CHAPTER II 



THE BIOSYNTHESIS OF TRIGLYCERIDES, 



PHOSPHOLIPIDS, AND FATTY ACIDS 



IN THE ANIMAL BODY 



1. Introduction 



Considerable evidence has accumulated to support the hypothesis that fat 

 is readily synthesized in the body from non-fatty materials such as carbo- 

 hydrates and proteins. Thus, it has been shown by the use of balance 

 experiments that the amount of carbon unaccounted for in the excreta 

 after the administration of starch is so great that it can be stored only in 

 the form of fat. Moreover, the respiratory quotients observed in animals 

 on a high-carbohydrate diet have been found in many cases to exceed 

 unity — a condition which should obviously occur when an oxygen-rich sub- 

 stance (such as carbohydrate) is being converted to an oxygen-poor com- 

 pound (such as fat). Probably the most convincing argument to prove the 

 change of carbohydrate to fat in the animal is the demonstration of iso- 

 topic carbon in fats after isotopically tagged carbohydrates have been fed. 



Proteins likewise are known to serve as a source of the triglycerides in 

 the body. The same basic experimental procedures have been employed 

 to demonstrate the change of protein -*■ carbohydrate, as well as the com- 

 plete transformation of protein -> fat. Since the proof that carbohj^drate 

 can be changed to fat is extensive and convincuig, the evidence that 

 protein can yield carbohydrate is a priori proof that it can also be converted 

 to fat. 



Experiments have likewise demonstrated the complete protein-fat re- 

 action. These procedures for esta}>lishing this change include the classical 

 type of balance experiments, as well as the response of the R.Q. in dogs 

 receiving large doses of protein ; these confirm the fact that the non-protein 

 moiety may be stored in the form of fat. The proof of the synthesis of fat 

 from carbohj'drate and protein has been detailed in Yo\. II, The Lipids 

 (pp. 538-553). The present chapter will be concerned chiefly Avith the 

 intermediaiy changes in these transformations. 



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