THE CARBOHYDKATES AND THEIR METABOLISM 269 



We have every reason to assume that all the lower fatty acids found in 

 milk are intermediary in the building up of the higher fatty acids. If 

 fatty acids were built up by the addition of one carbon we should .find 

 just as many odd carbon fatty acids as even. This consideration led 

 Xencki as far back as 1878 to suggest that fatty acids are built up by con- 

 secutive additions of two carbons, and that the two carbon compound is 

 probably. ajcetaldehyde which displays exceptional chemical reactivity. 



Support for this assumption may be found in the fact that in their 

 catabolism fatty acids undergo a series of P-oxidation, whereby they lose 

 two carbons in successive stages (Knoop (1910, 6), Ringer (1913, a). In 

 vitro, acetaldehyde will under certain conditions undergo what is known as 

 aldol condensation, whereby one acetaldehyde molecule combines with 

 another, forming aldol, which is a four carbon aldehyde. Raper (1907) 

 has succeeded in building up an eight carbon aldehyde in this way, which 

 he also easily oxidized to caprylic acid. 



CHO 



Acetaldehyde 



Smedley and Lubrynzka (1913) bring forth evidence that fat formation 

 in the body proceeds through the condensation of an acetaldehyde molecule 

 with that of pyruvic acid, forming first a four carbon aldehyde which 

 later combines with another pyruvic acid molecule, giving rise to a six 

 carbon aldehyde. The process thus repeats itself until the sixteen and 

 eighteen carbon fatty acids are reached. 



CH 3 CH 3 



I I I 



CHO + CH CH 



Acetaldehyde CH 3 Splitting off CH Decarboxilation CH 



I ofH 2 I | 



CO CO CHO 



I I 



COOH COOH 



Pyruvic Acid cc-Keto-angelic Crotonic 



Acid Aldehyde 



