254 VI. ACETIC, FORMIC, AND PROPIONIC ACIDS 



(7) The Occurrence of Acetic Acid in the Animal Body 



The presence of the acetate molecule in the tissues has been generally 

 accepted since the development of the theory of /3-oxidation. As a result 

 of the demonstration by Ivnoop- that phenyl-substituted fatty acids are 

 oxidized to benzoic or to phenylacetic acids, one must conclude that the 

 acetic acid would be a normal by-product of this reaction. The supposi- 

 tion is that two-carbon compounds, presumably molecules of acetic acid, 

 are successively spHt off as oxidation proceeds. The results of Dakin^ 

 likewise supported this hypothesis. 



In spite of the fact that acetic acid is believed to be continuously formed 

 as a result of metabolic reactions, the quantities in the blood are barely 

 detectable.^ This acid has been recognized as a normal constituent of 

 horse urine^ and of human urme.^''' In fact, Prousf^ recognized its presence 

 as early as 1800. The failure to demonstrate the presence of acetic acid in 

 tissues may be due to the fact that it is so rapidly metabolized. Smyth^ 

 reported that only mmimal amounts of acetic acid are excreted in the urine 

 after the mtra venous injection of acetate, and that this excretion accounts 

 for only a small proportion of the acetate disappearmg from the blood 

 stream. The liver was shown to be the tissue most active in destroymg the 

 acetate, while the kidney is no more important than other extrahepatic 

 organs in acetate metabolism. Acetate is used to some extent by muscle,^ 

 as well as by the beating heart. ^ Another suggestion to explain the failure 

 to demonstrate acetic acid in tissues is that it may be bound with protein. ^"-^^ 



The volatile acids, which mclude propionic and butyric acid, as well as 

 acetic, are present in the highest amount in the cecum and colon of the 

 horse, pig, and rabbit; in the sheep, which is a ruminant, an even greater 

 amount is present in the reticulum and rumen. Thus, Barcroft and 

 associates^'- found the following average weights for volatile acids in the 



2 F. Knoop, Beitr. chem. Physiol., Pathol., 6, 150-162 (1905). 



3 H. D. Dakin, /. Biol. Chem., 6, 203-219, 221-233 (1909). 



4 J. F. McClendon, /. Biol. Chem., 154, 357-360 (1944). 



5 C. Schotten, Z. phijsiol. Chem., 7, 375-383 (1882-1883). 



6 J. L. W. Thudichum, J. Chem. Soc, 23, 400-409 (1870); Arch. ges. Physiol. (Pflii- 

 ger's), 16, 12-26 (1877). 



7 M. Proust, Ann. chim. (ser. 1), 36, 258-277 (1800); D. L. Proust, Ibid. (ser. 2), 

 ^4, 257-288(1820). 



8 D. H. Smyth, /. Physiol., 105, 299-315 (1947). 



9 J. Barcroft, R. A. McAnally, and A. T. Phillipson, Biochem. J., 38, iv (1944). 

 i» A. L. Lehninger, /. Biol. Chem., 143, 147-157 (1942). 



11 E. M. MacKay, personal communication to A. L. Lehninger, /. Biol. Chem., 143, 

 147-157 (1942), p. 150. 



12 J. Barcroft, R. A. McAnally, and A. T. Phillipson, Biochem. J., 38, ii (1944). 



