338 ANIMAL METABOLISM 



alternate carbon beyond the y8-position toward the methyl end of the 

 chain. This results in a polyketo acid of the type, . . .COCH2COCH2- 

 COCH2COCH2COOH, which breaks down all at once to form acetic 

 (or acetoacetic) acid. It is still uncertain whether this type of oxida- 

 tion occurs extensively during fat metabolism in animals. 



Oxidation of Acetic Acid. The two-carbon fragment produced during 

 fat catabolism, as described above, can be further metabolized in a variety 

 of ways. Probably the bulk of it, under normal circumstances, is com- 

 pletely oxidized to carbon dioxide and water. This oxidation occurs 

 chiefly in the muscles and kidneys, the main energy-using organs. The 

 acetic acid condenses with oxalacetic acid to form citric acid (reaction 

 17, Fig. 13-4), which is then further metabolized by the reactions of the 

 citric acid cycle. This condensation, therefore, is a connecting link be- 

 tween fat and carbohydrate metabolism. Following the reactions from 

 acetic acid back to oxalacetic acid (Fig. 13-4), it may be seen that 

 8(H) and 2CO2 are removed, while 2H2O have been added. This means 

 that the acetic acid has been completely broken down: 



CH3COOH + 2H20-> 8(H) + 2CO2 



Since stearic acid was shown above to form nine acetic acid molecules, 

 the complete catabolism of this acid can now be represented as follows: 



CH3(CH2)i6COOH + 34H20^ 104(H) + ISCOo 



The hydrogen atoms, of course, are united with coenzymes as fast as they 

 are produced and are immediately transferred through the cytochrome 

 system to oxygen, thereby being converted into water. 



Ketosis. One of the most important features of fat metabolism is 

 the fact that fat is not oxidized efficiently to carbon dioxide and water 

 unless carbohydrate is also being oxidized at the same time. The reason 

 probably is that the supply of oxalacetic acid, which is formed from 

 pyruvic acid and carbon dioxide (reactions 15 and 25, Fig. 13-4), is 

 low when carbohydrates, and hence pyruvic acid, are not being metab- 

 olized. The essential relationships involved may be illustrated by an 

 hypothetical case. Suppose only one molecule of oxalacetic acid is pres- 

 ent in a cell which needs to oxidize three molecules of acetic acid. Three 

 separate "turns" of the citric acid cycle, one after another, will be required 

 to complete the job. However, if two molecules of pyruvic acid are 

 also present, they can be converted into two extra molecules of oxalacetic 

 acid, and hence all of the acetic acid — as well as the pyruvic — can be 

 metabolized at one turn of the cycle. At any rate, when carbohydrates 

 are not being metabolized, the acetic acid coming from fats is not 

 oxidized as fast as it is produced. Instead it piles up and is recombined 

 into acetoacetic acid: 



(32) 



CHjCOOH + CH3COOH ^-^ CH3COCH2COOH + H2O 



