256 VI. ACETIC, FORMIC, AND PROPIONIC ACIDS 



detection, is by the use of the isotope dilution technic. When an isotopi- 

 cally marked analogue of the metabolite under consideration is admin- 

 istered to an animal, or is added to an isolated system, and the isotopically 

 marked atoms are subsequently recovered in a free or combined state, the 

 test substance will be diluted with unlabeled compound in proportion to 

 that which is available in the tissues. Thus, Bernhardt® reported that, 

 when foreign amines such as sulfanilamide and deuterioacetic acid were 

 given to rabbits, the acetylated sulfanilamide excreted contained only 12% 

 of the labeled acetate. In tests on human subjects, it was found that 9% 

 of the acetyl excreted was derived from the deuterioacetic acid fed. The 

 conclusion was that the fraction of acetyl groups contibuted by exogenous 

 acetate was in proportion to its contribution to the total acetate available. 

 The acetyl precursor, therefore, was a mixture of endogenous acetate and 

 the labeled acetate which had been fed. A similar approach was made by 

 Bloch and Rittenberg" in determining the proportion of isotope present 

 in acetylamine (CH3-CONH2) in the intact animal after the injection of 

 deuterioacetic acid. The results obtained justified the conclusion that 

 the acetylation process can be attributed to a merging of labeled dietary 

 acetate and that normally arising during metabolism. On the basis of 

 the apphcation of the equation for isotope dilution, ^^ the quantity of ace- 

 tate formed in the rat over a twenty-four-hour period was calculated to 

 be 15 to 20 mM (0.9 to 1.2 g.) per 100 g., or approximately 1% of the 

 body weight. ^^ The accuracy of this figure is based upon the assumption 

 that ingested acetic acid mixes adequately with the acetate in the meta- 

 bolic pool. Moreover, the correctness of the figure is supported by the 

 fact that the isotope concentration of excreted acetyl was found to be 

 directly proportional to the dosage of isotopic acetate; this would indicate 

 that the concentration of acetate calculated as available in vivo was inde- 

 pendent of the dosage variation over a wide range. Thus, there was a 

 rapid merging of the ingested (exogenous) acetate with the acetic acid 

 formed in the tissues (endogenous). 



Lorber et al.^^ tested the acetate concentration in the intact rat, by 

 perfusion of the isolated heart with labeled acetic acid, and obtained 

 figures which correspond closely with those obtained by the isotope dilution 

 method. By means of calculations of the acetate required for fatty acid 

 regeneration over given periods, as determined by the method of Bernhard 



i« K. Bernhard, Z. physiol. Chem., 267, 91-98 (1940). 



1' K. Bloch and D. Rittenberg, J. Biol. Chem., 159, 45-58 (1945). 



18 D. Rittenberg and G. L. Foster, /. Biol. Chem., 1S3, 737-744 (1940). 



19 V. Lorber, N. Lifson, H. G. Wood, and J. Barcroft, Am. J. Physiol., U6, 557-560 

 (1946). 



