308 



ANIMAL BIOCHEMISTRY 



lu 



>^ 



o 



Oi 

 O 



Current 

 applied 



3 6 9 12 15 18 

 Time, sec 



FIGURE 13-4. Brain acetylcholine in young rats before and after shock. 



drops to 3 to 5 m/x moles. The loss occurs by a simple hydrolysis to 

 choline and acetic acid and is brought about by choline esterases. 

 Much of the acetylcholine in the brain is in a bound form and is 

 protected from hydrolysis by choline esterase until release by initiation 

 of the stimulus. In this respect the release of acetylcholine parallels 

 that of potassium ions (see above). Acetylcholine is synthesized from 

 acetyl coenzyme A and choline. The acetyl coenzyme A may arise 

 from glucose metabolism or from a synthesis involving coenzyme A, 

 ATP, and free acetate. The balance between synthesis and breakdown 

 is shown in Figure 13-5. 



The ATP required for recycling of acetate in the above synthesis is 



glucose 

 or pyruvate 

 metabolism 



coenzyme A + 



adenosine triphosphate 



+ acetate 



acetylcoenzyme A 



+ 



choline 



acetate | 

 choline J 



choline 



acetylase 



choline 



esterase 



combined 

 acetylcholine 



electrical impulses or 

 high concentration of K 



free acetylcholine 



FIGURE 13-5. Acetylcholine breakdown and synthesis in brain tissue. 



