PHAR 402, Dr. Lu     

Handout # 1                Nicotinic Agents and Anticholinesterases

 

Reading Assignment:

   (1).        Foye, W.O. “Principles of Medicinal Chemistry”, 4th ed., Chapter 17, pp. 327-343.

  (2).        Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 9th ed., McGraw Hill, 1996, New York, pp. 161-176 (Anticholinesterase Agents) & pp. 177-197 (Agents Acting at the Neuromuscular Junction and Autonomic Ganglia).

Additional References:

 (1).     Nogrady, Thomas “Medicinal Chemistry - A Biochemical Approach”, 2nd ed., Oxford University Press, New York, 1988, pp. 141-150.

  (2).        Ember, L.R. “The Nicotine Connection”, Chemical & Engineering News, November 28, 1994, pp. 8-18.

Educational Objectives and Study Guidelines

 A student should be able to:

 1.                 Understand the differences in the mechanism of action and structure-activity relationship of competitive (non-depolarizing) versus depolarizing neuromuscular blocking agents and explain why it is only possible to treat d-tubocurarine overdose but not decamethonium overdose with neostigmine?

2.                    Compare acetylcholinesterase (AChE) and butyrocholinesterase (BuChE) as to anatomical locations and functions and explain biochemically why some anticholinesterase is irreversible.

3.                    Understand the mechanism of reactivation of organophosphate poisoning by pralidoxime and recognize the role of enzyme aging in the enzyme-inhibitor interaction.

4.                    Understand the biochemical basis of selective toxicity of AChE inhibitors.

 

Nicotinic Cholinergic Receptor (nAChR)

                                                                           

·          A schematic representation of the nicotinic cholinergic receptor (nAChR).  The alpha subunits contain the ACh binding sites, shown as rectangles. The protein subunits surround the ion channel. (After Kaine, 1980).

·          Nicotinic receptor can be found in all autonomic ganglia and at the neuromuscular junctions of striated muscle.  Isolation of the nAChR glycoprotein was achieved with the aid of the electric organs of the electric eel and the specific antagonist, a-bungarotoxin (see its peptide structure on handout # 20) of the Indian cobra or the toxin of the Siamese cobra.

·          Nicotine produces a large variety of centrally induced physiological and behavior effects, including alteration of respiration, heart rate and blood pressure, as well as antinociception, suppression of appetite and increase in spontaneous activity.  Most of these effects are thought to result from interaction of nicotine with specific nAChRs (perhaps acting presynaptically to other transmitters to facilitate neurotransmitter release).

 

PHAR 402, Dr. Lu

Handout # 2                                            Nicotinic Cholinergic Receptor

 

            Biosynthesis, storage and release of ACh and their inhibition at the motor end plate (note: pay special attention to the mode of action of triethylcholine (TEC) & role of Ca++ ions).  

                             

            HC-3:  Hemicholinium-3;  TEC:    Triethylcholine;  TE-ACh:  Triethylacetylcholine; CoASH:  Co-enzyme A


PHAR 402, Dr. Lu

Handout # 3       A proposed mechanism attempting to explain the opening and closing of ion channels

 

            Amino Acid Sequence of Siamese Cobra Neurotoxin (a-bungarotoxin):  

 

 

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PHAR 402, Dr. Lu

Handout # 4                            Nicotinic Antagonists

 

Agents that Block ACh at the Ganglionic nAChRs  

                 

 

 Agents that Block ACh at the Neuromuscular Junction (Neuromuscular Blocking Agents)

 

a.         Competitive Neuromuscular Blocking Agents (Non-depolarizing agents)  

                  

b.         Depolarizing Agents (i.e., These drugs act by binding tightly to the ACh binding site on the nAChRs.  Can you see the Ach-like moiety in these molecules?)

 

                    


PHAR 402, Dr. Lu

Handout # 5                            Newer Nicotinic Drugs

Doxacurium Chloride (Nuromaxâ) - An injectible, noncummulative, non-depolarizing NM Blocker with minimal cardiovascular effects.  Duration of action appears to be similar to d-tubocurarine or pancuronium.

                            

 Mivacurium Chloride (Mivacronâ) - Marketed as a mixture of three stereoisomers.  It is a short-acting, non-depolarizing skeletal muscle relaxant introduced as an adjunct to general anesthesia.

                          

Practice problem:

 

            Both d-tubocurarine and decamethonium are potent neuromuscular blocking agents.  Explain why is it only possible to treat d-tubocurarine overdose but not the decamethonium overdose with neostigmine (an anticholinesterase)?


PHAR 402, Dr. Lu

Handout # 6                Drug Acting on Acetylcholinesterase (Anticholinesterase)

 

                                      Mechanism of ACh Hydrolysis by AChE:    

 Practice problem:

 

            If a single mutation substituted alanine [CH3CH(NH2)COOH] for serine at the active site of the AChE, why would cholinergic activity be increased?  Explain with the aid of ACh binding sites on the AChE.

 

PHAR 402, Dr. Lu

Handout # 7                               Mechanism of Action of AChE Inhibitors

 

                 

 

                                                Reversible AChE Inhibitors  

               

Handout # 7 (continued)         

                                    Irreversible AChE Inhibitors (Organophosphates)  

                   

By examing the chemical structures for each of the subclasses above, can you envision a chemical/biochemical reason why Malathion and the phosphoramide-type AChE inhibitors are the least toxic to mammals? (see handout #8)


PHAR 402, Dr. Lu

Handout # 8                Selective Toxicity of AChE Inhibitors

 

1.         All thiophosphate-type (P®S) AChE inhibitors need to be bioactivated by the cytochrome P-450 enzymes to the corresponding phosphate esters (P®O) (i.e., the P®S is not as electrophilic as P®O to allow nucleophilic attack by the Ser-OH). Since insects appear to have more reactive oxidative enzymes, some selectivity is observed for these agents.

                     

 

 2.        Phosphoramide-type AChE inhibitors are inactive due to the absence of a good leaving group within the molecule.  As illustrated below, selective toxicity is realized due to the differences in the metabolic biotransformations between insects and mammals.

                       

Handout # 8 (continued)

 

3.         Malathion is the least toxic organophosphates known to date due to the presence of ester functions in its molecule.  It is generally known that insects have much lower esterases activity than the mammals.

                           

                                 Antidote for Organophosphate Poisoning - Reactivation of AChE