460 ANIMAL BIOCHEMISTRY 



these compounds from acetyl CoA derived from carbohydrates and 

 amino acids. Whereas the mixture of fatty acids obtained in the diet 

 varies considerably with respect to chain length and unsaturation, the 

 process of lipogenesis apparently favors the production of 16- and 

 18-carbon chains. Moreover it appears from evidence based on isotope 

 data that saturated acids are formed in larger quantity and at a greater 

 rate of speed than are unsaturated acids. 



If isotopic stearic acid is fed to an animal a part of the isotope ac- 

 cumulates in oleic acid. In a similar fashion, feeding isotopic pal- 

 mitic acid results in an accumulation of palmitoleic acid. Moreover, 

 administration of isotopically marked palmitoleic or oleic acid results 

 in the appearance of "tagged" palmitic and stearic acids in the depot 

 fats. Thus it is evident that the animal is capable of introducing one 

 double bond in the carbon chain between carbons 9 and 10 and also 

 has the ability to convert such unsaturated acids to saturated ones. 

 The liver is the major site for such conversions, although evidence is 

 accumulating to indicate that these reactions can take place in other 

 tissues as well. 



Essential Fatty Acids 



It is significant that, when isotopic stearic or oleic acid is fed to 

 most animals, none of the isotope appears in linoleic or linolenic acid. 

 This suggests that introduction of a second double bond does not take 

 place, at least in some animals. Therefore, the inability to synthesize 

 these polyunsaturated acids makes it necessary for such animals to 

 ingest these compounds. Thus, linoleic, linolenic, and arachidonic 

 acids are said to be essential fatty acids. 



Although essential fatty acids cannot be produced de novo in any 

 common species of mammal, there is evidence that certain transforma- 

 tions of these acids can be effected. For example, linoleic acid can be 

 converted to y-linolenic acid (6,9,12-octadecatrienoic acid), which by 

 addition of 2 carbons and further dehydrogenation and isomerization 

 yields arachidonic acid (5,8,11,13-docosatetraenoic acid). Although 

 its role is not understood, pyridoxalphosphate appears to be essential 

 in the formation of arachidonic acid from y-linolenic acid. 



CHOLESTEROL METABOLISM 



The sterol found in greatest abundance in animal tissues is cho- 

 lesterol. Although foods such as eggs, milk, and animal fats furnish 

 some cholesterol, the quantity ingested is not sufficient to account for 



