montanine type; NI: not identified.

Fig. 4

Inhibitory activity of P. dubia (P. dub) and P. brevifolia (P. bre) alkaloid extracts against AChE and BuChE expressed as IC₅₀ (μg.mL⁻¹). The data presented are the mean values SD (n=3). GAL = galanthamine (reference compound).

Fig. 5

Graphical representations of the binding of (a) galathamine (9) and (b) crinine (6b) in the gorge of the active site of AChE.

Fig. 6

Graphical representations of the binding of (a) vittatine (6a) and (b) crinine (6b) in the gorge of the active site of hBuChE.