Formation of Fatty Acids 259 



Dr. A. J. P. Martin, F.R.S., of the National Institute for 

 Medical Research. Most of the work to be described has been 

 published in the Biochemical Journal. References to the 

 individual papers will be found in the text and in the biblio- 

 graphy at the end of this article. 



The experiments have shown that the mammary gland is 

 singularly suited for the investigation of biosynthetic pro- 

 cesses. It is not intended, however, to present the results 

 from the particular point of view of the metabolism of the 

 mammary gland, but to discuss their bearing on the more 

 general problem of the biosynthesis of fatty acids. Neverthe- 

 less, reference has to be made to certain salient features of 

 mammary gland secretion reviewed recently by Folley (1949). 



Milk is a remarkable biological product in that it provides 

 all the nutrients necessary for the growing young. In essence 

 it is a store of energy. A lactating rabbit may provide in 

 3-4 weeks an amount of milk equivalent to her own body 

 weight! We are convinced now that this remarkable output 

 of stored energy in milk is the result primarily of the metabolic 

 activities of the mammary gland cells. 



So far we have studied mainly the synthesis of milk fat 

 which, of all animal fats, is the only one that contains short- 

 chain fatty acids. The milk of ruminants is particularly 

 rich in these acids, containing every even numbered member 

 of the saturated series from butyric to stearic acid. The 

 origin of milk fat and particularly that of the short-chain 

 acids has been the subject of divergent opinions (for review 

 see Folley, 1949). 



Graham, Jones and Kay (1936) concluded that the amount 

 of glyceride fat which disappeared from the blood during 

 passage through the udder of lafctating cows was sufficient 

 to account for the production of milk fat. Voris, Ellis and 

 Maynard (1940), using a more specific method for the estima- 

 tion of blood glycerides, confirmed the uptake of blood fat 

 by the udder of lactating cows. On the other hand, the 

 finding of a high respiratory quotient for the active udder 

 in vivo led to the suggestion that part of the milk fat might 



