## 3 Discussion
laricifolium_ contained two separate enzyme systems, each capable of elaborating on a single enantiomer [51]. Thus, the formation of the separate enantiomers of linalyl pyrophosphate (LPP), (\(R\))-**6** and (\(S\))-**7**, from geranyl pyrophosphate (GPP) (**5**), followed by analogous carbocation reactions, would explain the production of the couples of monoterpene enantiomers **1/2** and **3/4**, as shown in Scheme 1. Moreover, the remarkable higher percent content in the EO of **1+4** than **2+3** (Table 2), which originated from each of the two LPP enantiomers, clearly indicated that \(K^{1}>K^{2}\) and/or \(K^{3}>K^{4}\). On the higher hand, the higher content of **4** compared to **1**, and **2** compared to **3** (Table 2), which are formed from the corresponding \(\alpha\)-terpynyl cation, demonstrated that \(K^{5}\) is greater than \(K^{6}/K^{7}\), but \(K^{9}/K^{10}\) are greater than \(K^{8}\) (Scheme 1). In conclusion, these findings clearly indicated that each step of the two biosynthetic pathways leading to **1-4** from GPP (**5**) was enzymatically controlled. Based on the IC\({}_{50}\) values (see Section 2.4), the EO from _H. laricifolium_ showed higher selective inhibitory activity against BuChE than against AChE. In fact, the former activity was about three times higher than the latter one and about ten times lower than that of the reference inhibitor donepezil hydrochloride. The relationship between anticholinesterase activity and structures of oil components, as well as their inhibitory effects mechanism(s), are far from having been firmly determined and even divergent results are observed. These discrepancies often depend on the fact that authors used different experimental parameters for testing cholinesterase inhibitory activity. Undoubtedly a notable trend in the predominant bioactive components is that essential oils with the higher potency for inhibiting AChE activity had monoterphere hydrocarbons, such as \(\alpha\)-pinene, \(\beta\)-pinene, \(\delta\)-3-carene, \(\gamma\)-terpinene, and limonene as their predominant components [37]. Due to the low molecular size and lipophilic nature, they can move into and across cell membranes, and have the potential to influence the fluidity and porosity of membrane structures and to cross the blood-brain barrier. However, polar oxygenated monoterpneoids such as 1,8-cineole, borneol, verbenone, carvacrol, and thymol also exhibited high inhibitory activity against AChE from _Electrophorus electricus_ (electric eel) [37]. Moreover, several studies have also attributed the AChE inhibition to the predominant occurrence of sesquiterpenes, especially germacrene-D and \(\alpha\)- and \(\beta\)-caryophyllene, in extracts, and the synergistic potential of sesquiterpenes [39]. In fact, although the bio-inhibitory activity of an essential oil could often be explained in terms of the individual effects of some main constituents, other reports have shown that the anti-AChE activity of essential oils is strongly dependent on the interaction of different terpenoid contents, which may include synergy between monoterpenoids and antagonistic relationships between monoterpenoids and sesquiterpenes. For example, limonene can act synergistically with other terpen to promote terpene absorption by facilitating trans-cellular membrane movement [39]. Recognizing the importance of BuChE in the later stages of AD, interest for BuChE inhibitory activity has recently intensified, although the studies are not as numerous as for the AChE inhibition. It should be noted that 55% of amino acid sequences are identical in the two enzymes; however, 6 of 14 aromatic amino acids contained in the AChE active site are replaced with aliphatic ones in the BuChE active site. Consequently, the BuChE active site is larger in volume than AChE, so that the two ChE enzymes have similar, however distinct, catalytic properties [39]. Monoterpenes \(\alpha\)-pinene, sabinene, and limonene showed relatively good inhibitory activity against equine serum BuChE [39].