Several evidences indicate that the accumulation of beta-amyloid, the formation of neurofibrillary tangles, oxidative stress, the reduction of acetylcholine (ACh) and estrogen play important roles in the pathoetiology of Alzheimer’s disease (AD). Based on multifactorial etiology of AD, controlling pathological processes of AD via multiple targets is more efficient than inhibition of single target. Therefore, a compound with multi-target action involved in pathological processes might be a potential candidate for AD. Pueraria candollei Wall. Ex Benth var. mirifica (Airy Shaw & Suvatabandhu) Niyomdhum, known as Kwaao kheur kao in Thai, is a rich source of phytoestrogen eg. genistein, daidzein, puerarin, mirificin, miroestrol and isomiroestrol2. Moreover, miroestrol was reported for highest estrogenic potency among all known phytoestrogen. Besides, Pueraria candollei extract also was claimed to have antioxidant action, since it contains abundant of flavonoids. The aim of our study was to evaluate the effects of phytoestrogen extracted from Pueraria candollei, miroestrol and isomiroestrol, on factors related AD. The test compounds were investigated for free radical scavenging and acetylcholinesterase (AChE) inhibitory activity in vitro. The interaction between the test compounds and alpha estrogenic receptor (ERα) was also studied using molecular modeling techniques.
The results showed that miroestrol and isomiroestrol were able to scavenge radical with IC50 of 67.70 and 32.75 µM, respectively. Isomiroestrol was more potent than trolox, a reference standard. As the result, Isomiroestrol showed two times potent than miroestrol. It might reason by the hydroxyl function group at 7 position of isomiroestrol could has less steric shield than miroestrol, though there are four groups of hydroxyl substituted on both structures. Both miroestrol and isomiroestrol have no effect on AChE activity. This is the first report for free radical scavenger activity and AChE activity of these compounds. Chemical structure of chromene, miroestrol and isomiroestrol are mimic to estradiol, especially hydroxyl group on 3 position and 17 position of estradiol. The phonolic A ring of estrogen have clarified as the critical chemical requirement for estrogenic antioxidant function in the central nervous system (CNS)7-8. The recent report showed that the phenolic ring structure is both necessary and sufficient for the antioxidant properties of estrogen. Thus, the phenolic ring structure of phytoestrogens, miroestrol and isomiroestrol revealed high potency of an estrogenic-like antioxidant property. For binding interaction studies, the result showed that miroestrol and isomiroestrol served as ligands bound to ERα active site. The test compounds tightly bound to ERα with the binding free energy of -12.92 and -12.08 kcal/mol, respectively, while estradiol, reference standard, showed the binding free energy of -10.08 kcal/mol. As the result of the difference contribution in predicted binding affinity on ER-α showed interaction between miroestrol to ERα is stronger (0.84 kcal/mol) than that of isomiroestrol. The remarkable differences between them are position of hydroxyl functional group at 14 position. Therefore at this position is play an important role for binding with ERα. This study showed the highest potency of an estrogenic-like antioxidant from precious natural source, Pueraria candollei Wall. Ex Benth var. mirifica like miroestrol and isomiroestrol, that might be powerful candidate for future studies on neuroprotective and estrogen function.
Wichitsak Sukhano, Chantana Boonyarat, Orawan Monthakantirat*
Faculty of Pharmaceutical Sciences, Khon Kaen University, Khon Kaen, Thailand
Full Research (PDF): Click here