TRIGLYCERIDES AND FATTY ACIDS 99 



COOH) labeled with deuterium, gave rise to labeled azelaic acid in the case 

 of dogs. This dicarboxylic acid could be isolated from the urine, proving 

 that a biologic splitting occurred at the triple bond of the fatty acid. 



g. Hydrogenation of Unsaturated Fatty Acids. The liver has the capac- 

 ity to alter fats by saturating them, as well as by bringing about the 

 reverse change. Probably the same enzymes are concerned with the 

 hydrogenation as are necessary to effect the dehydrogenation. 



(a) Hydrogenation of a, ^-Unsaturated Compounds. Linneweh^^* was the 

 first to observe that 7-crotonobetaine could be saturated to yield the cor- 

 responding butyrobetaine. 



+ + 



(CH3)3N-CH2-CH:CH-COO- > (CH3)3N-CH2-CH2-CH2-COO- 



7-Crotonobetaine 7-Butyrobetaine 



Kuhn, Kohler, and Kohler" reported that the hydrogenation of the 

 co,/3-double bond of geranic acid occurs together with co-oxidation when this 

 compound is metabolized. 



CH3 CH3 



CH3C:CH-CH2CH2C:CH-COOH (I) > 



CH3 CH3 



I I 



HOOCC:CHCH2CH2C:CHCOOH (II) 



CH3 CH, 



> HOOCC:CHCH2CH2-CHCH2COOH (III) 



Illustration of the Metabolism of Geranic Acid (I), with the Formation of Hilde- 

 brandt Acid (II) by w-Oxidation Alone, or of Dihydro-Hildebrandt Acid (III) by w- 

 Oxidation Together with a,/3-Hydrogenation" 



Fischer^-^ observed that hydrogenation is brought about by yeast acting 

 on unsaturated aldehydes and alcohols, but only when the double bond is in 

 the Q:,/3-position.^" Fischer and Bielig'" demonstrated that Q;,/3-saturation 

 may be effected in animals, luit only mider certain conditions. Thus, when 

 cinnamyl alcohol, CeHs-CHrCH -0112011, was fed to dogs, the following 

 changes were observed: a!,jS-hydrogenation alone resulting in the forma- 

 tion of /3-phenylpropanol, C6H5-CH2-CH2 -0112011 and, on the addition of 

 water, in the formation of j8-hydroxy-(8-phenylpropanol, OeHs-OHOH-- 

 OH2-OH20H. These end-products were present to the extent of only 3%; 

 i8-hydroxy-,3-phenylpropanol might have arisen from j8-oxidation of /3- 



1^ W. Linneweh, Z. physiol. Chem., 181, 54-57 (1929). 

 '^ F. G. Fischer, Angew. Chem., 53, 461-471 (1940). 

 "^ F. G. Fischer and H. J. Bielig, Z. physiol. Chem., 266, 73-98 (1940). 



