270 



G. POPJAK 



Table III shows that acetate is used for the synthesis of milk 

 fatty acids just as in the ruminant, the volatile acids having 

 the highest specific activity. It should, however, be pointed 

 out that in rabbit's milk the amount of butyric and caproic 

 acids is much less than in the goat, most (about 80 per cent) 

 of the water-soluble steam-volatile fatty acid fraction con- 

 sisting of octanoic acid. As far as the fatty acids and 



Table III 



Specific Activities of Carbon in Milk Constituents (Counts/Min. in o© 

 Thickness) from Rabbits after Administration of [Carboxy-l-^*C] Acetate 



OR i*C Carbohydrate 



i 



4 



•Milk collected during 12 hours; all others 6 hours. 



cholesterol are concerned the similarity between the results 

 obtained after the administration of carbohydrate and acetate 

 is striking. It is inferred that glucose also must yield C2 units, 

 which are indistinguishable from those derived from acetate. 

 Bloch and Rittenberg (1944) have concluded that only those 

 compounds which give rise to acetate (or Cg units) are used 

 for cholesterol synthesis. The only exception to this rule 

 seems to be i^o-valeric acid, the t^o-propyl part of which, 

 according to Zabin and Bloch (1950), is a better source of 

 carbon for cholesterol synthesis than acetate. 



I 



