INTERMEDIARY METABOLISM 



dilutions would be a measure of the amounts of acetic acid formed by 

 the intact animal. This assumj)tion proved to be valid for the acetyla- 

 tion of the aromatic amines, sulfanilamide and //-aminobenzoic acid 

 (unpublished results of Bloch and Rittenberg). The procedure em- 

 ployed represents an application of the isotope dilution technique to 

 in vivo systems. Dietary deuterioacetate and noni.sotopic endogenous 

 acetate merge, and a representative sample of the mixture becomes 

 fixed in a reaction product which resists attack by the animal cell and 

 is excreted in the urine. From the isotope concentration of the excreted 

 acetyl amino compound it can be estimated that, in 24 hours, the adult 

 rat produces a quantity of acetic acid equal to about one per cent of 

 its body weight. On the other hand, one can calculate from data on 

 fatty acid turnover (17) the amount of acetic acid which should arise 

 in the same period of time on the assumption that one molecule of 

 palmitic acid is broken down to eight molecules of acetate. The value 

 calculated on this basis is in close agreement with that found experi- 

 mentally by the isotope dilution method. These results suggest that, 

 during oxidation, most, if not all, carbon atoms of the fatty acid pass 

 through the acetic acid stage. 



By taking advantage of the fact that, in the acetylation of 

 foreign amines a sample of endogenous acetate is trapped and elimi- 

 nated in the form of a metabolically inert derivative, it becomes pos- 

 sible to demonstrate acetic acid formation from individual fatty acids. 

 Appearance of deuterioacetyl groups after administration of a labeled 

 fatty acid will be indicative of intermediary acetic acid formation. In 

 accord with the beta oxidation theory, labeled myristic, ^-valeric, 

 isovaleric, and butyric acids were found to result in the excretion of 

 isotopic acetyl groups, while propionic and isobutyric acids failed to 

 do so (5). Extension of these experiments to other fatty acids which 

 are labeled at specific positions with deuterium as well as isotopic 

 carbon seems desirable. 



As some of the intermediates in fatty acid catabolism contain 

 labile hydrogen which is subject to enolization and resultant exchange 

 with the hydrogen of the body fluids, deuterium may be partly lost. 

 Quantitative evaluation of the data on acetic acid formation from the 

 higher fatty acids may, therefore, be difficult and must await the 

 preparation of test substances containing isotopic carbon. On the 

 other hand, the lability of G — H bonds may often be of advantage by 



299 



