ANIMAL METABOLISM 337 



— COOH end of the fatty acid. These are then further oxidized to carbon 

 dioxide and water. The exact details of how the two-carbon piece is 

 broken off have not been completely worked out, but are probal)ly some- 

 what as follows: 



CHoCHnCHsCH.COOH ~|^ CH2CH,CH=CHC00H 



Carboxyl end of saturated Corresponding a, /3- 



"fatty acid molecule unsaturated acid 



O 



+ H2O II + HoO 



^ CH2-CH,-C— CH2COOH • 



-2(H) ^"^ ^"^•- ^ -.xxov.v.w.x ^3jj 



(30) Corresponding /3-keto acid 



CH2CH2COOH + CH3COOH 



Saturated fatty Acetic acid 



acid with two less 

 carbon atoms 



Note that it is the beta carbon atom (second from the — COOH group) 

 which is oxidized. This process is then thought to occur over and over 

 until the original fatty acid molecule has been broken down entirely 

 to acetic acid and hydrogen. For example, stearic acid, containing 

 eighteen carbon atoms, is split in 8 places to yield 9 molecules of acetic 

 acid as follows: 



CH3(CH2)i6COOH + 16HoO^ 9CH3COOH + 32(H) 



Block and Rittenberg have estimated the normal acetic acid production 

 in rats to be about 1 g. per 100 g. of body weight per day. The exact 

 amount will of course be influenced by the proportion of fat in the ration. 

 Thus acetic acid, and possibly also acetoacetic acid, represent the end 

 products of fat catabolism in the liver. These products are transported 

 by the blood to the muscles and kidneys, where the oxidation is completed. 



Other Theories of Fat Oxidation. There is evidence that the methyl 

 group of fatty acids (the "omega" carbon atom at the opposite end of 

 the chain from the — COOH) may be oxidized to a second carboxyl group. 

 This oxidation would produce a dibasic acid which could then undergo 

 )8-oxidation from each end. The "omega oxidation" probably occurs to 

 a minor extent, and only with fatty acids of intermediate chain length 

 (about 8-12 carbon atoms) . That it can occur, however, has been shown 

 by feeding dogs triglycerides of such fatty acids as undecanoic (saturated, 

 C-11 acid). The urine of these dogs was found to contain dibasic acids 

 of 11, 9, and 7 carbon atoms. Omega oxidation is probably not a major 

 pathway of normal fat catabolism. 



A third type of fat catabolism is 7nidtiple alternate oxidation. Accord- 

 ing to this idea the fatty acid is oxidized at the )8-carbon and at each 



