1. Indicate if the following statements are TRUE (T) or FALSE (F) about acetylcholine (ACh) and cholinergic neurotransmission.

a. (FALSE) AChE [CAT]is the enzyme that catalyzes the condensation of choline and acetate in the biosynthesis of Ach.

b. (FALSE) Muscarinic ACh receptors are G-protein linked sites involving the generation of the second messenger cyclic AMP [DAG and IP₃] through the catalytic action of adenylyl cyclase [phospholipase C].

c. (TRUE) ACh is a chemically reactive molecule that readily undergoes hydrolysis to choline and acetate in aqueous solution.

d. (FALSE) The plasma t½ of ACh is relatively short due to its rapid catabolism by the enzyme choline acEtyltransferase (CAT) [esterases].

e. (FALSE) ACh activation of muscarinic AChRs results in an increase in intracellular levels of inositol triphosphate which, in turn, triggers the activation of the enzyme phosphokinase C [protein kinase C].

f. (FALSE) Activation of niotinic AChRs results in increased efflux of Ca⁺⁺ [Na⁺ and Cl^-] through receptor-operated membrane channels.

g. (FALSE) Muscarinic AChR selectivity of ACh can be increased by addition of an alpha [ß]-CH₃ structural feature to choline esters.

h. (TRUE) A carbamate derivative of ACh, e.g. H₂NCOOCH₂CH₂N(CH₃)₃⁺, would be expected to possess a longer plasma t½ than ACh.

i. (TRUE) Direct-acting cholinomimetics possess miotic activity when administered intraocularly.

j. (FALSE) Diacylglycerol is a second-messenger of ACh in cells innervated by MAChRs and is involved in triggering the release of Ca⁺⁺ [activating protein kinase C].

2. The following questions refer to to the graphical depiction of the pH dependency of AChE inhibition by physostigmine and neostigmine (Figure 2).

a. Which drug, physostigmine or neostigmine, shows a dependency on pH for its inhibitory activity? Explain why?

Because of the need to possess a cationic N for effective interaction with the anionic site of AChE, physostigmine, a tertiary amine, will exhibit pH dependency in its inhibitory effects on AChE, i.e, greatest inhibitory activity at acidic pH where it is largely cationized.

b Which AChEI, physostigmine or neostigmine, shows no dependence on pH for its inhibitiory activity (line B)? Explain why?

Neostigmine, a quaternary ammonium salt, will be fully cationized regardless of pH and will therefore be able to effectively bind to AChE regardless of the pH of the medium.

c. Which drug, neostigmine or physostigmine, would be preferred in treating Alzheimer's disease? Explain why.

Because it is a tertiary amine and will better cross the blood-brain barrier, physostigmine would be preferred in the treatment of the CNS disorder, Alzheimer's Disease, compared to neostigmine which will NOT be distributed well to the CNS.

Explain why the analogue of neostigmine shown on the structure page (STRUCTURE 2d) is a much less potent AchEI.

The analogue structure, having a 1,4-relationship between the key carbamylating and quaternary ammonium functions, will not "fit" well at the active site of AChE and will therefore produce only weak inhibition.

3. Complete the following reaction by showing the structure of the complex formed between edrophonium and the active site of AChE.

a. Would you expect the interaction between edrophonium and AChE to be reversible or irreversible? Explain your answer.

As shown, edrophonium establishes bioreversible H-bonds and ionic bonds with the active site groups of AChE as opposed to highly stable covalent bonds. Therefore this constitutes a reversible (competitive) interaction with the enzyme.

b. Would you expect edrophonium to be a substrate or an inhibitor of AChE? Explain your answer.

Edrophonium has no functional groups subject to catalytic hydrolysis by AChE, i.e., this structure CANNOT function as a substrate but, rather, as an inhibitor.

---

Return to: PY420 Home Page