STRUCTURE OF THE CEREBROSIDES 475 



arc insoluble in water, as well as in diethyl ether and petroleum ether. 

 They are readily soluble in hot alcohol, but separate from it on cooling. 

 Pyridine is an excellent solvent when cold as well as when heated. Cere- 

 brosides can be precipitated from alcoholic or pyridine solutions by the 

 addition of paraldehyde.^^ They are optically active. 



These compounds have been referred to in the literature under a number 

 of designations, such as cerebrosides (Thudichum), galactolipins (Leathes 

 and MacLean), galactosides (Rosenheim), and sometimes glycolipids. 

 Since glucose-containing cerebrosides as well as galactose-containing cere- 

 brosides are kno^vn, it has been suggested that the term "galactolipid" be 

 reserved for those which contain galactose, while "glycolipid" be used to 

 designate only those members of the group which contain glucose. ^^^ 



2. Structure of the Cerebrosides 



The cerebrosides are believed to have a uniform structural pattern in 

 which the variations in the individual members involve only the substitu- 

 tion of different fatty acid molecules or the replacement of galactose by 

 glucose. When cerebrosides are hydrolyzed, sphingosine, one molecule of 

 sugar (c?(+)-galactose or c?( + )-glucose), and one fatty acid are set free. 

 The fact that the amino group of sphingosine is in combination, and that 

 the carboxyl grouping of the fatty acid cannot react, leads to the belief 

 that these components are joined together in a NH — CO linkage similar to 

 that which exists in the sphingomyelins. This is further confirmed by the 

 fact that the galactose can be split off, leaving a sphingosine fatty acid 

 compound or ceramide of which lignocerylsphingosine is an excellent ex- 

 ample. The galactose (or glucose) is combined with the ceramide fraction 

 through the aldehyde group of the sugar, since the cerebroside is a non- 

 reducing compound. That such a combination between the carbohydrate 

 and the ceramide occurs with the sphingosine rather than with the fatty 

 acid is indicated by the fact that a non-reducing derivative, psychosine, has 

 been prepared which consists of one molecule of sphingosine and one of 

 galactose, but from which the fatty acid residue is absent. In contra- 

 distinction to the cerebrosides, or in fact to the ceramides, the amino group 

 of psychosine is free; this constitutes further evidence that the union of 

 fatty acid to the sphingosine is through the amino group. The combination 

 of the carbohydrate with sphingosine must be through one of the hydroxyls 

 of the latter molecule. It is not loiown whether such a union occurs with 

 the hydroxyl on Ci or on C3. 



Four different cerebrosides of the galactolipid type are kno\\ii. These 

 consist of the following: 



Cerasine (referred to also as kerasine), which was discovered by Thudi- 

 chum^ in 1874, and which contains lignoceric acid (CH3(CH2)22COOH) as 

 the component acid. 



