# Terpenes as Potential Anti-Alzheimer's Disease Agents
## 2 Activities of Terpenes on the Mechanisms Associated with AD
### Inhibition of Acetylcholinesterase (AChE)
#### 2.4.4. Triterpenenes  
Three limonoids, extracted from the seeds of _T. velwitschia_, were identified as trichilia lactone D5 (**68**), rohituka 3 (**69**), and dreeganin DM4 (**70**). The IC\({}_{50}\) values for the inhibition of AChE were 19.13, 34.15, and 45.69 \(\upmu\)M, respectively. Thus, compound **70** has a moderate AChE and the others are even weaker inhibitors [114; 115]. As already mentioned above (Section 2.12), twelve sernate-type terpen (compounds **32**-**43**) were isolated from _L. carnu_, of Vietnam [76]. While 38,21\(\alpha\)-diacetoxyeratan-146-ol (**37**), 38,21\(\beta\)-trihydroxyerat-14-en-38-yl_p_-dihydroocumarate (**34**), 38,14\(\alpha\),15\(\alpha\),21\(\beta\)-tetrahydroxyeratan-24-oic acid-38-yl_(40-methoxy-50-hydroxybenzoate) (**33**), and 21\(\beta\)-hydroxyerat-14-en-3,16-dione (**36**) are active as inhibitors of AChE with IC\({}_{50}\) values of 0.91, 1.69, 9.98, and 10.67 \(\upmu\)M, respectively, all the other compounds were inactive (IC\({}_{50}\) > 30 \(\upmu\)M) [114]. Several other serrate-type triterpenes were extracted from _L. casaurinoides_, collected in China, identified as serrat-14-en-3\(\beta\),21\(\alpha\)-diol (**144**), serrat-14-en-3\(\beta\),21\(\beta\)-diol (**145**), \(\alpha\)-onocerin (**146**), 26-nor-8-oxo-\(\alpha\)-onocerin (**147**), and 26-nor-8-oxo-21-one-\(\alpha\)-onocerin (**148**). While compound **148** exhibits activity as an inhibitor of AChE with an IC\({}_{50}\) value of 1.01 \(\upmu\)M, all the other compounds are inactive (IC\({}_{50}\) > 50 \(\upmu\)M) [127]. From the fruits of _Citrallus colocynthia_ L. collected at Xinjiang, People's Republic of China, several cucurbitane-type triterpenes were extracted. The compounds were identified as colocynthenin A (**149**), colocynthenin B (**150**), colocynthenin C (**151**), colocynthenin D (**152**), cucurbitacin E (**153**), \(6^{\prime}\)-acetyl-2-O-\(\beta\)-\(D\)-glucocucurbitacin E (**154**), arvenin I (**155**), arvenin II (**156**), cucurbitacin B (**157**), 23,24-dihydroucurbitacin B (**158**), and colocynthoside A (**159**). Colocynthenin A (**149**) and colocynthenin C (**151**) are significant inhibitors of AChE, with IC\({}_{50}\) values of 2.6 and 3.1 \(\upmu\)M, respectively. The other compounds are inactive against AChE (IC\({}_{50}\) > 10 \(\upmu\)M) [114; 131]. Forty-three lanostane-type triterpenes (**160-202**) were extracted from the fruiting bodies of _Ganoderma lucidum_ ([PERSON] [PERSON] existing in China and Japan [114,132]. Considering the compounds that have conjugated double bonds on the side chain linked to C17 between C24 and C25 and the carboxylic group, such as ganolucidic acid E (**160**), 118-hydroxy-3,7-dioxo-5\(\alpha\)-lanosta-8,24(E)-dien-26-oic acid (**161**), 235-hydroxy-3,7,11,15-tetraoxo-lanost-8,24E-diene-26-oic acid (**162**), and ganoradic acid X1 (**163**), the IC\({}_{50}\) values for the AChE inhibition were 13.8, 10.8, \(>\)200, and \(>\)200 uM, respectively, suggesting that when rings B and C have a conjugated system between them comprising C7, C8, C9, and C11, and a carbonyl group at C15, such as on compounds **162** and **163**, the scaffold is too rigid and planar on that part of the molecule, and thus does not inhibit AChE.