Formation of Fatty Acids 269 



activities of the acids with increasing chain length as shown 

 in Fig. 2 are consistent with the view that they are being 

 synthesized by the same mechanism as caproic acid, i.e. 

 by the stepwise elongation of a shorter chain at the carboxyl 

 end (by the addition of a Cg unit) and that the intracellular 

 pools of the acids up to decanoic acid are very small. It 

 seems, however, probable that the pools of the acids from 

 lauric (dodecanoic) upwards are increasingly larger, an 

 assumption which would explain the differences in the specific 

 activities of the acids during the first and second part of the 

 experiment. 



In the introduction reference was made to the work of 

 Folley and French (1950) who found species differences in 

 respect of utilization of glucose and acetate for the in vitro 

 synthesis of fat by lactating mammary gland slices. Accord- 

 ing to their results, tissues of ruminants utilized acetate but 

 not glucose, and the tissues of rodents (rat, rabbit), glucose 

 and not acetate, for fat synthesis, although in the latter case 

 the addition of acetate to glucose in the incubation medium 

 had the effect of raising the respiratory quotients of the slices 

 above those observed with glucose alone. 



We have now carried out a comparison between acetate 

 and glucose as precursors of milk fat in rabbits (French and 

 Popjak, 1951). Rabbits during the third week of lactation 

 were given by stomach tube [carboxy-'^^C] acetate followed by 

 ^*C starch 3-5 days later, or were injected intravenously with 

 [carboxy-^*C] acetate followed by ^*C glucose three days later, 

 when the radioactivity in the milk fat due to acetate had 

 fallen to a negligibly low level. At the time of administration 

 of the labelled substance the mothers were separated from 

 their litters; they were milked six hours later. Usually 20 ml. 

 of milk were obtained. The milk fat was extracted with 

 boiling ethanol-ether (3:l,v/v) and the fatty acids fractionated 

 as in the experiment on the goat. In addition to the glyceride 

 fatty acids, glycerol was obtained as the tribenzoate from the 

 hydrolysate of glyceride fat, and lactose from the ethanol-ether 

 extract. Cholesterol was isolated as the digitonide. 



