274 G. PopjAK 



over the control figure. We think that an elongation of the 

 shorter acids occurred because the process of secretion into 

 milk did not interfere with the reactions. 



Stadtman, Stadtman and Barker (1949) have shown by 

 the use of isotopic carbon that Clostridium kluyveri synthesizes 

 caproic acid by the addition of a C2 unit, derived from ethanol, 

 to the carboxyl carbon of butyric acid. Stadtman and 

 Barker (1949a, b, c, d, e) have studied the reactions involved 

 in this synthesis in considerable detail by means of a cell-free 

 enzyme system. The anaerobic synthesis of fatty acids by 

 this organism is essentially an energy-yielding mechanism, 

 and it is not clear at present how far it is comparable to fat 

 synthesis in animals. It is quite probable, however, that 

 many of the intermediate steps are similar. The view, that 

 the short-chain fatty acids are intermediates in the synthesis 

 of the longer ones in the mammary gland, is not a new one 

 and has been expressed earlier by Folley and French (1948, 

 1950) on the basis of their studies on the in vitro utilization 

 of specific substrates by mammary gland slices. 



Little further need be said in considering the results 

 obtained with labelled carbohydrate, except that it does not 

 seem unlikely that the availability of glycerol from the meta- 

 bolism of glucose is one of the limiting factors in fatty acid 

 synthesis. 



The enhancing effect of carbohydrate on the in vitro 

 synthesis of fatty acids from acetate described by Bloch and 

 Kramer (1948) and by Folley and French (1949) may be due 

 to glycerol formation (see also Folley and French, 1950). 

 Balmain and Folley (1951) have actually demonstrated a 

 stimulating effect of glycerol on the in vitro synthesis of fat 

 by mammary gland slices, as judged by respiratory quotients 

 increased above those observed with glucose and/or acetate 

 as the sole substrates. 



REFERENCES 



ACHAYA, K. T., and Hilditch, T. P. (1950). Proc. roy, Soc.y B., 137» 



187. 

 Balmain, J. H., and Folley, S. J. (1951). Biochem. J., 48, i. 



