LIPIDE METABOLISM 457 



loinu'd to propanediol and thence to pyruvic acid, the metabolism of 

 which has been cHsciissed previously (Chapter 7). 



Under certain conditions of abnormal metabolism, such as during 

 starvation or diabetes, the concentration of ketone bodies increases to 

 the point where the animal is said to have ketosis. When an animal 

 is deprived of food for an extended period of time, the stores of gly- 

 cogen in the liver are rapidly depleted and the animal is forced to 

 depend largely on depot lipides for its energy. Accordingly the 

 degradation of fatty acids in the liver takes place at a greatly ac- 

 celerated rate with the result that there is formed an overabundance 

 of acetoacetyl CoA, which in turn favors the formation of acetoacetic 

 acid and its products, j5-hydroxybutyric acid and acetone. Thus, 

 ketosis develops. 



In diabetes, lipides are utilized for energy since normal carbohy- 

 drate metabolism is impaired owing to a lack of insulin. Hence once 

 again fatty acid degradation takes place at a very rapid rate, produc- 

 ing ketone bodies in increasing amounts. 



BIOSYNTHESIS OF FATTY ACIDS 



Feeding experiments have shown that animals do not acquire fatty 

 acids solely from dietary sources, but rather are capable of synthesiz- 

 ing some of them in the body. In fact it has been established that 

 mammals can synthesize the major portion of the fatty acids required 

 for growth and maintenance. In this way, the common saturated and 

 monoethenoid fatty acids are formed in the animal body from a com- 

 mon precursor, acetyl CoA. Therefore, any dietary constituent or 

 body tissue capable of producing acetyl CoA is at least theoretically 

 a potential source of carbon for fatty acid synthesis (lipogenesis). 

 Acetyl CoA is readily available from the metabolic reactions of carbo- 

 hydrates, proteins, or fats. Although some amino acids such as leucine 

 and alanine yield acetate directly, others are known to be glycogenic 

 and thereby yield acetate by the carbohydrate route. 



Mechanism of Lipogenesis 



At first it was assumed that all the major steps outlined for fatty 

 acid degradation were reversible, thereby affording a complete mecha- 

 nism for the synthesis of fatty acids. However, it was soon discovered 

 that lipogenesis differs from degradation in that the former has a 

 specific requirement for TPNH as coenzyme for one of the reactions. 

 Essentially, fatty acid synthesis may be described by 4 separate reac- 



