FAT METABOLISM 689 



of a somewhat more complex structure namely, the phospholipins. As 

 their name indicates, these consist chemically of phosphoric acid combined 

 with neutral fat and with a nitrogenous base, cholin. The best known of 

 the phospholipins is lecithin, which is widely distributed in the animal body 

 (present in blood and bile as well as in all cells). Other phospholipins 

 present in nervous tissue are cephalin, cuorin and sphingomyelin. There 

 are various lecithins distinguished from one another by the fatty-acid 

 radicles which they contain. Distearyl-lecithin, for example, has the 

 formula: 



CH 2 -0-OC(CH 2 ) 1I -CH 3 



CH -0-OC(CH 2 ) 14 -CH 3 

 (stearic acid) 

 CH 2 -0 O 



(glycerol) P 



OH OCH 2 -CH 2 -N(CH 3 ) 2 



(phosphoric 



acid) OH 



(choline) 



This complex molecule can readily be split up by hydrolysis (warming 

 with baryta water) into: 



glycero-phosphoric acid, CH 2 -OH 

 CH -OH 



C 2 H 4 OH 



CH, - O O 



\ / 



; choline, N 



(CH 3 ) 3 (oxy-ethyl-ammonium 



OH OH OH 



hydroxide) ; and fatty acids. 



With hydrochloric acid, choline forms a salt which readily forms a 

 double salt with platinic chloride. Since this double salt forms charac- 

 teristic crystals, it is used to identify and separate lecithins. For quan- 

 titative purposes, however, it is more suitable to determine lecithin in- 

 directly by the amount of phosphoric acid present in an ethereal ex- 

 tract of the organ or tissue. 



Evidence is constantly accumulating to show that lecithin is an ex- 

 tremely important constituent of cells; indeed, it seems to be the inter- 

 mediate stage in the utilization of neutral fats by protoplasm. Its phos- 

 phorus also probably serves as the source of this element for the con- 

 struction of nucleic acid (see page 637). In nervous tissues it is often 

 associated with carbohydrate molecules (galactose), forming the sub- 

 stance known as cerebrin. It may therefore have some role to play in 

 carbohvdrate metabolism. Some workers also attribute to lecithin an 



