SAPONIFIABLE LIPIDS 77 



No extensive surveys have been made to indicate how widespread the phosphatidic acids 

 are in plants. They have been isolated from spinach and cabbage leaves but may have 

 been artifacts of the isolation procedure. Reports of the occurrences of phosphatidic 

 acids in plants must be interpreted with caution since active hydrolytic enzymes are often 

 present which would break down other phosphatides to phosphatidic acids when the tissue 

 is disintegrated. The phosphatidic acids are oily liquids soluble in the usual fat solvents. 

 Since they contain a large proportion of unsaturated fatty acids, they are readily oxidized 

 in the air forming hard products which are insoluble in organic solvents. The barium and 

 calcium salts are insoluble in water and alcohol but soluble in ether. 



The best-known of all the phospholipids are lecithin and cephalin. These compounds 

 include the phosphatidic acid structure but in addition contain a nitrogenous compound 

 linked as an ester with the phosphate. In lecithin the nitrogen moiety is choline; in cepha- 

 lin it is either ethanolamine or serine. 



phosphatidylethanolamine phosphatidylserine 



cephalins 



Apparently at least one of the fatty acids in these compounds is always unsaturated. Lec- 

 ithin and cephalin are colorless, waxy solids which rapidly oxidize and darken on exposure 

 to light and air. They are soluble in the usual fat solvents with certain exceptions which 

 have been used in separating them from each other. These solubilities may be summa- 

 rized as follows: 



