THE CEPHALINS 449 



amino acids having a free amino and a free earboxyl group will respond to 

 the above reaction; ethanolamine, either alone or in the presence of oleic 

 acid, gave entirely negative results. Since Folch and Schneider^^^ carried 

 out these tests on unhydrolyzed cephalin, it is evident that the amino acid 

 must be combined in the cephalin molecule in such a way that both the 

 amino and the earboxyl groups are unattached. The amino acid could still be 

 quantitatively demonstrated in the water-soluble fraction after hydrolysis 

 by the use of the nhihydrin test. Further proof of the amino acid nature 

 of the product is afforded by the fact that, accompanying the liberation of 

 carbon dioxide, an equimolecular amount of ammonia is also released. The 

 liberation of both carbon dioxide and ammonia with ninhydrin is a reaction 

 characteristic only of the a-amino acids. 



The amino acid component was believed to be serine, on the basis of its 

 reaction with periodate.^^* The finding of glycolic aldehyde as an end prod- 

 uct of the ninhydrin reaction also adds weight to this hypothesis. The hy- 

 droxyl group on serine would afford a means for esterification with phos- 

 phoric acid while leaving both the amino and the earboxyl group free on the 

 unhydrolyzed molecule. The amino acid is not an adsorbed impurity, but 

 is actuallj^ an integral part of the cephalin molecule; this is indicated by the 

 large quantity which is present. Moreover, in the preparation of cephalin, 

 it flocculated from solution in such a way as to preclude the adsorption of wa- 

 ter-soluble impurities. The final proof of the identity of the acid was ob- 

 tained by the preparation of pure phosphatidylserine^^* and the identifica- 

 tion of the amino acid component as L-serine. The purified phosphatidyl- 

 serine contained 97% of its nitrogen in the form of amino nitrogen; on hy- 

 drolysis, 38% of the theoretical amount of serine was prepared from it in 

 crystalline form. Schuwirth'-^^'^°'' demonstrated the presence of serine in 

 human brain phosphatides following its extraction with butyl alcohol from 

 the barium hydroxide hydrolysate, as well as by the preparation of the (3- 

 naphthalene sulfonic acid derivative. The methods for the determination 

 of phosphatidylserine which are based upon the determination of serine are 

 discussed in the section on phosphatidylethanolamine. 



Folch^'^i was able to prepare phosphatidylserine in 92 to 97% purity from 

 human brain. Stearic and oleic acids were the chief fatty acids present. 

 On hydrolysis, glycerophosphoric acid, L-serine, and fatty acids were sepa- 

 rated in molecular ratios of 1:1:2. Phosphatidylserine was shown to com- 

 prise 60% of the nitrogenous lipid earboxyl in the brain. 



There are, in all probabilit.y, two types of phosphatidylserine, namely, a 

 and 13, depending upon the carbon atom of glycerol with which phosphoric 



»« J. Folch, /. Biol. Chem., 139, 973-974 (1941). 

 299 K. Schuwirth, Z. physiol Chem., 270, I-III (1941). 

 3"o K. S(;liu\virth, Z. plnj.siol. Chem., 277, 87-96 (1942). 

 3«i J. Folcli, ./. Biol. Chem., 174, 4139-450 (1948). 



