Choline esterase inhibitory properties of alkaloids from two Nigerian Crinum species

Bioassay-guided fractionation for acetylcholinesterase inhibition led to the isolation of four alkaloids from Crinum glaucum and Crinum jagus. Hamayne from C. glaucum was found to be the most active compound. The bulbs of Crinum jagus and Crinum glaucum are used in traditional medicine in southern Nigeria for memory loss and other mental symptoms associated with ageing. Alkaloidal extracts of bulbs from each species showed inhibition of acetylcholinesterase, an activity exploited therapeutically to raise the depressed levels of acetylcholine in the brain associated with Alzheimer’s disease. Using the in situ bioautographic test method for enzyme inhibition, a number of alkaloids were isolated and their activity quantified using the Ellman spectrophotometric test. The most active alkaloids isolated were hamayne (IC 50 250 μM) and lycorine (IC 50 450 μM) whilst other alkaloids were comparatively inactive with haemanthamane giving 3% inhibition and crinamine giving 4.4% inhibition at 50 mg ml −1 (174 μM). These contrast with the positive control physostigmine which gave IC 50 of 0.25 μM. Cholinesterase activity appears to be associated with the presence of two free hydroxy groups in this structural type of Amaryllidaceae alkaloid.