CHAPTER IV. 



CHOLINE AND ALLIED SUBSTANCES. 



THE previous chapters have dealt with basic substances derived from 

 the amino-acid units of proteins by various modifications. We must 

 next consider two bases which enter into the composition of the phos- 

 phatides ; they are units or " Bausteine " of these compounds, and are 

 analogous to the amino-acids (described in Plimmer's " Chemical Con- 

 stitution of the Proteins "). One of these units, choline, is apparently 

 present (in a combined form) in every living cell ; the other, amino- 

 ethyl alcohol, is probably the precursor of choline. 



Allied to choline there are two bases, neurine and muscarine, which 

 are derived from choline by dehydration and probably by esterifkation 

 respectively. These bases do not enter into the composition of phos- 

 phatides ; their physiological behaviour is different from that of 

 choline ; they are modified units and are therefore comparable to the 

 modified amino-acids with which we have been concerned so far. 



In this chapter are also included two other bases with pentavalent 

 nitrogen and without a carboxyl-group ; they are trimethylamine 

 oxide and neosine ; the latter is perhaps a homologue of choline. 



Betaine is generally grouped with choline on account of a more or 

 less accidental chemical connection, for it can be obtained in the 

 laboratory by oxidising choline. There is, however, a considerable 

 physiological difference between the two substances, for choline is a 

 structural unit of phosphatides, but betaine plays no such part either 

 in the phosphatide or in the protein molecule. Nor is a genetic 

 relationship between the two substances apparent in the organism. 

 It has been suggested that betaine is formed by the oxidation of choline, 

 but recent work has made the conclusion almost inevitable that 

 betaine is not formed in this way, but by the methylation of glycine 

 (glycocoll), like the other betaines described in Chapter III. Choline 

 and the substances derived from it further differ from the betaines in 

 being strong bases, having a marked physiological action. To em- 

 phasise all these points of difference the two groups of substances are 

 described in separate chapters. 



53 



