Lecithin (1918)
When the aim was to distill elementary phosphorus out of an organic material, it did not matter whether this was fresh or putrified. For obtaining lecithin out of egg yolk and similar materials, it was essential to use it in fresh condition. Otherwise, enzymes would have decomposed it. Through more recent work, four enzymes have been separated, which act specifically in decomposing lecithin. Enzyme A removes one fatty acid and leaves a complex residue, called lysolecithin, intact. Enzyme B attacks this residue and splits off the remaining fatty acid group from it, enzyme C liberates only the choline from lecithin, and enzyme D opens lecithin at the ester bond between glycerol and phosphoric acid. This is shown in the following diagram.
| Enzymatic Splitting of Lecithins | ||
|---|---|---|
| Enzyme | Substrate | Products |
| A | Lecithin | Lysolecithin and fatty acids. |
| B | Lysolecithin | Glycero-phospho-choline and fatty acids. |
| C | Lecithin | Phosphatidic acid and choline. |
| D | Lecithin | Phosphoryl choline and diglyceride. |
Several fatty acids can be present in lecithin from various sources: palmitic and oleic acid, besides the stearic acid which at first had been thought the only one involved. In another group of extracts from brain or nerve tissue, amino-ethanol H2NCH2CH2OH is found instead of the choline of lecithin. The variations include the alcohol, to which the fatty acids and choline phosphate are attached. For example, glycerol can be replaced by the so-called meat-sugar, inositol, which has six hydroxyl groups in its hexagon-shaped molecule C6H6(OH)6.
Figure 14.—Eduard Buchner (1860-1917) received the Nobel Prize in Chemistry for his discovery of cell-free fermentation, the first step in finding the role of phosphate in fermentations (1907).
The generally similar behavior of these phosphate-and fat-containing substances was emphasized by Ludwig Thudichum (1829-1901). He coined the name phosphatides for this group of substances from seeds and nerves.[29] His work on the phosphates in brain substance aroused particular interest. When William Crookes drew his highly imaginative picture of an “evolution” of the chemical elements, he put into it “phosphorus for the brain, salt for the sea, clay for the solid earth….” [30] But phosphatides occur in many places of organisms, in bacteria, in leaves and roots of plants, in fat and tissues of animals. And where phosphatides are found, there are also enzymes that specifically act on them. They are called phosphatases to imply that they split the phosphatides. In addition, enzymes are present, which transfer phosphate groups from one compound to another. They are more abundant in seeds of high fat content than in the more starch-containing seeds, but even potatoes and orange juice have phosphatases. [31]