GLYCEROL 47 



is derived, not from lactose, but from a hexose (glucose) which is in equilib- 

 rium with lactose. Thus, glucose originating from the newly-formed 

 lactose which, in turn, owes its origin to acetate, would serve as the mother 

 substance of glycerol. 



However, Balmain and associates'"' reported that, although the mammary 

 tissue of the living animal has the capacity to utilize acetate as a source of 

 glycerol, this conversion cannot occur in the mammary slices of the sheep 

 or rat in vitro. Under such conditions, with labeled acetate in the sub- 

 strate, the glycerol isolated from the newly-s>^lthesized fat was in all 

 cases non-radioactive. Thus, such isolated mammary tissue, although it 

 can effect a synthesis of fatty acids from acetate, cannot bring about a 

 conversion to glucose under the conditions so far investigated. 



While the foregoing evidence clearly indicates that glycerol is derived 

 from glucose in the mammary gland, the question which is still unanswered 

 is whether or not the rate of fat synthesis may be conditioned by the speed 

 of glycerol formation. It has been suggested that, once the fatty acid is 

 synthesized, it is "trapped" by being converted to the triglyceride. Thus, 

 it will immediately be removed from the site of synthesis. If a plentiful 

 supply of glycerol is present in mammary tissue, a maximal amount of 

 glyceride formation is to be expected under such conditions, both because 

 the synthesis is not delayed by lack of constituents, and particularly be- 

 cause the rate of synthesis of fatty acids is augmented by the rapid removal 

 of the end-product from the arena of activity. 



An answer to the question as to whether or not the speed of fat synthesis 

 is conditioned by the glycerol currently available has been given by Bal- 

 main and FoUey.^"^ These workers demonstrated that glycerol may be a 

 limiting factor in triglyceride sjmthesis in rat mammary tissue. When 

 glycerol was added to a system consisting of the mammary slices metabo- 

 lizing glucose plus acetate, a stimulation of R.Q., Qo^, and acetate uptake 

 similar to that obtained with insulin was noted. Balmain and associates^" 

 subsequently demonstrated that the addition of glycerol to mammary 

 slices containing labeled acetate increased the incorporation of the acetate 

 into fat. A similar stimulation of fat synthesis by glycerol was reported 

 in sheep udder when acetate was used, but not when both acetate and glu- 

 cose were present. 



It is now believed ^^^ that the increased fat synthesis resulting from the 

 addition of insulm in the presence of glucose is explained by the additional 

 glycerol thus made available as a result of the effect of the hormone on 

 glucose. The fact that the fat synthesis occurs with glycerol in rat mam- 

 mary tissue without an increase in glucose uptake is in harmony with this 



