98 



LIPIDES (fats and RELATED SUBSTANCES) 



group is attached to the center, or beta, carbon atom of glycerol. How- 

 ever, it seems very doubtful if such beta lecithins exist in nature. 



Many different lecithins have been found in brain, nerves, liver, pan- 

 creas, heart, blood, and other active tissues of the body. Egg yolk and 

 soybeans are especially rich (2 to 5 per cent) in lecithins and serve as 

 starting materials for commercial preparation. Soybean lecithin, pro- 

 duced in ton quantities, is used as an emulsifying agent in many food 

 products; for example, the addition of 0.2 per cent soybean lecithin to 

 oleomargarine gives a consistency that closely resembles that of butter. 

 To obtain the crude lecithin, the fat extracted from the original source 

 material is treated with acetone. True fats are soluble; lecithins and 

 cephaline, being insoluble, precipitate out. The precipitation can be 

 made more complete by adding magnesium chloride or cadmium chloride, 

 which form sparingly soluble addition compounds with lecithins and 

 cephalins. The cephalins are separated from lecithins by virtue of their 

 lower solubility in alcohol. 



Highly purified lecithins are colorless, greasy, paraffin-like substances. 

 Colorless preparations, however, are seldom seen, since they are difficult 

 to obtain and quickly darken on exposure to air. The outstanding char- 

 acteristic of the lecithins is their high chemical reactivity. They are 

 easily oxidized, easily hydrolyzed, and have a great capacity for com- 

 bining with other substances such as water, salts, proteins, and carbo- 

 hydrates. As indicated above they are also excellent emulsifying agents, 

 probably because their structure includes both the water-attracting salt 

 group and the long carbon chains of the fatty acid residues (compare 

 soaps and detergents, p. 87) . 



Hydrolysis of lecithins, with moderately strong acid or alkali, readily 

 breaks them down into the constituent fatty acids, choline, and a-glycero- 

 phosphoric acid. The attachment between the glycerol and the phos- 

 phoric acid is very strong. It is only broken by long boiling with strong 

 acid. 







CHaOH CH^OC-CsHj, 



I I 



CHOH CHOH 



O 

 II II 



CHjOP-OH CH^O-P-OCHoCHzNCCH,), 



1 I •*■ 

 OH 0- 



a-Glycerophosphoric acid A lysolecithin 



Enzymatic hydrolysis also occurs in living tissues as a result of the ac- 

 tion of lecithinases. One type of lecithinase (type B) splits off' both fatty 

 acid residues. Another (called type A) removes only one, leaving a 

 product, lysolecithin, which is poisonous because of its power to hemolyze 



