Formation of Fatty Acids 273 



matter of conjecture so far as the chemical form of the 

 reactants is concerned, but it can be concluded from the 

 results that the carbon chains of the volatile fatty acids form 

 the carbon skeleton of the intermediates for the synthesis of 

 the long-chain acids. The glyceride fatty acids of the milk 

 are perhaps best regarded as metabolic products which have 

 become stabilized by esterification with glycerol and secretion 

 into the milk ducts. The process of secretion might be regard- 

 ed as one which intervenes with great rapidity between the 

 formation of these acids and their further metabolism in the 

 cell, so as to trap in the milk-glycerides substances which 

 would otherwise be metabolized so rapidly as never to accum- 

 ulate to any great extent (Popjak, French, Hunter and 

 Martin, 1951). According to this argument then, the differ- 

 ence between milk and body fat formation might be more 

 apparent than real; the process of rapid secretion by the 

 mammary gland having only the effect of "preserving" the 

 intermediary products of synthesis. 



The above argument seems to be supported by the results 

 of an experiment which Dr. French and I carried out recently. 

 We have incubated, in Krebs-Henseleit solution for three 

 hours in the presence of glucose (0-3 per cent) and acetate 

 (0-02M.), washed slices of mammary gland from a lactating 

 rabbit, and measured the amount of short-chain fatty acids 

 before and after incubation. Table IV shows that while 

 there was no significant drop in the total titration value of 

 the short-chain acids, there was a significant decrease in the 

 amount of C4-C8 acids and an increase in the Cg-Cig acids 



Table IV 



Distribution of Volatile Fatty Acids- in Glyceride Fatty Acids of 

 Rabbit Mammary Gland Slices (mEq./g. Total Fatty Acid) before and 

 AFTER Incubation in Bicarbonate Buffer with Glucose and Acetate 



ISOTOPES 19 



