Formation of Fatty Acids 271 



In spite of the similarity of the results on fatty acid syn- 

 thesis from acetate and glucose, carbohydrate appears to be 

 a better source of "fat" since it also acts as a direct precursor 

 of glycerol. If the specific activity of the lactose can be taken 

 as the specific activity of the hexose available in the mammary 

 gland as substrate, then the specific activity found for glycerol 

 indicates that in six hours 75-100 per cent of the glycerol, 

 and about 25 per cent of the volatile acids in the milk are 

 newly formed from glucose in the mammary gland. 



From the results described the main conclusions which 

 might have a bearing on the general problem of biosynthesis 

 of fatty acids are: — 



(1) The synthesis of fatty acids proceeds by the stepwise 

 elongation of fatty acid chains at the carboxyl end by the 

 addition of a Cg unit; 



(2) Synthesis of fatty acids and cholesterol from glucose 

 occurs probably after the carbohydrate has been broken 

 down to Cg units (possibly via pyruvate, Krebs and Johnson, 

 1937); 



(3) Carbohydrate, however, is a better source of "fat" 

 than acetate because it also acts as a direct source of glycerol; 



(4) The participation of a C4 compound in the formation 

 of butyric acid in the milk of the goat has probably a signifi- 

 cance only in the metabolism of lactating ruminants (for 

 discussion of this point cf. Popjak, French, Hunter and 

 Martin, 1951). 



It is possible on the basis of these results to suggest the 

 main features of the biosynthetic process of fatty acids, and 

 to consider whether this is peculiar to the mammary gland or 

 whether it is a general process in mammalian tissues. The 

 important question is, if body fat and milk fat are synthesized 

 from the same precursor (s), why do the products differ in 

 composition? 



Acetate is known to be a building-stone available for all 

 positions of the fatty acid chain (see Bloch, 1947, 1948). The 

 interconversion of palmitic and stearic acids by the addition 

 or removal of C2 units was demonstrated by the experiments 



