Lecithins are fat-like bodies belonging to the group of phosphatides. They all consist of glyceryl esters containing two fatty acid radicals and the phosphoric acid radical in which one of the residual hydrogens is replaced by the choline group. The fatty acid may be palmitic, oleic or stearic and various combinations are known to exist; for example, distearyl lecithin, stearyl palmityl lecithin and so on. The commercial lecithins usually include the closely related kephalins.
On saponification the lecithins split more or less readily into choline, the fatty acids and glycerophosphoric acid, and by fusion with alkali nitrate and carbonate they yield alkali phosphate. They occur, free or in combination as lecithoproteins, most abundantly in certain animal tissues, but there are also vegetable lecithins. The lecithins of commerce are obtained usually from yolks of eggs or from calves’ or sheep’s brains.
Numerous processes have been devised for the preparation of lecithin from egg-yolk or animal tissue. From egg-yolk it may be obtained by making an alcoholic extract and precipitating by cadmium chloride. The precipitate is washed with alcohol and ether, mixed with 80 per cent. alcohol and warmed with the proper amount of ammonium carbonate to remove the cadmium. After filtering hot and concentrating the filtrate the lecithin is thrown down by cooling to a low temperature—10 C. or below. The precipitate is taken up in chloroform and reprecipitated by acetone.
From tissues it is obtained by extracting with warm alcohol and ether, concentrating the extract, precipitating with acetone and repeating the operations.
Pure lecithin is white, but the commercial preparations are yellowish-brown wax-like solids, which are not soluble in water but form milky emulsions which exhibit the myeline figures under the microscope. The solubility in cold alcohol or ether is slight, but heat aids it. Lecithins are not soluble in acetone. They are hygroscopic and the water mixtures undergo decomposition on standing. They darken on exposure to air and light.
The alcoholic solution is precipitated by platinum or cadmium chloride. It is decomposed by alkalies with the formation of choline and trimethylamine. The ash contains phosphoric acid. The different lecithins contain from 3.84 to 4.12 per cent. of phosphorus and 1.73 to 1.86 per cent. of nitrogen. The ratio of nitrogen to phosphorus should be at 1 to 2.21.
Lecithin is incompatible with alkalies; it should be kept in well-stoppered bottles and should be protected from the light.
The content of lecithin (plus kephalin) in tissues is about as follows:
| Per Cent. | ||
Egg-yolk | 8 to | 12 |
Egg-white | 0.1 to | 0.2 |
Liver | 2.0 to | 3.0 |
Kidney | 2.0 to | 3.6 |
Lung | 2.0 to | 3.0 |
Pancreas | 2.0 to | 3.0 |
Actions and Uses.—The lecithin preparations have been recommended in many pathologic conditions, especially in malnutrition and sexual debility. Moderate doses are said to bring about a marked retention of nitrogen and phosphorus, but satisfactory proof of this is lacking. It is extremely unlikely that the small doses which have been recommended in pill or tablet form or in emulsions can have any perceptible action, in view of the fact that many of our natural foods contain much greater weights of available lecithins than the medicinal doses provide. There is no good basis for the statement that the free lecithin has a greater food value or is more readily assimilated than is the substance as found in eggs or tissue. The reverse proposition is much more likely to be true, especially when it is considered that the commercial preparations are usually somewhat altered or decomposed in the process of separation.