LIPIDS PRESENT IN MILK FAT 809 



shown to have no similar action on fat synthesis in the mammary tissue 

 of the ruminant (sbeep). 618 



Folley 518 proposed an interesting hypothesis to explain the variability 

 of insulin in stimulating lipogenesis in in vitro tests in the mammary tissue 

 of the non-ruminant as contrasted with that of the ruminant. It is sug- 

 gested that there are differences in the energy-yielding systems in the two 

 classes of animals. In the case of the ruminant, acetate is the preferred 

 source of energy, while in the non-ruminant carbohydrate appears to be the 

 energy-yielding substrate of choice. Since insulin does not influence the 

 conversion of acetate to fat in the ruminant, and does so in the non-rumi- 

 nant only when glucose is present as well as acetate, it is postulated that 

 insulin stimulates fat synthesis by its effect on the utilization of carbohy- 

 drate for energy production. 



(c) The Importance of Glycerol in the Synthesis of Fats in the Mammary 

 Gland. In view of the fact that the main fatty end-products from the mam- 

 mary gland excreted in milk are triglycerides, it is possible that glycerol 

 may also present a bottleneck in the synthesis of this new fat. A plentiful 

 supply of this alcohol would probably accelerate fat synthesis, and would 

 tend to trap the fatty acids, thus preventing further changes. Folley and 

 French 557 suggested that the stimulatory effect of glucose on the net fat 

 synthesis by mammary tissue slices might in part be due to the formation 

 of glycerol from carbohydrate. That this is the case has been proved by 

 Balmain and Folley, 586 who reported that the addition of glycerol to a sys- 

 tem containing lactating mammary gland slices incubated in an acetate 

 plus glucose medium caused an insulin-like effect; the R.Q. was increased, 

 the oxygen uptake and acetate utilization rose, but the glucose breakdown 

 was not changed. In the last respect, the glycerol effect differed from the 

 action of insulin. Balmain and associates 572 proved, by the use of isotopic 

 acetate, that an increased fat synthesis resulted from the addition of glycerol. 

 Glycerol was likewise shown to promote fat synthesis from acetate in 

 media containing lactating sheep udder slices. In this case its stimulatory 

 effect was exerted only when acetate alone was present in the medium, and 

 not when both acetate and glucose were available. 



Folley 518 has offered as a possible explanation for the above findings the 

 hypothesis that the effect of insulin on the mammary synthesis of fat is 

 mediated by a system which forms glycerol from glucose rather than by 

 yielding energy derived from the breakdown of carbohydrate. The simi- 

 larity of the stimulation of fat synthesis by glycerol and by insulin is indi- 

 cated by the fact that the effects of glycerol and of insulin are not additive. 586 



686 J. H. Balmain and S. J. Folley, Biochem. J., 49, 663-670 (1951). 



