428 V. CHEMISTRY OF PHOSPHATIDES AND CEREBROSIDES 



has been treated by AUes^^^ and by Gaddum.^'^^ The importance of 

 choline is also emphasized by its inclusion as a component of the vitamin 

 B complex. Another important biochemical function is undoubtedly 

 related to its ability to act as a transmethylating agent. 



Another role which choline plays and which may be entirely distinct 

 from the other funclions is as an essential component of the important 

 phospholipid lecithin. 



One important compound of choline which occurs only in animal tissues 

 is acetylcholine. Baeyer^^^ first synthesized this compound from choline 

 chloride ("neurine" chloride) as indicated here. 



.0 /CH, O yCH, 



CH,C-CH-HOCH,CH.N-CH, CH,C-0CH,CH,N-CH,4-H,0 



OH^'^3 OH ^^5 



Acetyl Choline Acetyl- 



chloride chloride choline 



Although it was assumed at an early date that acetylcholine or similar 

 choline esters probably exist in animal tissues, acetylcholine was not isolated 

 from them in pure form until 1929, when Dale and Dudley^" succeeded 

 in this undertaking. The fact that acetylcholine is able to form a chloro- 

 platinate was given as proof by Baeyer that choline also contains an 

 alcoholic hydroxyl group, a suggestion which Wurtz had established earlier. 



Loewi^^* had discovered earlier that this same substance is liberated in 

 heart muscle when the vagus nerve is stimulated. It was later demon- 

 strated that a similar reaction occurs at the nerve endings of the parasym- 

 pathetic nerves when a voluntary muscle is stimulated. This liberated 

 substance has been found to be acetylcholine. The acetylcholine so pro- 

 duced occurs as a result of the activity of an enzyme, choline acetylase, while 

 another enzyme, choline esterase, controls the concentration of acetylcholine 

 by causing the hydrolysis of any excess. ^^* 



Choline was first prepared from hops as a crystalline product by Griess 

 and Harrow, ^^° who noted that it was a colorless, odorless compound with an 

 extremely bitter, caustic taste. One of the most outstanding properties of 

 choline is its great tendency to absorb water. In spite of such hydroscop- 

 icity, Meyer and HopfT^^^ were able to prepare crystals satisfactorily from 

 saturated aqueous solutions in a high-vacuum oven over phosphorus pent- 

 oxide. 



1" G. A. Alles, Physiol. Revs., U, 276-307 (1934). 

 "5 J. H. Gaddum, Ann. Rev. Biochem., 4, 311-330 (1935). 

 "6 A. Baeyer, Ann., 142, 322-326 (1867). 



1" H. H. bale and H. W. Dudley, /. Physiol, 68, 97-131 (1929). 

 »8 O. Loewi, Arch. ges. physiol. (Pfluger's), 189, 239-242 (1921); Chem. AbsL, 16, 1267 

 (1922). 



«9 G. L. Brown, Physiol. Revs., 17, 485-513 (1937). 



"0 P. Griess and G. Harrow, Ber., 18, 717-719 (1885). 



i»i K. H. Meyer and H. Hopff, Ber., 54, 2274-2282 (1921). 



