uct
a chemical formula for lecithin, glycerophosphoric acid combined with a
fatty acid and with choline (a hydrate of neurine).

{ OH }
N { (CH_3)_3 } Choline
{ C_2H_4O }

C_18H_33O_2 } HO }
} } PO
C_16H_31O_2 } C_3H_5O }

Fatty Acids Glycerophosphate
\--------v-------/
Lecithin
according to Strecker

This formula was not quite correct. Richard Willstaetter showed that an
internal neutralization takes place between the amino group and the free
acidic residue. This is expressed in his lecithin formula of 1918.

CH_{2}.O.R
|
CH_{2}.O.R_2
|
| O.CH_{2}.CH_{2}
| / \
CH_{2}.O--P=O N(CH_{3})_{3}
\ /
\---O----/

[Illustration: 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 H_{2}NCH_{2}CH_{2}OH is found
instead of the choline of lecithin. The variations i