TRIGLYCERIDES AND FATTY ACIDS 



129 



The mechanism of oxidation of the odd fatty acids is not kno^vn, but it 

 is presumed that the /3-oxidation-acetic acid condensation theory also 

 appUes. Under these conditions, one might postulate that heptanoic acid 

 would give rise to two acetic acid molecules (which could condense to 

 yield acetoacetate) and one molecule of propionic acid (which could be 

 changed to glucose). 



CH3(CH2)5-COOH 

 Heptanoic acid 



-* CHrCHoCH.CHs-COOH + 

 Valeric acid \ 



CH3CH2COOH 



Propionic acid 



CH3COOH 

 Acetic acid 



+ 



^.CHsCOOH 



Acetic acid 



CeHnOe CHj-COCHj-COOH 



Glucose Acetoacetic acid 



The Oxidation of Heptanoic Acid to Yield Acetoacetic Acid and Glucose 



Propionic acid is a strong glycogen former and does not give rise to any 

 ketone bodies. As the length of the odd-chain acid is increased, a larger 

 proportion of the molecule is available for acetoacetate formation from the 

 acetic acid set free, and a smaller proportion is convertible to propionic acid. 

 One would therefore expect that the higher odd fatty acids might exhibit 

 some degree of ketonuria. However, the excretion of ketone bodies is 

 suppressed to some extent by the presence of sugar being formed concomi- 

 tantly from the propionic acid moiety produced. 



The above mechanism for the metaboHsm of n-valeric acid has been con- 

 firmed by Siegel and Lorber,^^^ who made use of doubly-labeled valeric 

 acid. When carboxyl-C^^, 7-C'^-n- valeric acid was fed to fasted rats 

 along with glucose, about three times as much of the C'* was recovered in 

 the liver glycogen as of the C'^. Moreover, C^^ was found only on carbons 

 3 and 4 of glucose prepared from this glycogen, w^hile C^* was present on 

 carbons 1, 6, and 2, 5, predominantly and equally. According to these 

 workers, their findings correspond to an in vivo metabolic pathway for 

 n-valerate involving /3-oxidation to "acetate" and "propionate." 



On the basis of these principles, Kahn-^^ prepared an artificial triglyceride 

 composed of the odd-carbon acid, margaric acid (CH3-(CH2)i5-COOH), 

 which was called "intarvin" by Keefer et al.,'^^'' who reported that this sub- 

 stance was less ketogenic than were ordinary fats. However, their results 



*» I. Siegel and V. Lorber, /. Biol. Chem., 189, 571-576 (1951). 

 "« M. Kahn, Am. J. Med. Sci., 166, 826-833 (1923). 



*'" C. S. Keefer, W. A. Perlzweig, and W. S. McCann, Bull. Johns Hopkins Hosp., 35, 

 265-270(1924). 



